0001564590-21-012260 | 10-K | XBRL Viewer

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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2020	Mar. 05, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Registrant Name	Gritstone Oncology, Inc.
Entity Central Index Key	0001656634
Current Fiscal Year End Date	--12-31
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Trading Symbol	GRTS
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Voluntary Filers	No
ICFR Auditor Attestation Flag	false
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		48,951,561
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Public Float			$ 198.7
Entity File Number	001-38663
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-4859534
Entity Address, Address Line One	5959 Horton Street
Entity Address, Address Line Two	Suite 300
Entity Address, City or Town	Emeryville
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94608
City Area Code	510
Local Phone Number	871-6100
Document Annual Report	true
Document Transition Report	false
Documents Incorporated by Reference	Portions of the Registrant’s Definitive Proxy Statement relating to the Annual Meeting of Shareholders, scheduled to be held on June 18, 2021, are incorporated by reference into Part III of this Annual Report on Form 10-K where indicated. Such Definitive Proxy Statement will be filed with the Securities and Exchange Commission within 120 days after the fiscal year to which this report relates.

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 170,056	$ 57,408
Marketable securities	1,002	70,368
Prepaid expenses and other current assets	4,332	3,497
Total current assets	175,390	131,273
Property and equipment, net	22,105	26,911
Operating lease right-of-use assets	21,344	23,427
Deposits and other long-term assets	2,728	2,778
Total assets	221,567	184,389
Current liabilities:
Accounts payable	9,578	4,621
Accrued compensation	6,331	4,598
Accrued liabilities	677	1,041
Accrued research and development expenses	1,053	1,779
Lease liabilities, current portion	5,874	2,505
Deferred revenue, current portion	3,475	4,956
Total current liabilities	26,988	19,500
Other non-current liabilities	395	0
Lease liabilities, net of current portion	19,225	20,985
Deferred revenue, net of current portion	8,220	9,560
Total liabilities	54,828	50,045
Commitments and contingencies (Notes 6 and 7)
Stockholders’ equity:
Convertible preferred stock, $0.0001 par value; 10,000,000 shares authorized; no shares issued and outstanding	0	0
Common stock, $0.0001 par value; 300,000,000 shares authorized at December 31, 2020 and 2019; 47,552,693 and 36,332,956 shares issued and outstanding at December 31, 2020 and 2019, respectively	18	17
Additional paid-in capital	493,023	355,291
Accumulated other comprehensive gain	0	24
Accumulated deficit	(326,302)	(220,988)
Total stockholders’ equity	166,739	134,344
Total liabilities and stockholders’ equity	$ 221,567	$ 184,389

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	47,552,693	36,332,956
Common stock, shares outstanding	47,552,693	36,332,956

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Total revenues	$ 4,037	$ 4,365	$ 1,187
Operating expenses:
Research and development	88,643	82,896	54,965
General and administrative	21,411	19,409	11,806
Total operating expenses	110,054	102,305	66,771
Loss from operations	(106,017)	(97,940)	(65,584)
Interest income, net	703	3,507	809
Net loss	(105,314)	(94,433)	(64,775)
Other comprehensive gain (loss):
Unrealized gain (loss) on marketable securities	(24)	109	(11)
Net and comprehensive loss	$ (105,338)	$ (94,324)	$ (64,786)
Net loss per share, basic and diluted	$ (2.79)	$ (2.81)	$ (7.26)
Weighted-average number of shares used in computing net loss per share, basic and diluted	37,792,365	33,554,823	8,919,281
Collaboration [Member]
Total revenues	$ 3,462	$ 4,365	$ 1,187
Contribution [Member]
Total revenues	$ 575	$ 0	$ 0

Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	ATM Equity Offering Program [Member]	IPO [Member]	Follow-on Public Offering [Member]	Private Investment in Public Equity [Member]	Revision of Prior Period, Accounting Standards Update, Adjustment [Member]	Convertible Preferred Stock [Member]	Series C Convertible Preferred Stock [Member]	Series A B And C Convertible Preferred Stock	Common Stock [Member]	Common Stock [Member] ATM Equity Offering Program [Member]	Common Stock [Member] IPO [Member]	Common Stock [Member] Follow-on Public Offering [Member]	Common Stock [Member] Private Investment in Public Equity [Member]	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member] ATM Equity Offering Program [Member]	Additional Paid-In Capital [Member] IPO [Member]	Additional Paid-In Capital [Member] Follow-on Public Offering [Member]	Additional Paid-In Capital [Member] Private Investment in Public Equity [Member]	Accumulated Other Comprehensive Gain/(Loss) [Member]	Accumulated Deficit [Member]	Accumulated Deficit [Member] Revision of Prior Period, Accounting Standards Update, Adjustment [Member]
Balance at Dec. 31, 2017	$ 97,282						$ 156,937			$ 1					$ 2,045					$ (74)	$ (61,627)
Balance, shares at Dec. 31, 2017							17,797,529			2,152,525
Issuance of stock, net of issuance costs	20,935		$ 92,537					$ 20,935				$ 1					$ 92,536
Issuance of stock, net of issuance costs, shares								1,611,603				6,854,202
Conversion of Series A, B, and C convertible preferred stock into common stock upon initial public offering									$ (177,872)			$ 14					$ 177,858
Conversion of Series A, B, and C convertible preferred stock into common stock upon initial public offering, shares									(19,409,132)			19,409,132
Unrealized (loss) gain on marketable securities, net of tax	(11)																			(11)
Lapse of repurchase rights related to common stock issued pursuant to early exercises	97														97
Lapse of repurchase rights related to common stock issued pursuant to early exercises, shares										282,204
Issuance of common stock upon exercise of stock options	49														49
Issuance of common stock upon exercise of stock options, shares										80,463
Issuance of common stock for consulting services	36														36
Issuance of common stock for consulting services, shares										4,347
Exercise of common stock warrants	13														13
Exercise of common stock warrants, shares										40,257
Stock-based compensation	2,959														2,959
Net loss	(64,775)																				(64,775)
Balance at Dec. 31, 2018	$ 149,122					$ (153)				$ 16					275,593					(85)	(126,402)	$ (153)
Balance, shares at Dec. 31, 2018										28,823,130
Accounting Standards Update Extensible List	us-gaap:AccountingStandardsUpdate201602Member
Issuance of stock, net of issuance costs		$ 3,763		$ 69,709									$ 1			$ 3,763		$ 69,708
Issuance of stock, net of issuance costs, shares		490,880									490,880		6,500,000
Issuance of common stock under employee stock purchase plan (“ESPP”)	$ 413														413
Issuance of common stock under employee stock purchase plan ("ESPP"), shares										55,727
Unrealized (loss) gain on marketable securities, net of tax	109																			109
Lapse of repurchase rights related to common stock issued pursuant to early exercises	63														63
Lapse of repurchase rights related to common stock issued pursuant to early exercises, shares										182,195
Issuance of common stock upon exercise of stock options	452														452
Issuance of common stock upon exercise of stock options, shares										281,024
Stock-based compensation	5,299														5,299
Net loss	(94,433)																				(94,433)
Balance at Dec. 31, 2019	134,344									$ 17					355,291					24	(220,988)
Balance, shares at Dec. 31, 2019										36,332,956
Issuance of stock, net of issuance costs		$ 9,670			$ 32,122									$ 1		$ 9,670			$ 32,121
Issuance of stock, net of issuance costs, shares		1,160,963									1,160,963			9,586,478
Issuance of pre-funded warrants, net of issuance costs	87,704														87,704
Issuance of common stock under employee stock purchase plan (“ESPP”)	910														910
Issuance of common stock under employee stock purchase plan ("ESPP"), shares										243,878
Unrealized (loss) gain on marketable securities, net of tax	(24)																			$ (24)
Lapse of repurchase rights related to common stock issued pursuant to early exercises	11														11
Lapse of repurchase rights related to common stock issued pursuant to early exercises, shares										30,874
Issuance of common stock upon exercise of stock options	$ 206														206
Issuance of common stock upon exercise of stock options, shares	197,544									197,544
Stock-based compensation	$ 7,110														7,110
Net loss	(105,314)																				(105,314)
Balance at Dec. 31, 2020	$ 166,739									$ 18					$ 493,023						$ (326,302)
Balance, shares at Dec. 31, 2020										47,552,693

Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
ATM Equity Offering Program [Member]
Payment for issuance cost	$ 100	$ 300
Common Stock [Member] \| IPO [Member]
Payment for issuance cost			$ 10,276
Shares issued price per share			$ 15.00
Common Stock [Member] \| Follow-on Public Offering [Member]
Payment for issuance cost		$ 556
Shares issued price per share		$ 11.50
Common Stock [Member] \| ATM Equity Offering Program [Member]
Payment for issuance cost	104	$ 301
Common Stock [Member] \| Private Investment in Public Equity [Member]
Payment for issuance cost	$ 1,394
Shares issued price per share	$ 3.71
Warrant [Member]
Payment for issuance cost	$ 3,806
Series C Convertible Preferred Stock [Member]
Payment for issuance cost			$ 81
Shares issued price per share			$ 13.04

Condensed Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
Condensed Statements of Cash Flows - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities
Net loss	$ (105,314)	$ (94,433)	$ (64,775)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	6,644	4,745	3,961
Net amortization of premiums and discounts on marketable securities	(122)	(1,349)	(552)
Stock-based compensation	7,110	5,299	2,995
Non-cash operating lease expense	7,511	6,382	0
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(852)	37	(1,781)
Operating lease right-of-use assets	0	(3,328)	0
Deposits and other long-term assets	49	(351)	(818)
Accounts payable	826	45	3,249
Accrued compensation	1,731	650	1,724
Accrued and other non-current liabilities	662	9	(582)
Accrued research and development expenses	(725)	1,526	0
Deferred rent	0	0	(396)
Lease liability	(3,801)	54	0
Deferred revenue	(2,821)	(4,297)	18,813
Net cash used in operating activities	(89,102)	(85,011)	(38,162)
Investing activities
Purchase of marketable securities	(8,809)	(80,979)	(102,160)
Maturities of marketable securities	72,872	112,993	48,720
Sales of marketable securities	5,401	0	0
Purchase of property and equipment	(3,515)	(16,173)	(5,663)
Net cash provided by (used in) investing activities	65,949	15,841	(59,103)
Financing activities
Proceeds from issuance of common stock, net of issuance costs	10,879	75,202	92,586
Proceeds from issuance of common stock and pre-funded warrants	125,026	0	0
Payments of deferred financing costs	(104)	(807)	(3,080)
Proceeds from issuance of convertible preferred stock, net of issuance costs	0	0	20,935
Net cash provided by financing activities	135,801	74,395	110,441
Net increase in cash, cash equivalents and restricted cash	112,648	5,225	13,176
Cash, cash equivalents and restricted cash at beginning of period	58,400	53,175	39,999
Cash, cash equivalents and restricted cash at end of period	171,048	58,400	53,175
Supplemental disclosures of non-cash investing and financing information
Property and equipment purchases accrued but not yet paid	316	1,232	1,482
Deferred financing costs included in accrued liabilities and accounts payable	5,200	0	0
Remeasurement of operating lease right-of-use asset for lease modification	$ 3,174	$ 1,878	$ 0

Organization

12 Months Ended

Dec. 31, 2020

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization

1.	Organization

Description of Business

Gritstone Oncology, Inc. (“Gritstone” or “the Company”) is a biotechnology company developing targeted immunotherapies for cancer and infectious disease. The Company was incorporated in the state of Delaware in August 2015, and is based in Emeryville, California and Cambridge, Massachusetts, with a manufacturing facility in Pleasanton, California. The Company operates in one segment.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2020

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2.	Summary of Significant Accounting Policies

Basis of Presentation

The accompanying consolidated financial statements have been prepared in accordance with United States generally accepted accounting principles (“U.S. GAAP”) and the rules and regulations of the SEC for reporting. The consolidated financial statements are comprised of the consolidation of the Company and its wholly-owned subsidiary. All intercompany balances and transactions have been eliminated in consolidation. The Company has no unconsolidated subsidiaries or investments accounted for under the equity method.

Need for Additional Capital

The Company has incurred operating losses and has an accumulated deficit as a result of ongoing efforts to develop drug product candidates, including conducting preclinical and clinical trials and providing general and administrative support for these operations. The Company had an accumulated deficit of $326.3 million and $221.0 million as of December 31, 2020 and 2019, respectively. The Company had net losses of $105.3 million, $94.4 million, and $64.8 million for the years ended December 31, 2020, 2019 and 2018, respectively, and net cash used in operating activities of $89.1 million, $85.0 million, and $38.2 million for years ended December 31, 2020, 2019 and 2018, respectively. To date, none of the Company’s drug candidates have been approved for sale and therefore the Company has not generated any revenue from sales of commercial products. Management expects operating losses to continue for the foreseeable future. The Company has evaluated and concluded there are no conditions or events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern for a period of one year following the date that these consolidated financial statements are issued.

Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and judgments that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities as of the date of the financial statements and the reported amounts of expenses during the reporting period. On an ongoing basis, management evaluates its estimates, including those related to preclinical and clinical study trial accruals, fair value of assets and liabilities, the fair value of right-of-use assets (“ROU Assets”) and lease liabilities, fair value of pre-funded warrants, revenue recognition, and the fair value of stock-based compensation awards. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could differ from those estimates.

Fair Value of Financial Instruments

U.S. GAAP establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability and are developed based on the best information available in the circumstances.

Fair value is established as the exchange price, or exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, an established three-tier fair value hierarchy distinguishes between the following:

•

Level 1 inputs are quoted prices in active markets that are accessible at the market date for identical assets or liabilities.

•

Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly.

•

Level 3 inputs are unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the assets or liability. Financial assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement.

To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value instrument.

The carrying amounts reflected on the consolidated balance sheets for cash and cash equivalents, prepaid expenses and other current assets, accounts payable, accrued compensation and accrued liabilities approximate their fair values due to their short-term nature.

Cash, Cash Equivalents and Restricted Cash

Cash equivalents, which consist primarily of highly liquid investments with original maturities of three (3) months or less when purchased, are stated at cost which approximates fair value. These assets include investments in money market funds that invest in U.S. Treasury obligations and certificates of deposit which are stated at fair value.

The Company has issued a letter of credit under a lease agreement which has been collateralized by a cash deposit for an equal amount and is recorded within deposits and other long-term assets on the consolidated balance sheets based on the term of the underlying lease. The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the balance sheets that sum to the total of the same amounts shown in the consolidated statements of cash flows (in thousands).

	December 31,
	2020		2019
Cash and cash equivalents	$	170,056	$	57,408
Restricted cash		992		992
Total cash, cash equivalents and restricted cash	$	171,048	$	58,400

Marketable Securities

The Company invests its excess cash in investment grade short-term fixed income securities. Such investments in marketable securities are considered available for sale, and reported at fair value, with unrealized gains and losses included as a component of accumulated other comprehensive income (loss). Marketable securities with maturities of greater than three (3) months from the date of purchase but less than one year from the consolidated balance sheet date are classified as short-term, while marketable securities with maturities in one year or beyond one year from the consolidated balance sheet date are classified as long term. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity, which is included in interest income on the consolidated statements of operations and comprehensive loss. Realized gains and losses and declines in value judged

to be other than temporary, if any, on available-for-sale securities are included in interest income, net. The cost of securities sold is determined using specific identification method.

The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other than temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Factors considered include quoted market prices, recent financial results and operating trends, implied values from any recent transactions or offers of investee securities, credit quality of debt instrument issuers, other publicly available information that may affect the value of the marketable security, duration and severity of the decline in value, and the Company’s strategy and intentions for holding the marketable security. To date, the Company has not recorded any impairment charges on its marketable securities related to other-than-temporary declines in market value. No significant facts or circumstances have arisen to indicate that there has been any significant deterioration in the creditworthiness of the issuers of the securities held by us, thus there has been no recognition of any other-than-temporary impairment in the year ended December 31, 2020, 2019 or 2018. The Company considered the current and expected future economic and market conditions surrounding the COVID-19 pandemic and determined that the estimate of credit losses was not significantly impacted. Additionally, the Company has determined that it has the ability and intent to hold all marketable securities that have been in a continuous loss position until maturity or recovery, thus there has been no recognition of any other-than-temporary impairment in the year ended December 31, 2020.

Concentrations of Credit Risk

Financial instruments that potentially subject the Company to concentrations of credit risk consist of cash, cash equivalents, and marketable securities. Cash, cash equivalents and marketable securities are invested through banks and other financial institutions in the United States. Such deposits may be in excess of federally insured limits. The Company maintains cash equivalents and marketable securities with various high-credit-quality and capitalized financial institutions. The Company has not experienced any credit losses in such accounts and does not believe it is exposed to any significant credit risk on these funds.

The Company’s investment policy limits investments to certain types of securities issued by the U.S. government, its agencies, and institutions with investment-grade credit ratings and places restrictions on maturities and concentration by type and issuer. The Company is exposed to credit risk in the event of a default by the financial institutions holding its cash, cash equivalents, and marketable securities and issuers of marketable securities to the extent recorded on the consolidated balance sheets. Through December 31, 2020, the Company has no off-balance sheet concentrations of credit risk.

Other Risks and Uncertainties

The Company is subject to a number of risks similar to those of other clinical-stage immunotherapy companies, including dependence on key individuals; the need to develop commercially viable therapeutics; competition from other companies, many of which are larger and better capitalized; and the need to obtain adequate additional financing to fund the development of its products. The Company currently depends on third-party suppliers for key materials and services used in its research and development manufacturing process and is subject to certain risks related to the loss of these third-party suppliers or their inability to supply the Company with adequate materials and services.

In March 2020, the World Health Organization declared the global novel coronavirus disease (“COVID-19”) outbreak a pandemic. To date, the Company’s business has not been materially impacted by the COVID-19 pandemic. However, the Company has experienced slowing of patient recruitment and sample collection in its ongoing clinical trials and cannot at this time predict the specific extent, duration, or full impact that the COVID-19 pandemic will have on its financial condition and operations, including ongoing and planned clinical trials. The impact of the COVID-19 pandemic on the financial performance of the Company will depend on future developments, including the duration and spread of the outbreak and related governmental advisories and restrictions. These developments and the impact of COVID-19 on the financial markets and the overall economy are highly uncertain. If the financial markets and/or the overall economy are impacted for an extended period, the Company’s results may be adversely affected.

Property and Equipment, Net

Property and equipment are stated at cost, less accumulated depreciation and amortization. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred.

Depreciation is calculated using the straight-line method over the estimated useful lives of the respective assets, which are as follows:

Asset		Estimated Useful Life
Computer equipment and software		3 to 5 years
Furniture and fixtures		5 years
Laboratory equipment		5 to 7 years
Leasehold improvements		Shorter of useful life or lease term

Long-Lived Assets

The Company evaluates long-lived assets, including property and equipment and ROU assets, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the asset may not be fully recoverable. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of the asset and its eventual disposition are less than its carrying amount. Impairment, if any, is measured as the amount by which the carrying amount of a long-lived asset exceeds its fair value. There were no indicators of impairment of long-lived assets and no impairment losses have been recorded as of and for the year ended December 31, 2020, 2019 or 2018.

Revenue Recognition

The Company analyzes its collaboration agreements to assess whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities. This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. For collaboration arrangements that are considered to be in the scope of the collaboration guidance and that contain multiple elements, the Company first determines which elements of the collaboration are deemed to be within the scope of the collaboration guidance and those that are more reflective of a vendor-customer relationship and, therefore, within the scope of the revenue with contracts with customer guidance. For elements of collaboration arrangements that are accounted for pursuant to the revenue from contracts with customer guidance, an appropriate recognition method is determined and applied consistently, generally by analogy to the revenue from contracts with customers guidance.

The terms of the licensing and collaboration agreements entered into typically include payment of one or more of the following: non-refundable, up-front fees; development, regulatory, and commercial milestone payments; payments for manufacturing supply services; and royalties on net sales of licensed products. Each of these payments results in license, collaboration, and other revenues, except for revenues from royalties on net sales of licensed products, which are classified as royalty revenues. The core principle of the accounting for revenue from contracts with customers guidance is to recognize revenues when promised goods or services are transferred to customers in an amount that reflects the consideration that is expected to be received in exchange for those goods or services.

In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under each of its agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation.

Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company’s consolidated balance sheets. If the related performance obligation is expected to be satisfied within the next twelve (12) months this will be classified in current liabilities. Amounts recognized as revenue prior to receipt are recorded as contract assets in the Company’s consolidated balance sheets. If the Company expects to have an unconditional right to receive consideration in the next twelve (12) months, this will be classified in current assets. A net contract asset or liability is presented for each contract with a customer.

At contract inception, the Company assesses the goods or services promised in a contract with a customer and identifies those distinct goods and services that represent a performance obligation. A promised good or service may not be identified as a performance obligation if it is immaterial in the context of the contract with the customer, if it is not separately identifiable from other promises in the contract (either because it is not capable of being separated or because it is not separable in the context of the contract), or if the performance obligation does not provide the customer with a material right.

The Company considers the terms of the contract and its customary business practices to determine the transaction price. The transaction price is the amount of consideration to which the Company expects to be entitled in exchange for transferring promised goods or services to a customer. The consideration promised in a contract with a customer may include fixed amounts, variable amounts, or both. Variable consideration will only be included in the transaction price when it is not considered constrained, which is when it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur.

If it is determined that multiple performance obligations exist, the transaction price is allocated at the inception of the agreement to all identified performance obligations based on the relative standalone selling prices. The relative selling price for each deliverable is estimated using objective evidence if it is available. If objective evidence is not available, the Company uses its best estimate of the selling price for the deliverable.

Revenue is recognized when, or as, the Company satisfies a performance obligation by transferring a promised good or service to a customer. An asset is transferred when, or as, the customer obtains control of that asset, which for a service, is considered to be as the services are received and used. The Company recognizes revenue over time by measuring the progress toward complete satisfaction of the relevant performance obligation using an appropriate input or output method based on the nature of the good or service promised to the customer.

After contract inception, the transaction price is reassessed at every period end and updated for changes such as resolution of uncertain events. Any change in the transaction price is allocated to the performance obligations on the same basis as at contract inception.

Management may be required to exercise considerable judgment in estimating revenue to be recognized. Judgment is required in identifying performance obligations, estimating the transaction price, estimating the stand-alone selling prices of identified performance obligations, which may include forecasted revenue, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success, and estimating the progress towards satisfaction of performance obligations.

Stock-Based Compensation

The Company measures and recognizes compensation expense for all stock-based awards made to employees, directors, and non-employees based on the grant date estimated fair value of each award. Such expense is recognized on a straight-line basis over the requisite service period which is generally the vesting period for the entire award. Expense is adjusted for estimated forfeitures. Forfeitures of awards are estimated based on historical forfeiture experience and the experience of other companies in the same industry. The estimate of forfeitures will be adjusted over the service period to the extent that actual forfeitures differ, or are expected to differ, from prior estimates.

The Company estimates the fair value of stock option grants and ESPP purchase rights using the Black-Scholes option-pricing model (“the Black-Scholes model”). The Black-Scholes model requires management to make assumptions and judgments about the variables used in the calculation, including the expected term (weighted-average period of time that the options granted are expected to be outstanding), the expected volatility of common stock, an assumed risk-free interest rate, and expected dividends the Company may pay. Management uses the simplified calculation of the expected term for its stock options. Volatility is based on an average of the historical volatilities of the common stock of entities with characteristics similar to the Company’s. The risk-free rate is based on the U.S. Treasury yield curve in effect at the time of grant for periods corresponding with the expected life of the option. The Company uses an assumed dividend yield of zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock.

Research and Development Expenses

All research and development costs, including work performed by third parties, are expensed as incurred. Research and development costs consist of salaries and other personnel-related expenses, including associated non-cash stock-based compensation, consulting fees, laboratory supplies, and facility costs, as well as external research and development expenses incurred under arrangements with third parties, fees paid to other entities that conduct certain research and development activities on behalf of the Company, and costs incurred related to our collaboration agreements. Costs to develop the Company’s technologies are recorded as research and development expense unless certain costs which meet the criteria to be capitalized as internal-use software costs is met. Payments made prior to the receipt of goods or services to be used in research and development are capitalized until the goods are received or services are realized or consumed. Such payments are evaluated for current or long-term classification based on when they will be realized.

The Company has and may continue to enter into license agreements to access and utilize certain technology. In each case, the Company evaluates of the license agreement results in the acquisition of an asset or a business. To date, none of the Company’s license agreements have been considered to be acquisitions of businesses. For asset acquisitions, the upfront payments to acquire such licenses, as well as any future milestone payments, are immediately recognized as research and development expense when paid, provided that there is no alternative future use of the rights in other research and development projects. These license agreements may also include contingent consideration in the form of cash payments to be made for future milestone events. The Company assesses whether such contingent consideration meets the definition of a derivative and, to date, the Company has determined that such contingent consideration are not derivatives.

Clinical and pre-clinical costs are a component of research and development expense. The Company accrues and expenses clinical and pre-clinical trial activities performed by third parties based upon actual work completed in accordance with agreements established with its service providers. The Company determines the actual costs through discussions with internal personnel and external service providers as to the progress or stage of completion of services and the agreed-upon fee to be paid for such services.

Leases

Prior to January 1, 2019, the Company rented its office space and facilities under non-cancelable operating lease agreements and recognizes related rent expense on a straight-line basis over the term of the lease. The Company’s lease agreements contained rent holidays, scheduled rent increases, and renewal options. Rent holidays and scheduled rent increases were included in the determination of rent expense to be recorded ratably over the lease term. The Company did not assume renewals in its determination of the lease term unless they were deemed to be reasonably assured at the inception of the lease. The Company began recognizing rent expense on the date that it obtained the legal right to use and control the leased space. Deferred rent consisted of the difference between cash payments and the recognition of rent expense on a straight-line basis for the buildings the Company occupied.

Funding of leasehold improvements by the Company’s landlord was accounted for as a tenant improvement allowance and recorded as current and non-current deferred rent liabilities and amortized on a straight-line basis as a reduction of rent expense over the term of the lease.

In certain arrangements, the Company was involved in the construction of improvements to buildings it is leasing. To the extent the Company was involved with the structural improvements of the construction project or takes construction risk, the Company was considered to be the owner of the building and related improvements for accounting purposes during the construction period. The Company recorded the fair value of the building and related improvements subject to the lease within property and equipment on the consolidated balance sheet. The Company also recorded a corresponding lease financing obligation on its consolidated balance sheet representing the amounts financed by the lessor for the building and lessor financed improvements. Once a construction project was complete, the Company considered the requirements for sale-leaseback accounting treatment. If the Company concludes the arrangement does not qualify for sale-leaseback accounting treatment, the building and related improvements remained on the Company’s consolidated balance sheets and were subject to depreciation and assessment of impairment.

For such arrangements, at both pre and post the construction period, the Company bifurcated its lease payments into a portion allocated to the building and a portion allocated to the parcel of land on which the building had been built. The portion of the lease payments allocated to the land was treated for accounting purposes as operating lease payments, and therefore was recorded as rent expense in the consolidated statements of operations and comprehensive loss. The portion of the lease payments allocated to the building was further bifurcated into a portion allocated to interest expense and a portion allocated to reduce the lease financing obligation. The interest rate used for the lease financing obligation represents the Company’s estimated incremental borrowing rate at the inception of the lease, adjusted to reduce any built-in loss.

Subsequent to January 1, 2019, the Company determines whether the arrangement is or contains a lease at the inception of the arrangement and if such a lease is classified as a financing lease or operating lease. All of the Company’s leases are classified as operating leases. Leases with a term greater than one year are included in operating lease ROU assets, lease liabilities, current portion, and lease liabilities, net of current portion in our consolidated balance sheets at December 31, 2020 and 2019. The Company has elected not to recognize on the consolidated balance sheets leases with terms of one year or less. Lease liabilities and their corresponding ROU assets are recorded based on the present value of lease payments over the expected lease term. In determining the net present value of lease payments, the interest rate implicit in lease contracts is typically not readily determinable. As such, the Company utilizes the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment. Certain adjustments to the ROU assets may be required for items such as initial direct costs paid or incentives received and impairment charges if we determine the ROU asset is impaired.

The Company considers a lease term to be the noncancelable period that it has the right to use the underlying asset, including any periods where it is reasonably assured the Company will exercise the option to extend the contract. Periods covered by an option to extend are included in the lease term if the lessor controls the exercise of that option.

The Company recognizes lease expense on a straight-line basis over the expected lease term.

The Company has elected to not separate lease and non-lease components for its leased assets and accounts for all lease and non-lease components of its agreements as a single lease component. The lease components resulting in a ROU asset have been recorded on the consolidated balance sheet and amortized as lease expense on a straight-line basis over the lease term.

Income Taxes

The Company accounts for income taxes under the liability method. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates that will be in effect when the differences are expected to reverse. Valuation allowances are established when necessary to reduce deferred tax assets to the amounts expected to be realized.

The Company recognizes and measures uncertain tax positions using a two—step approach set forth in authoritative guidance. The first step is to evaluate the tax position taken or expected to be taken by determining whether the weight of available evidence indicates that it is more likely than not that the tax position will be sustained in an audit, including resolution of any related appeals or litigation processes. The second step is to measure the tax benefit as the largest amount that is more than 50% likely to be realized upon ultimate settlement. Significant judgment is required to evaluate uncertain tax positions. The Company evaluates uncertain tax positions on a regular basis. The evaluations are based on a number of factors, including changes in facts and circumstances, changes in tax law, correspondence with tax authorities during the course of the audit, and effective settlement of audit issues. The provision for income taxes includes the effects of any accruals that the Company believes are appropriate. It is the Company’s policy to recognize interest and penalties related to income tax matters in income tax expense. Through December 31, 2020, the Company had not accrued interest or penalties related to uncertain tax positions.

On March 18, 2020, the Families First Coronavirus Response Act (“FFCR Act”), and on March 27, 2020, the Coronavirus Aid, Relief, and Economic Security Act (“CARES Act”) were each enacted in response to the COVID-19 pandemic. The FFCR Act and the CARES Act contain numerous income tax provisions relating to refundable payroll tax credits, deferment of employer side social security payments, net operating loss carryback periods, alternative minimum tax credit refunds, modifications to the net interest deduction limitations and technical corrections to tax depreciation methods for qualified improvement property.

On June 29, 2020, Assembly Bill 85 (“A.B. 85”) was signed into California law. A.B. 85 provides for a three-year suspension of the use of net operating losses for medium and large businesses and a three-year cap on the use of business incentive tax credits to offset no more than $5.0 million of tax per year. A.B. 85 suspends the use of net operating losses for taxable years 2020, 2021 and 2022 for certain taxpayers with taxable income of $1.0 million or more. The carryover period for any net operating losses that are suspended under this provision will be extended. A.B. 85 also requires that business incentive tax credits including carryovers may not reduce the applicable tax by more than $5.0 million for taxable years 2020, 2021 and 2022.

The FFCR Act, CARES Act and A.B. 85 did not have a material impact on the Company’s consolidated financial statements as of December 31, 2020; however, the Company continues to examine the impacts the FFCR Act, CARES Act and A.B. 85 may have on its business, results of operations, financial condition, liquidity and related disclosures.

Comprehensive Loss

Comprehensive loss includes net loss and certain changes in stockholders’ equity that are excluded from net loss, primarily unrealized gains and losses on the Company’s marketable securities.

Net Loss Per Share

Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is the same as basic net loss per share, since the effects of potentially dilutive securities are antidilutive given the net loss for each period presented.

Recently Adopted Accounting Pronouncements

In June 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standard Update (“ASU”) 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which represents a new credit loss standard that will change the impairment model for most financial assets and certain other financial instruments. The FASB subsequently issued amendments to ASU 2016-13, which have the same effective and transition date of January 1, 2020. Specifically, this guidance will require entities to utilize a new “expected loss” model as it relates to trade and other receivables. In addition, entities will be required to recognize an allowance for estimated credit losses on available-for-sale debt securities, regardless of the length of time that a security has been in an unrealized loss position. The standard is effective for interim and annual periods beginning after December 15, 2019. The Company adopted this new standard on January 1, 2020 with no material impact on its consolidated financial statements and related disclosures.

In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement. ASU 2018-13 eliminates, adds and modifies certain disclosure requirements for fair value measurements and requires companies to disclose certain information. The Company adopted this standard on January 1, 2020 with no material impact on its consolidated financial statements and related disclosures.

In August 2018, the FASB issued ASU 2018-15, Intangibles (Topic 350): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract, which aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. This new standard also requires customers to expense the capitalized implementation costs of a hosting arrangement that is a service contract over the term of the hosting arrangement. This standard is effective for annual reporting periods beginning after December 15, 2019, and interim periods within that year. This new standard can be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. The Company adopted this standard on January 1, 2020 with no material impact on its consolidated financial statements and related disclosures.

In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606. This standard provides guidance on the interaction between Revenue Recognition (Topic 606) and Collaborative Arrangements (Topic 808) by aligning the unit of account guidance between the two topics and clarifying whether certain transactions between collaborative participants should be accounted for as revenue under Topic 606. The standard is effective for interim and annual periods beginning after December 15, 2019. The Company adopted this standard on January 1, 2020 with no material impact on its consolidated financial statements and related disclosures.

In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes (“ASU 2019-12”), which is intended to simplify various aspects related to accounting for income taxes by removing certain exceptions to the general principles in Topic 740. The pronouncement is effective for the Company for fiscal years beginning after December 15, 2021, and for interim periods within fiscal years beginning after December 15, 2022. Early adoption is permitted. The Company early adopted this standard on April 1, 2020 commencing on January 1, 2020 on a prospective basis. The adoption of this standard did not have a material impact on the Company’s consolidated financial statements and related disclosures.

Recently Issued Accounting Pronouncements Not Yet Adopted

In August 2020, the FASB issued ASU No. 2020-06, Debt - Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging - Contracts in Entity’s Own Equity. The standard eliminates the beneficial conversion and cash conversion accounting models for convertible instruments. It also amends the accounting for certain contracts in an entity’s own equity that are currently accounted for as derivatives because of specific settlement provisions. In addition, the standard modifies how particular convertible instruments and certain contracts that may be settled in cash or shares impact the diluted EPS computation. The amendments in ASU 2020-06 are effective for the Company as defined by the SEC for fiscal years beginning after December 15, 2023, including interim periods within those fiscal years. Early adoption is permitted, but not earlier than fiscal years beginning after December 15, 2020. The Company is currently evaluating the impact of ASU 2020-06 on its consolidated financial statements and related disclosures.

In October 2020, the FASB issued ASU 2020-10, Codification Improvements. The standard contains improvements to the Codification by ensuring that all guidance that requires or provides an option for an entity to provide information in the notes to financial statements is codified in the disclosure section of the Codification. The standard also improves various topics in the Codification so that entities can apply guidance more consistently on codifications that are varied in nature where the original guidance may have been unclear. The amendments in ASU 2020-10 are effective for the Company for fiscal years beginning after December 15, 2021, and interim periods within fiscal years beginning after December 15, 2022. Early adoption is permitted. The Company does not expect the adoption of ASU 2020-10 to have a material impact on its consolidated financial statements and related disclosures.

Cash Equivalents and Marketable Securities

12 Months Ended

Dec. 31, 2020

Cash And Cash Equivalents [Abstract]

Cash Equivalents and Marketable Securities

3.	Cash Equivalents and Marketable Securities

The amortized cost, unrealized gains and losses and fair values of cash equivalents and marketable securities were as follows (in thousands):

	December 31, 2020
Description	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value
Cash equivalents:
Money market funds	$	36,801	$	—	$	—	$	36,801
Total cash equivalents		36,801		—		—		36,801
Short-term marketable securities:
U.S. treasuries		1,002		—		—		1,002
Total short-term marketable securities		1,002		—		—		1,002
Total	$	37,803	$	—	$	—	$	37,803

	December 31, 2019
Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents:
Money market funds	$	42,133	$	—	$	—		$	42,133
Commercial paper		1,749		—		—			1,749
Corporate debt securities		2,500		—		—			2,500
Total cash equivalents		46,382		—		—			46,382
Short-term marketable securities:
Certificates of deposit		631		—		—			631
Commercial paper		31,476		15		—			31,491
Corporate debt securities		38,237		15		(6	)		38,246
Total short-term marketable securities		70,344		30		(6	)		70,368
Total	$	116,726	$	30	$	(6	)	$	116,750

As of December 31, 2020 and 2019, the Company had a total of $171.1 million and $127.8 million in cash, cash equivalents and marketable securities, which includes $170.1 million and $57.4 million in cash and cash equivalents and $1.0 million and $70.4 million in marketable securities, respectively.

All marketable securities held as of December 31, 2020, had contractual maturities of less than one year. There have been no material realized gains or losses on marketable securities for the periods presented. As of December 31, 2020, the Company did not hold any individual securities in an unrealized loss position. Thus, the Company has not recorded an allowance for credit losses as of December 31, 2020. The Company has the ability and intent to hold all marketable securities that have been in a continuous loss position until maturity or recovery. No significant facts or circumstances have arisen to indicate that there has been any significant deterioration in the creditworthiness of the issuers of the securities held by us, thus there has been no recognition of any other-than-temporary impairment in the year ended December 31, 2020, 2019 or 2018. The Company considered the current and expected future economic and market conditions surrounding the COVID-19 pandemic and determined that the estimate of credit losses was not significantly impacted. The Company will continue to assess the current and expected future economic and market conditions surrounding the COVID-19 pandemic, as further development arises.

See Note 4 for further information regarding the fair value of the Company's financial instruments.

Fair Value Measurements

12 Months Ended

Dec. 31, 2020

Fair Value Disclosures [Abstract]

Fair Value Measurements

4.	Fair Value Measurements

The Company’s financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements were as follows (in thousands):

	December 31, 2020
Description	Total		Level 1		Level 2		Level 3
Cash equivalents:
Money market funds	$	36,801	$	36,801	$	—	$	—
Total cash equivalents		36,801		36,801		—		—
Short-term marketable securities:
U.S. treasuries		1,002		1,002		—		—
Total short-term marketable securities		1,002		1,002		—		—
Total	$	37,803	$	37,803	$	—	$	—

	December 31, 2019
Description	Total		Level 1		Level 2		Level 3
Cash equivalents:
Money market funds	$	42,133	$	42,133	$	—	$	—
Commercial paper		1,749		—		1,749		—
Corporate debt securities		2,500		—		2,500		—
Total cash equivalents		46,382		42,133		4,249		—
Short-term marketable securities:
Certificates of deposit		631		—		631		—
Commercial paper		31,491		—		31,491		—
Corporate debt securities		38,246		—		38,246		—
Total short-term marketable securities		70,368		—		70,368		—
Total	$	116,750	$	42,133	$	74,617	$	—

The Company measures the fair value of money market funds based on quoted prices in active markets for identical securities. Commercial paper, certificates of deposit, and corporate debt securities are valued taking into consideration valuations obtained from third-party pricing services. The pricing services utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include reported trades of and broker/dealer quotes on the same or similar securities, issuer credit spreads; benchmark securities; prepayment/default projections based on historical data; and other observable inputs.

There were no transfers between Level 1 and Level 2 during the periods presented.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Property and Equipment, Net

12 Months Ended

Dec. 31, 2020

Property Plant And Equipment [Abstract]

Property and Equipment, Net

5.	Property and Equipment, Net

Property and equipment and related accumulated depreciation and amortization are as follows (in thousands):

	December 31,
	2020			2019
Computer equipment and software	$	971		$	863
Furniture and fixtures		2,054			2,007
Laboratory equipment		22,213			21,025
Leasehold improvements		13,726			11,102
		38,964			34,997
Less accumulated depreciation and amortization		(16,925	)		(10,310	)
Construction-in-progress		66			2,224
Total property and equipment, net	$	22,105		$	26,911

Depreciation and amortization expense was $6.6 million, $4.7 million, and $4.0 million for the periods ended December 31, 2020, 2019 and 2018, respectively.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2020

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

6.	Commitments and Contingencies

Leases

In November 2015, the Company entered into an 84-month non-cancelable operating lease, effective March 2016, for a new facility in Emeryville, California, with laboratory and office space. In conjunction with signing the lease, the Company paid a cash security deposit of $50,000. The lease agreement includes an escalation clause for increased rent and a renewal provision allowing the Company to extend this lease for an additional three years at the prevailing rental rate. In September 2018, the Emeryville lease was amended whereby the Company entered into a 12-month operating lease for additional temporary space. The Company may terminate the temporary space lease agreement with 30 days advanced written notice to the Landlord.

In January 2019, the Company entered into a 120-month operating lease for a new facility in Emeryville, California with office and laboratory space for the Company’s new principal executive offices. In conjunction with signing the lease, the Company paid a cash security deposit of $0.6 million, which is recorded as a deposit on the Company’s consolidated balance sheets as of December 31, 2020 and 2019. The lease agreement includes a free rent period, an escalation clause for increased rent and a renewal provision allowing the Company to extend this lease for an additional two five-year periods at the then market rental rate. The lessor provided the Company a tenant improvement allowance for a total of $4.0 million to complete the laboratory and office renovation. The Company’s obligation to pay rent commenced on November 1, 2019. The Company has determined the tenant improvements to be lessee owned and therefore has recorded a $9.3 million ROU Asset and a $14.8 million lease liability on the consolidated balance sheet as of December 31, 2020.

In connection with the new lease agreement, the Company also entered into an agreement (the “Lease Termination Agreement”) to early terminate the Company’s existing lease dated November 2015, for its current premises. The then existing lease terminated in October 2019. The Company accounted for the Lease Termination Agreement as a separate contract and recorded an adjustment of $1.8 million to the ROU asset and lease liability to reflect the remaining term of the modified agreement through October 2019.

In February 2016, the Company entered into a 67-month non-cancellable operating lease effective October 2016 for a new facility in Cambridge, Massachusetts, with laboratory and office space. In conjunction with signing the lease, the Company paid a cash security deposit of $0.3 million, which is recorded in deposits and other long-term assets on the Company’s consolidated balance sheet as of December 31, 2020. The lease agreement includes an escalation clause for increased rent and a renewal provision allowing the Company to extend this lease for an additional three years at the prevailing rental rate. The lessor provided the Company a tenant improvement allowance for a total of $2.1 million to complete laboratory and office renovation.

In March 2017, the Company entered into a noncancelable lease (the “Pleasanton Lease”) to lease 42,620 square feet of office, cleanroom, and laboratory support manufacturing space in Pleasanton, California (the Pleasanton Facility). Subsequently, in April 2017, the Company took possession of the space. The Pleasanton Lease includes a free rent period, escalating rent payments and a term that expires on November 30, 2024. The Company has the option to extend the lease term for a period of five years at the then market rental rate. The Company’s obligation to pay rent commenced on December 1, 2017. The Company obtained an irrevocable letter of credit in March 2017 in the initial amount of $1.0 million as a security deposit to the Pleasanton Lease, which may be drawn down by the landlord in the event the Company fails to fully and faithfully perform all of its obligations. The letter of credit may be reduced based on certain levels of cash and cash equivalents the Company holds. As of December 31, 2020, none of the irrevocable letter of credit amount has been drawn. The Pleasanton Lease further provides that the Company is obligated to pay to the landlord its proportionate share of certain basic operating costs, including taxes and operating expenses.

In connection with the Pleasanton Lease, the Company received a tenant improvement allowance of $1.2 million from the landlord for the costs associated with the design, development and construction of tenant improvements for the Pleasanton Facility building. The unamortized tenant improvement balance as of January 1, 2019 has been recognized as a component of operating lease ROU assets on the consolidated balance sheets as of December 31, 2020 and 2019.

In September 2018, the Company entered into a 24-month non-cancellable operating lease for an additional facility in Cambridge, Massachusetts with laboratory and office space. In conjunction with signing the lease, the Company prepaid the first twelve months base rent in the amount of $1.3 million of which $0.9 million as of January 1, 2019 was reclassified to the ROU Asset on the consolidated balance sheet upon adoption of Topic 842. The Company also paid a cash security deposit of $0.3 million, which included $0.1 million for the last month’s rent and has been classified as part of the operating lease ROU assets on the consolidated balance sheets as of December 31, 2020 and 2019. The remaining security deposit is recorded in deposits and other long-term assets on the Company’s consolidated balance sheet as of December 31, 2020.

In July 2019, the Company amended its Cambridge, Massachusetts laboratory and office space facility lease. The amendment extended the original 24-month lease term ending in August 2020 by another 12 months through August 2021 and added additional leased laboratory and office space. Upon six months written notice, the Company has the right to terminate the amended lease agreement. The amendment provides for annual base rent of approximately $3.4 million, effective July 2019. In conjunction with signing the lease amendment, the Company prepaid an additional twelve months base rent for both the original leased space and the additional leased space in the amount of $3.2 million, which was reclassified to operating lease right-of-use assets on the consolidated balance sheets. The Company also paid a cash security deposit of $0.3 million, which included $0.1 million for the last month’s rent and has been classified as part of the operating lease right-of-use assets on the consolidated balance sheets as of December 31, 2020 and 2019. The remaining security deposit is recorded in deposits and other long-term assets on the Company’s consolidated balance sheets as of December 31, 2020 and 2019.

In May 2020, the Company executed an additional amendment to its Cambridge, Massachusetts laboratory and office space facility lease. The amendment extended the lease term by an additional 12 months through August 2022. Upon six months written notice, the Company has the right to terminate the amended lease agreement. The amendment provides for annual base rent of approximately $3.6 million, effective September 2021. This amendment resulted in an increase to the ROU assets and lease liabilities of $3.1 million.

In May 2019, the Company entered into a 64-month non-cancellable operating lease for additional office space in Pleasanton, California. The lessor provided the Company a tenant improvement allowance for a total of $0.1 million to complete the office renovation. The Company’s obligation to pay rent commenced on August 1, 2019. The Company has determined the tenant improvements to be lessee owned and therefore has recorded a $0.3 million ROU Asset and a $0.4 million lease liability on the consolidated balance sheet as of December 31, 2020.

The Company’s operating leases include various covenants, indemnities, defaults, termination rights, security deposits and other provisions customary for lease transactions of this nature.

The components of lease costs, which were included in our consolidated statements of operations and comprehensive loss, were as follows (in thousands):

	Year ended December 31,
	2020		2019
Lease cost
Operating lease cost	$	7,511	$	6,382
Short-term lease cost		7		271
Total lease cost	$	7,518	$	6,653

Supplemental information related to leases was as follows (in thousands):

	Year ended December 31,
	2020			2019
Cash paid for amounts included in the measurement of lease liabilities (in thousands):
Operating cash flows from operating leases	$	5,389		$	2,495
New right-of-use assets obtained in exchange for lease obligations (in thousands):
Operating leases	$	3,174		$	12,331
Weighted average remaining lease term (years):
Operating leases		6.10			7.20
Weighted average discount rate:
Operating leases		9.0	%		9.0	%

As of December 31, 2020, minimum annual payments under the Company’s operating lease agreements are as follows (in thousands):

	Lease Financing Obligation
Year ending December 31,
2021	$	7,863
2022		5,836
2023		3,459
2024		3,459
2025		2,456
Thereafter		10,076
Total minimum payments		33,149
Less: Amounts representing interest expense		(8,050	)
Present value of future minimum lease payments		25,099
Less: Current portion of lease liability		(5,874	)
Noncurrent portion of lease liability	$	19,225

Rent expense was $7.5 million, $6.7 million and $1.8 million for the years ended December 31, 2020, 2019 and 2018, respectively.

Agreements with CROs

In September 2017, the Company entered into a contract research and development agreement with a third-party contract research organization (“CRO”) to provide research, analysis and antibody samples to further the Company’s development of its drug candidates. The Company is obligated to pay the CRO certain milestone payments of up to $36.4 million on achievement of specified events. None of these events had occurred as of December 31, 2020. During the year ended December 31, 2020, the Company had no research and development expense under the agreement. During the year ended December 31, 2019, the Company recognized an insignificant amount of research and development expense under the agreement. During 2018, the Company recognized a total of $1.0 million of research and development expense under the agreement.

In May 2019, the Company entered into a contract research and testing agreement with a third-party contract research organization to provide antibody discovery related services. The Company is obligated to pay the CRO certain milestone payments of up to $34.8 million on achievement of specified events. None of these events had occurred as of December 31, 2020. During the year ended December 31, 2020, the Company recognized an insignificant amount of research and development expense under the agreement. The Company recognized a total of $1.0 million in research and development expense under the agreement during the year ended December 31, 2019.

Guarantees and Indemnifications

The Company, as permitted under Delaware law and in accordance with its certification of incorporation and bylaws, and pursuant to indemnification agreements with certain of its officers and directors, indemnifies its officers and directors for certain events or occurrences, subject to certain limits, which the officer or director is or was serving at the Company’s request in such capacity. The term of the indemnification period lasts as long as an officer or director may be subject to any proceeding arising out of acts or omissions of such officer or director in such capacity. The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director and officer liability insurance. This insurance limits the Company’s exposure and may enable it to recover a portion of any future amounts paid. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, the Company has not recognized any liabilities relating to these obligations for any period presented.

Collaboration and License Agreements

12 Months Ended

Dec. 31, 2020

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Collaboration and License Agreements

7.	Collaboration and License Agreements

bluebird bio, Inc.

In August 2018, the Company entered into a Research Collaboration and License Agreement, or the bluebird Collaboration Agreement with bluebird bio, Inc., or bluebird”. Under the terms of the bluebird Collaboration Agreement, the Company will provide to bluebird tumor-specific targets across several tumor types and, in certain cases, T-cell receptors (“TCR”) directed to those targets. The Company received a non-refundable upfront payment of $20.0 million and bluebird also concurrently acquired 768,115 shares of the Company’s Series C convertible preferred stock for $10.0 million at $13.04 per share. Per the bluebird Collaboration Agreement, bluebird was also provided an option to acquire shares of the Company’s common stock at the same price as all other investors in connection with the IPO. In October 2018, bluebird purchased 666,667 shares of the Company’s common stock at the price to the public of $15.00 per share for a total of $10.0 million. Under the terms of the bluebird Collaboration Agreement, the Company is eligible to earn development, regulatory, and sales-based milestones in an amount of up to $1.2 billion, and single-digit royalties on sales of products that utilize the technology subject to the bluebird Collaboration Agreement. None of these events had occurred as of December 31, 2020 and no royalties were due from the sale of licensed products.

In August 2019, the Company entered into a First Amendment to the bluebird Collaboration Agreement, which extended the timeline for the Company and bluebird to execute a Patient Selection Services Agreement from within one year to within two years after the Effective Date of the bluebird Collaboration Agreement. In August 2020, the Company entered into a Second Amendment, which extended the timeline of the Patient Selection Services Agreement to within three years and also extended the Tissue Analysis Period from February 28, 2021 to June 30, 2021. Both amendments were entered into for administrative purposes, and the Company determined the amendments were not a modification of contract under the contract with customers guidance.

bluebird may terminate the bluebird Collaboration Agreement by giving a 120-day prior written notice to the Company at any time after the effective date of the agreement. Unless terminated early, the agreement has a term that ends upon the last payment owed by Gritstone on a licensed product. The bluebird Collaboration Agreement may be terminated for cause by either party based on uncured material breach by the other party or bankruptcy of the other party. Upon early termination, all ongoing activities under the agreement and all mutual collaboration, development and commercialization licenses and sublicenses will terminate. The licenses granted by the Company to bluebird under the licensed intellectual property will remain in effect in accordance with their respective terms. Additionally, all of bluebird’s payment obligations that have not yet accrued related to future milestone and royalty payments will be reduced by 50% for the remainder of the agreement term.

The Company concluded that bluebird is a customer, and the contract is not subject to guidance on collaborative arrangements. This is because the Company granted to bluebird a license to its intellectual property, and provided research and development services, all of which are outputs of the Company’s ongoing activities, in exchange for consideration.

The Company identified the following three material promises under the bluebird Collaboration Agreement: 1) transfer of a license to intellectual property and related technology know-how (“License and Know-How”); 2) the obligation to perform target selection and TCR generation services (“Research and Development Services”); and 3) participation on the Joint Steering Committee (“JSC”). The Company provided to bluebird standard indemnification and protection of licensed intellectual property, which is part of assurance that the license meets the contract’s specifications and is not an obligation to provide goods or services.

The Company considered that the License and Know-How has standalone functionality, was considered to be functional intellectual property, and is capable of being distinct. However, the Company determined that the License and Know-How is not distinct from the Research and Development Services or participation on the JSC within the context of the agreement, because bluebird is dependent on the Company to execute the Research and Development Services and participate on the JSC in order for bluebird to benefit from the License and Know-How. As such, the License and Know-How is combined with the Research and Development Services and participation on the JSC into a single performance obligation, and the transaction price under this arrangement will be allocated to this single performance obligation.

The Company has also determined that all other goods or services that are contingent upon bluebird reaching various milestones are not considered performance obligations at the inception of the arrangement.

The transaction price at the inception of the bluebird Collaboration Agreement consisted of the upfront payment of $20.0 million and the $10.0 million received from bluebird for the purchase of the Company’s Series C convertible preferred stock. The sale of the Series C convertible preferred stock was not considered to be a performance obligation as it was a separate financing component of the transaction. Accordingly, $10.0 million of the transaction price was allocated to the issuance of 768,115 shares of Series C convertible preferred stock at fair value of $13.04 per share and recorded in stockholders’ equity.

The variable consideration related to the remaining development, regulatory, and sales-based milestones payments has not been included in the initial transaction price and continues to be fully constrained as of December 31, 2020. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt of the milestones is outside the control of the Company and contingent upon initiation of clinical trials for early-stage targets and bluebird’s development efforts. Any variable consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur, as they were determined to relate predominantly to the License and Know-How granted to bluebird. The Company will re-evaluate the transaction price in each reporting period and as uncertain events are resolved or other changes in circumstances occur.

For revenue recognition purposes, the Company determined that the duration of the contract began on the effective date in August 2018 and ends upon completion of the Research and Development Services, which is also when the participation on the JSC is no longer an obligation. The contract duration is defined as the period in which parties to the contract have present enforceable rights and obligations. The Company also analyzed the impact of bluebird terminating the agreement prior to August 2023 and determined, considering both quantitative and qualitative factors, that there were substantive non-monetary penalties to bluebird for doing so. We considered quantitative and qualitative factors to reach this conclusion.

Revenue is recognized when, or as, the Company satisfies its performance obligation by transferring the promised services to bluebird. Revenue will be recognized over time using a cost-based input method, based on internal labor cost effort to perform the research services, since the internal labor cost incurred over time is thought to best reflect the transfer of services to bluebird. In applying a cost-based input method of revenue recognition, we use actual costs incurred relative to budgeted costs to fulfill the combined performance obligation. A cost-based input method of revenue recognition requires us to make estimates of costs to complete the performance obligation. The cumulative effect of any revisions to estimated costs to complete the performance obligation will be recorded in the period in which changes are identified and amounts can be reasonably estimated. A significant change in these assumptions and estimates could have a material impact on the timing and amount of revenue recognized in future periods.

For the years ended December 31, 2020 and 2019, the Company recognized $2.8 million and $4.3 million, respectively, as collaboration revenue as a result of satisfying its performance obligation by transferring the promised services estimated by the labor cost incurred. A deferred revenue balance of $11.7 million is recorded on the consolidated balance sheet in both current and long-term liabilities as of December 31, 2020, which relates to the performance obligation identified, with such amounts to be recognized over the period the performance obligation is expected to be satisfied, which is currently expected to be through mid-2023.

Changes in the deferred revenue balance during the year ended December 31, 2020 are as follows (in thousands):

	Deferred Revenue
Balance at December 31, 2019	$	14,516
Additions		—
Deductions		(2,821	)
Balance at December 31, 2020	$	11,695

There were no receivables or net contract assets recorded as of December 31, 2020 associated with the bluebird Collaboration Agreement.

The Company expensed all incremental costs of obtaining the bluebird Collaboration Agreement in 2018, as such amounts were insignificant.

Arbutus Biopharma Corporation

In October 2017, the Company entered into an Exclusive License Agreement with Arbutus Biopharma Corporation, or Arbutus, and its wholly-owned subsidiary, Protiva Biotherapeutics Inc. Certain terms of the agreement were modified by amendment in July 2018. Under the license agreement, the Company has an exclusive license to utilize certain Arbutus intellectual property, including patents and know-how relating to immunotherapy. Under this license agreement, the Company made an upfront payment of $5.0 million, which was included in research and development expenses during 2017. During the year ended December 31, 2020, the Company had no research and development expense under the agreement. During 2019, the Company reimbursed Arbutus for materials and personnel costs totaling $0.4 million. The Company is obligated to pay Arbutus for services rendered and certain milestone payments up to an aggregate of $123.5 million on achievement of specified events, and royalties on sales of its licensed products. Following the acceptance of our investigational new drug application for GRANITE by the U.S. Food and Drug Administration, the Company made a $2.5 million development milestone payment to Arbutus in September 2018 that was recorded as research and development expense. In August 2019, a milestone was met following the initial patient treatment of SLATE in the Company’s GO-005 clinical trial. In 2019, the Company recorded $3.0 million as research and development expense in connection with the milestone. None of the other events had occurred as of December 31, 2020, and no royalties were due from the sale of licensed products.

Non-Profit Hospital Cancer Center

In January 2016, the Company entered into an Exclusive License Agreement with a non-profit hospital cancer center. Under the license agreement, the Company has an exclusive license to utilize certain patents and know-how relating to immunotherapy for an insignificant upfront payment, cash milestone payments on achievement of specified events, and a low single digit royalty on sales of licensed products. The achievement of the milestones and payment of royalties is dependent upon obtaining regulatory approval. Upon achievement of a milestone related to the Company’s Phase 1 clinical trial for GRANITE, GO-004, in December 2018 the Company recorded an insignificant amount to research and development expense for amounts owed to the Hospital Cancer Center, which was paid to the hospital in February 2019. None of the other events had occurred as of December 31, 2020 and no royalties were due from the sales of licensed products.

Genevant Sciences GmbH

In October 2020, the Company entered into an Option and License and Development Agreement, or the license agreement, with Genevant Sciences GmbH, or Genevant, pursuant to which Genevant granted the Company exclusive license rights under certain intellectual property related to Genevant’s lipid nanoparticle, or LNP, technology for a single therapeutic indication, and the Company agreed to pay Genevant an initial payment of $2.0 million, up to an aggregate of $71.0 million in specified development, regulatory, and commercial milestones, and low to mid-single digit royalties on net sales of licensed products. The upfront payment of $2.0 million was included in research and development expense for the year ended December 31, 2020. The license agreement expands Gritstone’s intellectual property rights to such LNP technology originally obtained pursuant to the Company’s license agreement with Arbutus. Genevant is a spin-off of Arbutus. Prior to this agreement, the Company licensed Arbutus’ LNP technology for indications in the oncology space. The remainder of Arbutus’ IP portfolio was transferred to Genevant in the spin-off.

Pursuant to the license agreement, Genevant also granted the Company certain options to license the LNP technology for additional therapeutic indications of up to $1.5 million for each indication and $1.0 million to extend the option term. The license agreement with Genevant continues in effect until the last to expire royalty term or early termination. The license agreement is terminable by the Company for convenience with 90 days prior written notice or immediately if based on certain product safety or efficacy or regulatory criteria. Either party may terminate the agreement for material breach, subject to a cure period, and Genevant may terminate the agreement if the Company challenges a licensed patent.

Balance Sheet Components

12 Months Ended

Dec. 31, 2020

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Balance Sheet Components

8.	Balance Sheet Components

Prepaid Expenses and Other Assets

Prepaid expenses and other current assets consist of the following (in thousands):

	December 31,
	2020		2019
Prepaid insurance	$	1,480	$	1,252
Prepaid research and development-related expenses		2,584		1,473
Other		268		772
Total prepaid expenses and other current assets	$	4,332	$	3,497

Deposits and Other Long-Term Assets

Deposits and other long-term assets consist of the following (in thousands):

	December 31,
	2020		2019
Lease security deposits	$	1,218	$	1,201
Prepaid research and development-related expenses		518		585
Restricted cash		992		992
Total deposits and other long-term assets	$	2,728	$	2,778

Accrued Liabilities

Accrued current liabilities consist of the following (in thousands):

	December 31,
	2020		2019
Property and equipment	$	96	$	761
Other		581		280
Total accrued current liabilities	$	677	$	1,041

Stockholders' Equity

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Stockholders' Equity

9.	Stockholders’ Equity

In connection with the completion of its IPO, on October 2, 2018, the Company’s certificate of incorporation was amended and restated to provide for 300,000,000 authorized shares of common stock with a par value of $0.0001 per share and 10,000,000 authorized shares of preferred stock with a par value of $0.0001 per share.

The Company has 10,000,000 shares of preferred stock authorized for issuance, par value of $0.0001 per share. As of December 31, 2020 and 2019, no shares of preferred stock were issued and outstanding.

The Company has 300,000,000 shares of common stock authorized for issuance, par value of $0.0001 per share. Holders of the Company’s common stock are entitled to one vote per share. As of December 31, 2020 and 2019, respectively, there were 47,552,693 and 36,332,956 shares of common stock issued and outstanding.

Sale of Common Stock, Preferred Stock and Pre-Funded Warrants

The Company entered into a Series C preferred stock purchase agreement with certain investors in June 2018, and upon approval by the Company’s Board of Directors, the Company completed a Series C convertible preferred stock financing at a price per share of $13.04. The net cash proceeds totaled $8.9 million and 690,128 shares of Series C convertible preferred stock were issued. Issuance costs totaled $0.1 million and were recorded as a reduction of the proceeds. In July 2018, the Company sold an additional 153,360 shares of Series C convertible preferred stock at a price of $13.04 per share for net cash proceeds of $2.0 million. In August 2018, in conjunction with the bluebird Collaboration Agreement, the Company sold bluebird 768,115 shares of Series C convertible preferred stock at a price of $13.04 per share for gross proceeds of $10.0 million.

In October 2018, the Company closed its initial public offering, or IPO, of 6,854,202 shares of common stock, including 187,535 shares sold pursuant to the underwriters’ partial exercise of their option to purchase additional shares, at an offering price to the public of $15.00 per share. The Company received net proceeds of approximately $92.5 million, after deducting underwriting discounts and commissions and offering costs. In connection with the IPO, all of the Company’s outstanding shares of convertible preferred stock were automatically converted into 19,409,132 shares of common stock. The related carrying value of $177.9 million was reclassified to common stock and additional paid-in capital.

In April 2019, the Company completed an underwritten public offering and sold and issued an aggregate of 6,500,000 shares of common stock at a price to the public of $11.50 per share. The Company received aggregate net proceeds from the offering of approximately $69.7 million, after deducting underwriting discounts and commissions and offering costs.

In October 2019, the Company filed a Registration Statement on Form S-3, or the Shelf Registration Statement, with the SEC, covering the offering of up to $250.0 million of common stock, preferred stock, debt securities, warrants and units. The Shelf Registration Statement included a prospectus covering the offering, issuance and sale of up to $75.0 million of the Company’s common stock from time to time through an “at the market offering” under the Securities Act, or the ATM Offering Program. The SEC declared the Shelf Registration Statement effective on November 8, 2019.

In October 2019, the Company also entered into the Sales Agreement with Cowen and Company, LLC, or Cowen, to sell shares of the Company’s common stock, from time to time, with aggregate gross sales proceeds of up to $75.0 million, through an “at the market offering” under which Cowen will act as its sales agent. Cowen is entitled to compensation for its services equal to up to 3.0% of the gross proceeds of any shares of common stock sold through Cowen under the Sales Agreement. In addition, the Company has agreed to reimburse a portion of the expenses of Cowen in connection with the offering up to a maximum of $50,000. During the year ended December 31, 2019, the Company issued and sold 490,880 shares of its common stock through its ATM Offering Program and received net proceeds of approximately $3.8 million, after deducting commissions of $0.1 million and other offering expenses of $0.3 million. During the year ended December 31, 2020, the Company issued and sold 1,160,963 shares of its common stock through its ATM Offering Program and received net proceeds of approximately $9.8 million, after deducting commissions of $0.3 million and $0.1 million of other offering expenses.

In December 2020, the Company entered into an agreement to sell 5,543,351 shares of its common stock at a price of $3.34 per share and pre-funded warrants (the “Warrants”) to purchase 27,480,719 shares of common stock at a public offering price of $3.33 through a private investment in public equity, or PIPE, financing. The Company received aggregate net proceeds of approximately $110.0 million after deducting underwriting discounts and commissions and offering expenses payable by the Company. The Warrants are exercisable immediately upon issuance at an exercise price of $0.01 per share. Per their terms, the outstanding Warrants to purchase shares of the Company’s common stock generally may not be exercised if the holder’s aggregate beneficial ownership would be more than 9.99% of the total issued and outstanding shares of the Company’s common stock following such exercise. The exercise price and number of shares of common stock issuable upon the exercise of the Warrants (“Warrant Shares”) are subject to adjustment in the event of any stock dividends and splits, reverse stock split, recapitalization, reorganization or similar transaction, as described in the Warrant agreements. Under certain circumstances, the Warrants may be exercisable on a “cashless” basis. In connection with the issuance and sale of the common stock and Warrants, the Company granted the Warrant holders certain registration rights with respect to the Warrants and the Warrant Shares.

The Warrants were classified as a component of permanent stockholders’ equity within additional paid-in-capital and were recorded at the issuance date using a relative fair value allocation method. The Warrants are equity classified because they are freestanding financial instruments that are legally detachable and separately exercisable from the equity instruments, are immediately exercisable, do not embody an obligation for the Company to repurchase its shares, permit the holders to receive a fixed number of common shares upon exercise, are indexed to the Company’s common stock and meet the equity classification criteria. In addition,

such Warrants do not provide any guarantee of value or return. The Company valued the Warrants at issuance, concluding their sales price approximated their fair value, and allocated net proceeds from the sale proportionately to the common stock and Warrants, of which $87.7 million, net of issuance costs, was allocated to the Warrants and recorded as a component of additional paid-in-capital.

In December 2020, the Company entered into an additional agreement and sold an additional 4,043,127 shares of its common stock through a PIPE financing at a price per share of $3.71 for aggregate gross proceeds of $15.0 million. No additional Warrants were issued in connection with this second PIPE financing.

The underwriter placement and legal fees of $5.2 million related to these two PIPE financing transactions were included in accounts payable and accrued liabilities on the consolidated balance sheet as of December 31, 2020.

Common Stock Warrants

As of December 31, 2020, the following warrants to purchase shares of the Company’s common stock were issued and outstanding:

Issue Date		Expiration Date		Exercise Price				Number of Warrants Outstanding
December 28, 2020		The first date on which no shares of common stock are issuable upon a cash exercise of the Warrant		$	3.34				27,480,719

There were no warrants exercised during the year ended December 31, 2020. There were no warrants issued or outstanding in previous years.

Stock-Based Compensation

12 Months Ended

Dec. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

10.	Stock-Based Compensation

Award Incentive Plans

In August 2015, the Board of Directors approved the 2015 Equity Incentive Plan (“2015 Plan”). In connection with the Company’s IPO and the effectiveness of the 2018 Award Incentive Plan (“2018 Plan”), the 2015 Plan terminated, and no further awards will be granted under the 2015 Plan. The 92,815 shares of common stock shares that were then unissued and available for future issuance under the 2015 Plan became available under the 2018 Plan.

In September 2018, the Company’s Board of Directors approved the 2018 Plan. Under the 2018 Plan, a total of 2,690,000 shares of our common stock were initially reserved for issuance under the 2018 Plan, plus the number of shares remaining available for future awards under the 2015 Plan, as of the effective date of the 2018 Plan. The number of shares of common stock reserved for issuance under the 2018 Plan will automatically increase on January 1 of each year, beginning on January 1, 2019 and continuing through and including January 1, 2028, by 4% of the total number of shares of our stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the Company’s Board of Directors. The maximum number of shares that may be issued upon the exercise of stock options under the 2018 Plan is 45,000,000.

Prior to the Company’s IPO, the grant date fair value of the Company’s common stock was determined by the Company’s Board of Directors with the assistance of management and an independent third-party valuation specialist.

Subsequent to the Company’s IPO, the grant date fair value of each share of common stock underlying stock option awards is based on the closing price of our common stock as reported by the Nasdaq Select Global Market on the date of grant of the award.

The Company’s Board of Directors has the authority to determine to whom options will be granted, the number of shares, the term, and the exercise price.

If an individual owns stock representing 10% or more of the outstanding shares, the price of each share shall be at least 110% of the fair market value, as determined by the board of directors. Options granted have a term of up to 10 years and generally vest over a 4-year period with a straight-line vesting.

2018 Employee Stock Purchase Plan

In September 2018, the Company’s Board of Directors approved the 2018 Employee Stock Purchase Plan (“2018 ESPP”). The 2018 ESPP also became effective in September 2018. A total of 282,334 shares were initially reserved for issuance under the 2018 ESPP. Additionally, the number of shares of common stock reserved for issuance under the 2018 ESPP will increase automatically each year, beginning on January 1, 2019 and continuing through and including January 1, 2028, by the lesser of (1) 1% of the shares of common stock outstanding on December 31 of the preceding calendar year or (2) such lesser number of shares determined by the Company’s Board of Directors. The maximum number of shares that may be issued under the 2018 ESPP is 5,000,000. The offering periods are scheduled to start on the first trading day on or after June 1 or December 1 of each year. Contributions under the 2018 ESPP are limited to a maximum of 15% of an employee’s eligible compensation.

The estimated fair value of stock purchase rights granted under the 2018 ESPP were calculated using the Black-Scholes option-pricing model using the following assumptions:

	Year ended			Year ended
	December 31, 2020			December 31, 2019
Expected dividend yield		—	%		—	%
Expected term	0.48 years			0.50 years
Risk-free interest rate		0.8	%		2.2	%
Expected volatility		81.0	%		74.0	%

Valuation of Stock Options

The fair value of each stock option granted to an employee or a director was estimated as of the date of grant using the Black-Scholes model with the following weighted-average assumptions:

	Year Ended December 31,
	2020			2019			2018
Expected dividend yield		—	%		—	%		—	%
Expected term	6.01 years			6.01 years			6.04 years
Risk-free interest rate		1.1	%		2.3	%		2.8	%
Expected volatility		73.0	%		83.0	%		88.0	%

Management’s calculations are based on a grant date valuation approach. Using the Black-Scholes model, the weighted-average grant-date fair value of employee stock options granted was $5.39, $8.08, and $6.59 per share during the years ended December 31, 2020, 2019, and 2018, respectively.

Stock Option Activity

A summary of the 2015 Plan and 2018 Plan activity is as follows:

	Options Outstanding
	Number of Shares Available for Issuance			Number of Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Balance at December 31, 2019		2,851,883			3,165,961		$	7.83		8.45	$	8,875
Authorized		1,454,553			—		$	—
Granted		(1,822,300	)		1,822,300		$	8.41
Exercised		—			(197,544	)	$	1.06
Cancelled		624,276			(624,276	)	$	9.66
Balance at December 31, 2020		3,108,412			4,166,441		$	8.13		8.15	$	2,092
Vested and exercisable – December 31, 2020					1,941,879		$	6.99		7.52	$	1,874
Vested and expected to vest – December 31, 2020					3,894,070		$	8.06		8.10	$	2,078

For the years ended December 31, 2020, 2019, and 2018, the total intrinsic value of stock option awards exercised was $0.9 million, $3.0 million, and $0.8 million, respectively, determined at the date of option exercise, and the total cash received upon exercise of stock options was not significant for either period. The aggregate intrinsic value was calculated as the difference between the exercise prices of the underlying stock option awards and the estimated fair value of the common stock on the date of exercise.

As of December 31, 2020, $11.4 million of total cost related to non-vested employee and consultant options is expected to be recognized over a weighted-average period of 2.4 years. The total fair value of shares vested during the year ended December 31, 2020 was $6.8 million.

Stock-based compensation expense and awards granted to non-employees was insignificant for the years ended December 31, 2020, 2019 and 2018.

Stock-Based Compensation Expense

Total stock-based compensation for all awards granted to employees and consultants and our 2018 ESPP, before taxes, is as follows (in thousands):

	Year Ended December 31,
	2020		2019		2018
Research and development expenses	$	4,453	$	3,437	$	2,081
General and administrative expenses		2,657		1,862		914
Total	$	7,110	$	5,299	$	2,995

Income Taxes

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Income Taxes

11.	Income Taxes

The effective tax rate for the years ended December 31, 2020, 2019 and 2018 is different from the federal statutory rate primarily due to the valuation allowance against deferred tax assets as a result of insufficient sources of income. The effective tax rate of the Company’s provision for income taxes differs from the federal statutory rate as follows:

	Year Ended December 31,
	2020			2019			2018
Statutory federal income tax rate		21.0	%		21.0	%		21.0	%
State tax, net of federal benefit		7.9			4.2			4.3
Permanent differences		(0.6	)		(0.8	)		(1.0	)
Research and development tax credits		2.8			2.6			4.4
Other		(0.2	)		(0.8	)		(0.2	)
Change in state tax apportionment		9.4			—			—
Change in valuation allowance		(40.3	)		(26.3	)		(28.5	)
Total provision for income taxes		—	%		—	%		—	%

The Company uses the liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. The Company assesses the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized.

In assessing the realizability of deferred tax assets, the Company considers whether it is more likely than not that some portion or all of the deferred tax assets will not be realized. The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income during the periods in which the temporary differences representing net future deductible amounts become deductible. Due to the Company’s history of losses, and lack of other positive evidence, the Company has determined that it is more likely than not that its net deferred tax assets will not be realized, and therefore, the net deferred tax assets are fully offset by a valuation allowance at December 31, 2020 and 2019. The deferred tax assets were primarily comprised of federal and state tax net operating losses and tax credit carryforwards. The increase in valuation allowance of $42.4 million and $24.9 million during 2020 and

2019, respectively, was primarily attributable to the Company’s current year taxable loss and increase in the effective state tax rate due to changes in the Company’s state apportionment factors.

The components of the net deferred tax assets/liabilities are as follows (in thousands):

	December 31,
	2020			2019
Deferred tax assets:
Federal and state net operating loss carryforwards	$	74,278		$	45,228
Research and development tax credits		9,247			6,205
Lease liabilities		7,327			5,920
Accruals and other		3,354			2,172
Amortization		11,155			2,700
Deferred revenue		3,414			3,659
Deferred tax liabilities:
Other depreciation		(763	)		(638	)
Operating lease right-of-use assets		(6,231	)		(5,904	)
Total net deferred tax assets		101,781			59,342
Less valuation allowance		(101,781	)		(59,342	)
Deferred tax assets, net of allowance	$	—		$	—

At December 31, 2020, the Company’s federal and state income tax net operating loss carryforwards were approximately $277.1 million and $242.9 million, respectively, which may be subject to limitations as described below. If not utilized, the federal tax loss carryforwards will begin to expire in 2035 and the state tax loss carryforwards will begin to expire in 2035. Under the Tax Act, federal net operating losses generated after 2017 and in future years may be carried forward indefinitely, but the deductibility of such federal net operating losses is limited. The federal net operating losses generated after December 31, 2017 of $226.5 million are carried forward indefinitely. In addition, the Company has certain federal, California and Massachusetts research and development income tax credit carryforwards of $8.3 million, $5.1 million and $0.7 million, respectively. If not utilized, the federal research and development income tax credit carryforwards will begin to expire in 2035. The California research and development income tax credit carryforwards do not expire and can be carried forward indefinitely. The Massachusetts research and development income tax credit carryforwards will begin to expire in 2035.

Net operating loss and tax credit carryforwards as of December 31, 2020 are as follows (in thousands):

	Amount		Expiration Years
Net operating losses, federal (post December 31, 2017)	$	226,510	Do not expire
Net operating losses, federal (pre January 1, 2018)		50,587	2035 - 2037
Net operating loss, state		242,922	2035 - 2040
Tax credits, federal		8,252	2035 - 2040
Tax credits, state		5,772	CA: credits do not expire MA: 2035 - 2040

The net operating loss and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities and may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant stockholders over a three-year period in excess of 50%, as defined under IRC Sections 382 and 383. This could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. Subsequent ownership changes may further affect the limitation in future years. The Company completed an analysis through December 31, 2020 under IRC Sections 382 and 383 to determine if the Company’s net operating loss carryforwards and research and development credits are limited due to a change in ownership. In connection with the Company’s IPO, which closed in October 2018, the Company did experience an ownership change pursuant to Section 382. There was no reduction in federal or California net operating loss carryforwards or research and development income tax credits as a result of this ownership change. Other than the change identified in connection with the Company’s IPO, through December 31, 2020, the Company did not experience any other ownership change pursuant to Section 382.

The following table summarizes the activity related to the Company’s unrecognized tax benefits (in thousands):

	December 31,
	2020		2019			2018
Beginning of year—unrecognized tax benefits	$	2,610	$	2,077		$	1,089
Decrease for tax positions taken during prior periods		—		(711	)		(453	)
Increases for tax positions taken during current period		1,415		1,244			1,441
End of year—unrecognized tax benefits	$	4,025	$	2,610		$	2,077

If recognized, none of the unrecognized tax benefits as of December 31, 2020 and 2019 would impact the annual effective tax rate, primarily due to corresponding adjustments to the valuation allowance. The Company does not expect any material changes to the estimated amount of liability associated with its uncertain tax positions within the next twelve (12) months.

During the years ended December 31, 2020, 2019, and 2018, the Company did not recognize accrued interest and penalties related to unrecognized tax benefits.

The Company files income tax returns in the U.S. federal jurisdiction and various state jurisdictions. The Company is not currently under audit by the Internal Revenue Service or other similar state or local authorities. Due to the net operating loss carryforwards, all years remain open for income tax examination by tax authorities in the United States, various states and foreign tax jurisdictions in which the Company files tax returns.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References + Details

Net Loss Per Common Share

12 Months Ended

Dec. 31, 2020

Earnings Per Share [Abstract]

Net Loss Per Common Share

12.	Net Loss Per Common Share

The following table sets forth the computation of the basic and diluted net loss per share (in thousands, except for share and per share amounts):

	Year Ended December 31,
	2020			2019			2018
Numerator:
Net loss	$	(105,314	)	$	(94,433	)	$	(64,775	)
Denominator:
Weighted-average common shares outstanding, basic and diluted		37,792,365			33,554,823			8,919,281
Net loss per share, basic and diluted	$	(2.79	)	$	(2.81	)	$	(7.26	)

In December 2020, the Company issued and sold Warrants to purchase 27,480,719 shares of common stock at a nominal exercise price of $0.01 per share (see Note 9). The shares of common stock into which the Warrants may be exercised are considered outstanding for the purposes of computing earnings per share because the shares may be issued for little or no consideration, they are fully vested, and are immediately exercisable upon their issuance date.

Since the Company was in a loss position for all periods presented, basic net loss per share is the same as diluted net loss per share for all periods, as the inclusion of all potential common shares outstanding would have been anti-dilutive. Potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive were as follows:

	December 31,
	2020		2019		2018
Options issued and outstanding and ESPP shares issuable and outstanding		4,213,738		3,179,041		2,429,859
Early exercised common stock subject to future vesting		—		30,874		226,967
Total		4,213,738		3,209,915		2,656,826

Related-Party Transactions

12 Months Ended

Dec. 31, 2020

Related-Party Transactions

13.	Related-Party Transactions

During the year ended December 31, 2018, the Company issued 333,333 shares of common stock for total net proceeds of $5.0 million to certain stockholders considered to be related parties.

Defined Contribution Plan

12 Months Ended

Dec. 31, 2020

Compensation And Retirement Disclosure [Abstract]

Defined Contribution Plan

14.	Defined Contribution Plan

The Company began sponsoring a 401(k) Plan in 2017, which provides that eligible employees can elect to contribute to the 401(k) Plan, subject to certain limitations, on a pretax basis. The Company matches up to 50% of the first 4% of each employee’s contribution. During the years ended December 31, 2020, 2019 and 2018, expenses recognized for the 401(k) Plan were insignificant.

- Definition

The entire disclosure for retirement benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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Reference 8: http://www.xbrl.org/2003/role/disclosureRef
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Reference 10: http://www.xbrl.org/2003/role/disclosureRef
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Reference 13: http://www.xbrl.org/2003/role/disclosureRef
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Reference 14: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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+ Details

Subsequent Event

12 Months Ended

Dec. 31, 2020

Subsequent Events [Abstract]

Subsequent Event

15.	Subsequent Event

In January 2021, the Company entered into a Non-Exclusive License and Development Agreement (the “License Agreement”) with Genevant Sciences GmbH (“Genevant”). Pursuant to the License Agreement, the Company obtained a nonexclusive license to Genevant’s lipid nanoparticle technology (“LNP Technology”) to develop and commercialize self-amplifying RNA (“SAM”) vaccines against SARS-CoV-2, the virus that causes COVID-19. Under the License Agreement, the Company made a $1.5M upfront payment to Genevant and Genevant is eligible to receive from the Company up to $191.0 million in contingent milestone payments per product, plus certain royalties on future product sales or licensing. In certain scenarios, in lieu of milestones and royalties, Genevant will be entitled to a percentage of amounts that the Company receives from sublicenses under the License Agreement, subject to certain conditions.

In January 2021, the Company entered into a Collaboration, Option and License Agreement with Gilead Sciences, Inc., or Gilead, to research and develop a vaccine-based immunotherapy as part of Gilead’s efforts to find a curative treatment for HIV infection. Pursuant to the terms of this agreement, Gilead will be responsible for conducting a Phase 1 study of a HIV-specific therapeutic vaccine utilizing the Company’s technology and has made a $30.0 million upfront cash payment to the Company. The Company also granted Gilead an option to acquire an exclusive license to develop and commercialize the HIV-specific therapeutic vaccine beyond the Phase 1 study. The Company will be eligible to receive up to an aggregate of $725.0 million if this option is exercised and if certain clinical, regulatory and commercial milestones are achieved, as well as tiered royalties ranging from the mid single-digits to low double-digits on net sales of a therapeutic product utilizing its technology. In connection with this agreement, the Company and Gilead entered into a Stock Purchase Agreement in January 2021, pursuant to which the Company issued and sold to Gilead, in an unregistered offering, 1,169,591 shares of the Company’s common stock at a purchase price of approximately $25.65 per share for aggregate gross proceeds to the Company of $30.0 million. In addition, the Company and Gilead entered into a Stockholder Agreement in January 2021, pursuant to which the Company granted certain registration rights to Gilead in respect of the shares of common stock purchased in this private placement, and Gilead agreed to certain standstill and market stand-off provisions.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2020

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

16.	Selected Quarterly Financial Data (Unaudited)

The following tables show a summary of the Company’s quarterly financial information for each of the four quarters of 2020 and 2019 and has been prepared in accordance with U.S. GAAP for interim financial reporting (in thousands, except per share amounts). In the opinion of management, all adjustments (consisting of normal recurring accruals) considered necessary for a fair presentation have been included.

	Quarter Ended
	December 31, 2020			September 30, 2020			June 30, 2020			March 31, 2020
Revenue	$	1,348		$	939		$	488		$	1,262
Loss from operations	$	(27,147	)	$	(26,142	)	$	(26,057	)	$	(26,671	)
Net loss	$	(27,167	)	$	(26,073	)	$	(25,868	)	$	(26,206	)
Net loss per share, basic and diluted	$	(0.69	)	$	(0.69	)	$	(0.69	)	$	(0.71	)

	Quarter Ended
	December 31, 2019			September 30, 2019			June 30, 2019			March 31, 2019
Revenue	$	884		$	984		$	1,150		$	1,347
Loss from operations	$	(28,313	)	$	(28,484	)	$	(22,214	)	$	(18,929	)
Net loss	$	(27,704	)	$	(27,548	)	$	(21,172	)	$	(18,009	)
Net loss per share, basic and diluted	$	(0.77	)	$	(0.77	)	$	(0.63	)	$	(0.62	)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2020

Accounting Policies [Abstract]

Basis of Presentation

Need for Additional Capital

Use of Estimates

Fair Value of Financial Instruments

•

Level 1 inputs are quoted prices in active markets that are accessible at the market date for identical assets or liabilities.

•

Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly.

•

Cash, Cash Equivalents, and Restricted Cash

Cash, Cash Equivalents and Restricted Cash

	December 31,
	2020		2019
Cash and cash equivalents	$	170,056	$	57,408
Restricted cash		992		992
Total cash, cash equivalents and restricted cash	$	171,048	$	58,400

Marketable Securities

to be other than temporary, if any, on available-for-sale securities are included in interest income, net. The cost of securities sold is determined using specific identification method.

Concentrations of Credit Risk

Other Risks and Uncertainties

Property and Equipment, Net

Depreciation is calculated using the straight-line method over the estimated useful lives of the respective assets, which are as follows:

Asset		Estimated Useful Life
Computer equipment and software		3 to 5 years
Furniture and fixtures		5 years
Laboratory equipment		5 to 7 years
Leasehold improvements		Shorter of useful life or lease term

Long-Lived Assets

Revenue Recognition

Share-Based Compensation

Stock-Based Compensation

Research and Development Expenses

Leases

The Company recognizes lease expense on a straight-line basis over the expected lease term.

Income Taxes

Comprehensive Loss

Comprehensive loss includes net loss and certain changes in stockholders’ equity that are excluded from net loss, primarily unrealized gains and losses on the Company’s marketable securities.

Net Loss per Share

Net Loss Per Share

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements Not Yet Adopted

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2020

Accounting Policies [Abstract]

Cash, Cash Equivalents, and Restricted Cash

The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the balance sheets that sum to the total of the same amounts shown in the consolidated statements of cash flows (in thousands).

	December 31,
	2020		2019
Cash and cash equivalents	$	170,056	$	57,408
Restricted cash		992		992
Total cash, cash equivalents and restricted cash	$	171,048	$	58,400

Estimated Useful Lives of Assets

Depreciation is calculated using the straight-line method over the estimated useful lives of the respective assets, which are as follows:

Asset		Estimated Useful Life
Computer equipment and software		3 to 5 years
Furniture and fixtures		5 years
Laboratory equipment		5 to 7 years
Leasehold improvements		Shorter of useful life or lease term

Cash Equivalents and Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2020

Cash And Cash Equivalents [Abstract]

Cash Equivalents and Marketable Securities

The amortized cost, unrealized gains and losses and fair values of cash equivalents and marketable securities were as follows (in thousands):

	December 31, 2020
Description	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value
Cash equivalents:
Money market funds	$	36,801	$	—	$	—	$	36,801
Total cash equivalents		36,801		—		—		36,801
Short-term marketable securities:
U.S. treasuries		1,002		—		—		1,002
Total short-term marketable securities		1,002		—		—		1,002
Total	$	37,803	$	—	$	—	$	37,803

	December 31, 2019
Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents:
Money market funds	$	42,133	$	—	$	—		$	42,133
Commercial paper		1,749		—		—			1,749
Corporate debt securities		2,500		—		—			2,500
Total cash equivalents		46,382		—		—			46,382
Short-term marketable securities:
Certificates of deposit		631		—		—			631
Commercial paper		31,476		15		—			31,491
Corporate debt securities		38,237		15		(6	)		38,246
Total short-term marketable securities		70,344		30		(6	)		70,368
Total	$	116,726	$	30	$	(6	)	$	116,750

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2020

Fair Value Disclosures [Abstract]

Assets and Liabilities Measured at Fair Value

The Company’s financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements were as follows (in thousands):

	December 31, 2020
Description	Total		Level 1		Level 2		Level 3
Cash equivalents:
Money market funds	$	36,801	$	36,801	$	—	$	—
Total cash equivalents		36,801		36,801		—		—
Short-term marketable securities:
U.S. treasuries		1,002		1,002		—		—
Total short-term marketable securities		1,002		1,002		—		—
Total	$	37,803	$	37,803	$	—	$	—

	December 31, 2019
Description	Total		Level 1		Level 2		Level 3
Cash equivalents:
Money market funds	$	42,133	$	42,133	$	—	$	—
Commercial paper		1,749		—		1,749		—
Corporate debt securities		2,500		—		2,500		—
Total cash equivalents		46,382		42,133		4,249		—
Short-term marketable securities:
Certificates of deposit		631		—		631		—
Commercial paper		31,491		—		31,491		—
Corporate debt securities		38,246		—		38,246		—
Total short-term marketable securities		70,368		—		70,368		—
Total	$	116,750	$	42,133	$	74,617	$	—

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2020

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment, Net

Property and equipment and related accumulated depreciation and amortization are as follows (in thousands):

	December 31,
	2020			2019
Computer equipment and software	$	971		$	863
Furniture and fixtures		2,054			2,007
Laboratory equipment		22,213			21,025
Leasehold improvements		13,726			11,102
		38,964			34,997
Less accumulated depreciation and amortization		(16,925	)		(10,310	)
Construction-in-progress		66			2,224
Total property and equipment, net	$	22,105		$	26,911

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2020

Commitments And Contingencies Disclosure [Abstract]

Schedule of Components of Lease Costs

The components of lease costs, which were included in our consolidated statements of operations and comprehensive loss, were as follows (in thousands):

	Year ended December 31,
	2020		2019
Lease cost
Operating lease cost	$	7,511	$	6,382
Short-term lease cost		7		271
Total lease cost	$	7,518	$	6,653

Schedule of Supplemental Information Related to Leases

Supplemental information related to leases was as follows (in thousands):

	Year ended December 31,
	2020			2019
Cash paid for amounts included in the measurement of lease liabilities (in thousands):
Operating cash flows from operating leases	$	5,389		$	2,495
New right-of-use assets obtained in exchange for lease obligations (in thousands):
Operating leases	$	3,174		$	12,331
Weighted average remaining lease term (years):
Operating leases		6.10			7.20
Weighted average discount rate:
Operating leases		9.0	%		9.0	%

Schedule of Minimum Annual Payments Under Operating Lease Agreements

As of December 31, 2020, minimum annual payments under the Company’s operating lease agreements are as follows (in thousands):

	Lease Financing Obligation
Year ending December 31,
2021	$	7,863
2022		5,836
2023		3,459
2024		3,459
2025		2,456
Thereafter		10,076
Total minimum payments		33,149
Less: Amounts representing interest expense		(8,050	)
Present value of future minimum lease payments		25,099
Less: Current portion of lease liability		(5,874	)
Noncurrent portion of lease liability	$	19,225

Collaboration and License Agreements (Tables)

12 Months Ended

Dec. 31, 2020

Collaborative And License Agreements [Abstract]

Schedule of Changes in Deferred Revenue Balance

Changes in the deferred revenue balance during the year ended December 31, 2020 are as follows (in thousands):

	Deferred Revenue
Balance at December 31, 2019	$	14,516
Additions		—
Deductions		(2,821	)
Balance at December 31, 2020	$	11,695

Balance Sheet Components (Tables)

12 Months Ended

Dec. 31, 2020

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Schedule of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consist of the following (in thousands):

	December 31,
	2020		2019
Prepaid insurance	$	1,480	$	1,252
Prepaid research and development-related expenses		2,584		1,473
Other		268		772
Total prepaid expenses and other current assets	$	4,332	$	3,497

Schedule of Deposits and Other Long-Term Assets

Deposits and other long-term assets consist of the following (in thousands):

	December 31,
	2020		2019
Lease security deposits	$	1,218	$	1,201
Prepaid research and development-related expenses		518		585
Restricted cash		992		992
Total deposits and other long-term assets	$	2,728	$	2,778

Schedule of Accrued Current Liabilities

Accrued current liabilities consist of the following (in thousands):

	December 31,
	2020		2019
Property and equipment	$	96	$	761
Other		581		280
Total accrued current liabilities	$	677	$	1,041

Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Schedule of Warrants to Purchase Shares of Common Stock Issued and Outstanding

Common Stock Warrants

As of December 31, 2020, the following warrants to purchase shares of the Company’s common stock were issued and outstanding:

Issue Date		Expiration Date		Exercise Price				Number of Warrants Outstanding
December 28, 2020		The first date on which no shares of common stock are issuable upon a cash exercise of the Warrant		$	3.34				27,480,719

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Assumptions Used for Estimating the Fair Value of Stock Purchase Rights Granted Under 2018 Employee Stock Purchase Plan

The estimated fair value of stock purchase rights granted under the 2018 ESPP were calculated using the Black-Scholes option-pricing model using the following assumptions:

	Year ended			Year ended
	December 31, 2020			December 31, 2019
Expected dividend yield		—	%		—	%
Expected term	0.48 years			0.50 years
Risk-free interest rate		0.8	%		2.2	%
Expected volatility		81.0	%		74.0	%

Schedule of Weighted-Average Assumptions Used in Black-Scholes Option-Pricing Model to Estimate Fair Value of Employee Stock Options at Grant Date

The fair value of each stock option granted to an employee or a director was estimated as of the date of grant using the Black-Scholes model with the following weighted-average assumptions:

	Year Ended December 31,
	2020			2019			2018
Expected dividend yield		—	%		—	%		—	%
Expected term	6.01 years			6.01 years			6.04 years
Risk-free interest rate		1.1	%		2.3	%		2.8	%
Expected volatility		73.0	%		83.0	%		88.0	%

Schedule of Stock Options Activity

A summary of the 2015 Plan and 2018 Plan activity is as follows:

	Options Outstanding
	Number of Shares Available for Issuance			Number of Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Balance at December 31, 2019		2,851,883			3,165,961		$	7.83		8.45	$	8,875
Authorized		1,454,553			—		$	—
Granted		(1,822,300	)		1,822,300		$	8.41
Exercised		—			(197,544	)	$	1.06
Cancelled		624,276			(624,276	)	$	9.66
Balance at December 31, 2020		3,108,412			4,166,441		$	8.13		8.15	$	2,092
Vested and exercisable – December 31, 2020					1,941,879		$	6.99		7.52	$	1,874
Vested and expected to vest – December 31, 2020					3,894,070		$	8.06		8.10	$	2,078

Schedule of Stock Based Compensation Expense

Total stock-based compensation for all awards granted to employees and consultants and our 2018 ESPP, before taxes, is as follows (in thousands):

	Year Ended December 31,
	2020		2019		2018
Research and development expenses	$	4,453	$	3,437	$	2,081
General and administrative expenses		2,657		1,862		914
Total	$	7,110	$	5,299	$	2,995

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2020

Income Tax Disclosure [Abstract]

Schedule of Reconciliation of Statutory Federal Income Tax Rate to Company's Effective Tax Rate

	Year Ended December 31,
	2020			2019			2018
Statutory federal income tax rate		21.0	%		21.0	%		21.0	%
State tax, net of federal benefit		7.9			4.2			4.3
Permanent differences		(0.6	)		(0.8	)		(1.0	)
Research and development tax credits		2.8			2.6			4.4
Other		(0.2	)		(0.8	)		(0.2	)
Change in state tax apportionment		9.4			—			—
Change in valuation allowance		(40.3	)		(26.3	)		(28.5	)
Total provision for income taxes		—	%		—	%		—	%

Schedule of Components of Net Deferred Tax Assets/Liabilities

The components of the net deferred tax assets/liabilities are as follows (in thousands):

	December 31,
	2020			2019
Deferred tax assets:
Federal and state net operating loss carryforwards	$	74,278		$	45,228
Research and development tax credits		9,247			6,205
Lease liabilities		7,327			5,920
Accruals and other		3,354			2,172
Amortization		11,155			2,700
Deferred revenue		3,414			3,659
Deferred tax liabilities:
Other depreciation		(763	)		(638	)
Operating lease right-of-use assets		(6,231	)		(5,904	)
Total net deferred tax assets		101,781			59,342
Less valuation allowance		(101,781	)		(59,342	)
Deferred tax assets, net of allowance	$	—		$	—

Schedule of Net Operating Loss and Tax Credit Carryforwards

Net operating loss and tax credit carryforwards as of December 31, 2020 are as follows (in thousands):

	Amount		Expiration Years
Net operating losses, federal (post December 31, 2017)	$	226,510	Do not expire
Net operating losses, federal (pre January 1, 2018)		50,587	2035 - 2037
Net operating loss, state		242,922	2035 - 2040
Tax credits, federal		8,252	2035 - 2040
Tax credits, state		5,772	CA: credits do not expire MA: 2035 - 2040

Schedule of Unrecognized Tax Benefits

The following table summarizes the activity related to the Company’s unrecognized tax benefits (in thousands):

	December 31,
	2020		2019			2018
Beginning of year—unrecognized tax benefits	$	2,610	$	2,077		$	1,089
Decrease for tax positions taken during prior periods		—		(711	)		(453	)
Increases for tax positions taken during current period		1,415		1,244			1,441
End of year—unrecognized tax benefits	$	4,025	$	2,610		$	2,077

Net Loss Per Common Share (Tables)

12 Months Ended

Dec. 31, 2020

Earnings Per Share [Abstract]

Computation of Basic and Diluted Net Loss Per Share

The following table sets forth the computation of the basic and diluted net loss per share (in thousands, except for share and per share amounts):

	Year Ended December 31,
	2020			2019			2018
Numerator:
Net loss	$	(105,314	)	$	(94,433	)	$	(64,775	)
Denominator:
Weighted-average common shares outstanding, basic and diluted		37,792,365			33,554,823			8,919,281
Net loss per share, basic and diluted	$	(2.79	)	$	(2.81	)	$	(7.26	)

Computation of Potentially Anti-Dilutive Securities

	December 31,
	2020		2019		2018
Options issued and outstanding and ESPP shares issuable and outstanding		4,213,738		3,179,041		2,429,859
Early exercised common stock subject to future vesting		—		30,874		226,967
Total		4,213,738		3,209,915		2,656,826

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2020

Quarterly Financial Information Disclosure [Abstract]

Summary of Company's Quarterly Financial Information

	Quarter Ended
	December 31, 2020			September 30, 2020			June 30, 2020			March 31, 2020
Revenue	$	1,348		$	939		$	488		$	1,262
Loss from operations	$	(27,147	)	$	(26,142	)	$	(26,057	)	$	(26,671	)
Net loss	$	(27,167	)	$	(26,073	)	$	(25,868	)	$	(26,206	)
Net loss per share, basic and diluted	$	(0.69	)	$	(0.69	)	$	(0.69	)	$	(0.71	)

	Quarter Ended
	December 31, 2019			September 30, 2019			June 30, 2019			March 31, 2019
Revenue	$	884		$	984		$	1,150		$	1,347
Loss from operations	$	(28,313	)	$	(28,484	)	$	(22,214	)	$	(18,929	)
Net loss	$	(27,704	)	$	(27,548	)	$	(21,172	)	$	(18,009	)
Net loss per share, basic and diluted	$	(0.77	)	$	(0.77	)	$	(0.63	)	$	(0.62	)

Organization - Additional Information (Detail)	12 Months Ended
Organization - Additional Information (Detail)	Dec. 31, 2020 Segment
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Number of operating segments	1

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)		3 Months Ended								12 Months Ended
	Jun. 29, 2020	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit		$ 326,302,000				$ 220,988,000				$ 326,302,000	$ 220,988,000
Net loss		$ 27,167,000	$ 26,073,000	$ 25,868,000	$ 26,206,000	$ 27,704,000	$ 27,548,000	$ 21,172,000	$ 18,009,000	105,314,000	94,433,000	$ 64,775,000
Net cash used in operating activities										89,102,000	85,011,000	38,162,000
Marketable securities recognition of other-than-temporary impairment										0	0	0
Impairment losses										$ 0	$ 0	$ 0
Percentage of realization of tax benefit upon ultimate settlement										50.00%
Use of net operating losses suspension period for medium and large business	3 years
Use of business incentive tax credits period cap	3 years
ASU 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted		true								true
Change in accounting principle, accounting standards update, adoption date		Jan. 01, 2020								Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect		true								true
ASU 2018-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted		true								true
Change in accounting principle, accounting standards update, adoption date		Jan. 01, 2020								Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect		true								true
ASU 2018-15 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted		true								true
Change in accounting principle, accounting standards update, adoption date		Jan. 01, 2020								Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect		true								true
ASU 2018-18 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted		true								true
Change in accounting principle, accounting standards update, adoption date		Jan. 01, 2020								Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect		true								true
ASU 2019-12 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted		true								true
Change in accounting principle, accounting standards update, adoption date		Apr. 01, 2020								Apr. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect		true								true
ASU 2020-10 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted		false								false
Change in accounting principle, accounting standards update, adoption date		Dec. 15, 2021								Dec. 15, 2021
Change in accounting principle, accounting standards update, immaterial effect		true								true
Taxable Years 2020, 2021 and 2022 [Member] \| Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Business incentive tax credits to offset against taxes	$ 5,000.0
Taxable Years 2020, 2021 and 2022 [Member] \| Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Net operating carryforward loss for certain tax payers with taxable income where use of net operating losses is suspended	$ 1,000.0

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 170,056	$ 57,408
Restricted cash	992	992
Total cash, cash equivalents and restricted cash	$ 171,048	$ 58,400

Summary of Significant Accounting Policies - Schedule of Straight Line Method Over Estimated Useful Lives of Respective Assets (Detail)	12 Months Ended
	Dec. 31, 2020
Computer Equipment and Software [Member] \| Minimum [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful life	3 years
Computer Equipment and Software [Member] \| Maximum [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful life	5 years
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful life	5 years
Laboratory Equipment [Member] \| Minimum [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful life	5 years
Laboratory Equipment [Member] \| Maximum [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful life	7 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful lives	Shorter of useful life or lease term

Cash Equivalents and Marketable Securities - Schedule of Amortized Cost, Unrealized Gains and Losses and Fair Value of Cash Equivalent and Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Cash equivalents and marketable securities [Line Items]
Amortized Cost	$ 170,056	$ 57,408
Unrealized Losses		(6)
Amortized Cost	37,803	116,726
Unrealized Gains		30
Fair Value	37,803	116,750
Cash Equivalents [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost	36,801	46,382
Fair Value	36,801	46,382
Cash Equivalents [Member] \| Money Market Funds [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost	36,801	42,133
Fair Value	36,801	42,133
Cash Equivalents [Member] \| Commercial Paper [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost		1,749
Fair Value		1,749
Cash Equivalents [Member] \| Corporate Debt Securities [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost		2,500
Fair Value		2,500
Short-Term Marketable Securities [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost	1,002	70,344
Unrealized Gains		30
Unrealized Losses		(6)
Fair Value	1,002	70,368
Short-Term Marketable Securities [Member] \| U.S. Treasuries [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost	1,002
Fair Value	$ 1,002
Short-Term Marketable Securities [Member] \| Commercial Paper [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost		31,476
Unrealized Gains		15
Fair Value		31,491
Short-Term Marketable Securities [Member] \| Corporate Debt Securities [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost		38,237
Unrealized Gains		15
Unrealized Losses		(6)
Fair Value		38,246
Short-Term Marketable Securities [Member] \| Certificates of Deposit [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost		631
Fair Value		$ 631

Cash Equivalents and Marketable Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020 USD ($) Security	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Cash equivalents and marketable securities [Line Items]
Cash and cash equivalents and marketable securities	$ 171,100,000	$ 127,800,000
Cash and cash equivalents	170,056,000	57,408,000
Marketable securities	$ 1,002,000	70,368,000
Number of individual securities in an unrealized loss position \| Security	0
Allowance for credit losses	$ 0
Marketable securities recognition of other-than-temporary impairment	$ 0	$ 0	$ 0
Maximum [Member]
Cash equivalents and marketable securities [Line Items]
Marketable securities contractual maturities period	1 year

Fair Value Measurements - Additional Information (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value, Assets, Level 1 to Level 2 Transfers	$ 0	$ 0
Fair Value, Liabilities, Level 1 to Level 2 Transfers	0	0
Fair Value, Assets, Level 2 to Level 1 Transfers	0	0
Fair Value, Liabilities, Level 2 to Level 1 Transfers	$ 0	$ 0

Property and Equipment, Net - Summary of Property and Equipment and Related Accumulated Depreciation and Amortization (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 38,964	$ 34,997
Less accumulated depreciation and amortization	(16,925)	(10,310)
Total property and equipment, net	22,105	26,911
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	971	863
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,054	2,007
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	22,213	21,025
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	13,726	11,102
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, net	$ 66	$ 2,224

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 6,644	$ 4,745	$ 3,961

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended						12 Months Ended
	May 31, 2020 USD ($)	Jul. 31, 2019 USD ($)	Jan. 31, 2019 USD ($) renewal_term	Sep. 30, 2018 USD ($)	Mar. 31, 2017 USD ($) ft²	Nov. 30, 2015 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	May 31, 2019 USD ($)	Jan. 01, 2019 USD ($)	Sep. 30, 2017 USD ($)	Feb. 28, 2016 USD ($)
Commitments and Contingencies Disclosure [Line Items]
Operating lease cash security deposit							$ 1,218,000	$ 1,201,000
Operating lease right-of-use assets							21,344,000	23,427,000
Lease liability							25,099,000
Rent expense							7,500,000	6,700,000	$ 1,800,000
Research and Development Expense							88,643,000	82,896,000	54,965,000
Contract Research Organization [Member]
Commitments and Contingencies Disclosure [Line Items]
Maximum milestone payments										$ 34,800,000		$ 36,400,000
Research and Development Expense							0	1,000,000.0	$ 1,000,000.0
Emeryville California [Member]
Commitments and Contingencies Disclosure [Line Items]
Operating leasing term			120 months			84 months
Operating lease cash security deposit						$ 50,000	600,000	$ 600,000
Operating leasing renewal option to extend lease			5 years			3 years
Operating lease period or additional temporary space				12 months
Temporary space lease termination notice period				30 days
Operating leases number of renewal terms \| renewal_term			2
Tenant improvement allowance			$ 4,000,000.0
Operating lease right-of-use assets							9,300,000
Lease liability							14,800,000
Operating lease termination date						2019-10
Adjustment to right-of-use assets and lease liability							1,800,000
Pleasanton [Member]
Commitments and Contingencies Disclosure [Line Items]
Operating leasing term										64 months
Operating leasing renewal option to extend lease					5 years
Tenant improvement allowance					$ 1,200,000					$ 100,000
Operating lease right-of-use assets							300,000
Lease liability							400,000
Net rentable area \| ft²					42,620
Lease expiration date					Nov. 30, 2024
Letters of credit outstanding, amount					$ 1,000,000.0
Withdrawal from irrevocable letters of credit							$ 0
Cambridge Massachusetts [Member]
Commitments and Contingencies Disclosure [Line Items]
Operating leasing term		24 months		24 months									67 months
Operating lease cash security deposit		$ 300,000		$ 300,000									$ 300,000
Operating leasing renewal option to extend lease	12 months	12 months											3 years
Tenant improvement allowance													$ 2,100,000
Operating lease right-of-use assets	$ 3,100,000
Lease liability	$ 3,100,000
Operating lease termination date	2022-08	2021-08		2020-08
Base rent paid for the year		$ 3,200,000		$ 1,300,000
Operating leases rent expense		100,000		$ 100,000
Annual base rent	$ 3,600,000	$ 3,400,000
Cambridge Massachusetts [Member] \| Accounting Standards Update 2016-02 [Member]
Commitments and Contingencies Disclosure [Line Items]
Operating lease right-of-use assets											$ 900,000

Commitments and Contingencies - Schedule of Components of Lease Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Lease cost
Operating lease cost	$ 7,511	$ 6,382	$ 0
Short-term lease cost	7	271
Total lease cost	$ 7,518	$ 6,653

Commitments and Contingencies - Schedule of Supplemental Information Related to Leases (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash paid for amounts included in the measurement of lease liabilities (in thousands):
Operating cash flows from operating leases	$ 5,389	$ 2,495
New right-of-use assets obtained in exchange for lease obligations (in thousands):
Operating leases	$ 3,174	$ 12,331
Weighted average remaining lease term (years):
Operating leases	6 years 1 month 6 days	7 years 2 months 12 days
Weighted average discount rate:
Operating leases	9.00%	9.00%

Commitments and Contingencies - Schedule of Minimum Annual Payments Under Operating Lease Agreements (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Operating Leases
2021	$ 7,863
2022	5,836
2023	3,459
2024	3,459
2025	2,456
Thereafter	10,076
Total minimum payments	33,149
Less: Amounts representing interest expense	(8,050)
Present value of future minimum lease payments	25,099
Less: Current portion of lease liability	(5,874)	$ (2,505)
Noncurrent portion of lease liability	$ 19,225	$ 20,985

Collaboration and License Agreements - Additional Information (Detail) - USD ($)	1 Months Ended						12 Months Ended
	Oct. 31, 2019	Oct. 31, 2018	Sep. 30, 2018	Aug. 31, 2018	Jul. 31, 2018	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Oct. 31, 2020
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Preferred stock shares issued							0	0
Proceeds from issuance of preferred stock							$ 0	$ 0	$ 20,935,000
Purchase of common stock upon Company IPO							10,879,000	75,202,000	92,586,000
Collaboration and license revenue							2,821,000
Deferred revenue							11,695,000	14,516,000
Research and development							$ 88,643,000	82,896,000	$ 54,965,000
Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Purchase of common stock upon Company IPO	$ 75,000,000.0
Series C [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of common shares sold				768,115	153,360	690,128
Bluebird Bio Inc [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement early termination notice period							120 days
Early termination reduction of future milestone and royalty payments, description							Additionally, all of bluebird’s payment obligations that have not yet accrued related to future milestone and royalty payments will be reduced by 50% for the remainder of the agreement term.
Collaboration Agreement [Member] \| Bluebird Bio Inc [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from non refundable upfront payment				$ 20,000,000.0
Proceeds from issuance of preferred stock				$ 10,000,000.0
Offering price per share		$ 15.00
Purchase of common stock upon Company IPO		$ 10,000,000.0
Number of common shares sold		666,667
Maximum development, regulatory, and commercial milestones							$ 1,200,000,000
Revenue recognition contract start month year							2018-08
Revenue recognition contract end month year							2023-08
Collaboration and license revenue							$ 2,800,000	4,300,000
Deferred revenue							11,700,000
Receivables							0
Contract assets							0
Collaboration Agreement [Member] \| Bluebird Bio Inc [Member] \| Series C [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Preferred stock shares issued				768,115
Offering price per share				$ 13.04
Collaboration Agreement [Member] \| Arbutus Biopharma Corporation and Protiva Biotherapeutics Inc [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment										$ 5,000,000.0
Research and development							0
Maximum milestone consideration payable							123,500,000
Milestone payment included in research and development expense			$ 2,500,000					3,000,000.0
Reimbursements expense materials and personnel costs								$ 400,000
Option and License and Development Agreement [Member] \| Genevant Sciences GmbH [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment											$ 2,000,000.0
Amount to extend the option term							1,000,000.0
Option and License and Development Agreement [Member] \| Genevant Sciences GmbH [Member] \| Research and Development Expense [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment							2,000,000.0
Option and License and Development Agreement [Member] \| Genevant Sciences GmbH [Member] \| Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate initial payment in specified development regulatory and commercial milestone											$ 71,000,000.0
Payments for options to license for each indication							$ 1,500,000

Collaboration and License Agreements - Schedule of Changes in Deferred Revenue Balance (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)
Revenue From Contract With Customer [Abstract]
Balance at December 31, 2019	$ 14,516
Additions	0
Deductions	(2,821)
Balance at December 31, 2020	$ 11,695

Balance Sheet Components - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Prepaid Expense And Other Assets Current [Abstract]
Prepaid insurance	$ 1,480	$ 1,252
Prepaid research and development-related expenses	2,584	1,473
Other	268	772
Total prepaid expenses and other current assets	$ 4,332	$ 3,497

Balance Sheet Components - Schedule of Deposits and Other Long-Term Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deposit Assets And Other Assets Noncurrent Disclosure [Abstract]
Lease security deposits	$ 1,218	$ 1,201
Prepaid research and development-related expenses	518	585
Restricted cash	992	992
Total deposits and other long-term assets	$ 2,728	$ 2,778

Balance Sheet Components - Schedule of Accrued Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables And Accruals [Abstract]
Property and equipment	$ 96	$ 761
Other	581	280
Total accrued current liabilities	$ 677	$ 1,041

Stockholders' Equity - Additional Information (Detail) - USD ($)	1 Months Ended							12 Months Ended
	Dec. 31, 2020	Oct. 31, 2019	Apr. 30, 2019	Oct. 31, 2018	Aug. 31, 2018	Jul. 31, 2018	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Oct. 02, 2018
Subsidiary Sale Of Stock [Line Items]
Common stock, shares authorized	300,000,000							300,000,000	300,000,000
Common stock, par value	$ 0.0001							$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000							10,000,000	10,000,000
Preferred stock, par value	$ 0.0001							$ 0.0001	$ 0.0001
Preferred stock, shares issued	0							0	0
Preferred stock, shares outstanding	0							0	0
Common stock, shares issued	47,552,693							47,552,693	36,332,956
Common stock, shares outstanding	47,552,693							47,552,693	36,332,956
Common stock voting rights								Holders of the Company’s common stock are entitled to one vote per share.
Net proceeds from issuance of stock								$ 10,879,000	$ 75,202,000	$ 92,586,000
Common stock, preferred stock, debt securities, warrants and units covered in Registration Statement		$ 250,000,000.0
Percentage of gross proceeds as underwriter compensation		3.00%
Maximum offering expense agreed to be reimbursed		$ 50,000
Issuance of pre-funded warrants, net of issuance costs								$ 87,704,000
Warrant exercise price	$ 3.34							$ 3.34
Class of warrant issued									0
Warrants exercised								0
Warrants outstanding									$ 0
Maximum [Member]
Subsidiary Sale Of Stock [Line Items]
Net proceeds from issuance of stock		$ 75,000,000.0
Series C Preferred Stock [Member]
Subsidiary Sale Of Stock [Line Items]
Shares issued price per share					$ 13.04	$ 13.04	$ 13.04
Proceed from issuance of stock					$ 10,000,000.0	$ 2,000,000.0	$ 8,900,000
Stock issued, number of shares					768,115	153,360	690,128
Payment for issuance cost							$ 100,000
Amended and Restated Certificate of Incorporation [Member]
Subsidiary Sale Of Stock [Line Items]
Common stock, shares authorized											300,000,000
Common stock, par value											$ 0.0001
Preferred stock, shares authorized											10,000,000
Preferred stock, par value											$ 0.0001
Underwritten Public Offering [Member]
Subsidiary Sale Of Stock [Line Items]
Shares issued price per share			$ 11.50
Net proceeds from issuance of stock			$ 69,700,000
ATM Equity Offering Program [Member]
Subsidiary Sale Of Stock [Line Items]
Stock issued, number of shares								1,160,963	490,880
Payment for issuance cost								$ 100,000	$ 300,000
Net proceeds from issuance of stock								9,800,000	3,800,000
Payment of commissions on sale of shares								$ 300,000	$ 100,000
Common Stock [Member] \| IPO [Member]
Subsidiary Sale Of Stock [Line Items]
Shares issued price per share				$ 15.00						$ 15.00
Stock issued, number of shares				6,854,202						6,854,202
Payment for issuance cost										$ 10,276,000
Net proceeds from issuance of stock				$ 92,500,000
Conversion of preferred stock shares into common stock shares				19,409,132
Reclassified common stock and additional paid in capital				$ 177,900,000						$ 14,000
Common Stock [Member] \| Over-Allotment Option [Member]
Subsidiary Sale Of Stock [Line Items]
Stock issued, number of shares				187,535
Common Stock [Member] \| Underwritten Public Offering [Member]
Subsidiary Sale Of Stock [Line Items]
Stock issued, number of shares			6,500,000
Common Stock [Member] \| ATM Equity Offering Program [Member]
Subsidiary Sale Of Stock [Line Items]
Stock issued, number of shares								1,160,963	490,880
Payment for issuance cost								$ 104,000	$ 301,000
Common Stock [Member] \| Private Investment in Public Equity [Member]
Subsidiary Sale Of Stock [Line Items]
Shares issued price per share	$ 3.71							$ 3.71
Proceed from issuance of stock	$ 15,000,000.0
Stock issued, number of shares	4,043,127							9,586,478
Payment for issuance cost								$ 1,394,000
Class of warrant issued	0							0
Underwriter placement and legal fees	$ 5,200,000
Common Stock and Warrants [Member] \| Private Investment in Public Equity [Member]
Subsidiary Sale Of Stock [Line Items]
Shares issued price per share	$ 3.34							$ 3.34
Proceed from issuance of stock	$ 110,000,000.0
Sale of common stock	5,543,351
Warrant exercise price	$ 0.01							0.01
Offering price per share	$ 3.33							$ 3.33
Warrants to purchase shares of common stock	27,480,719
Maximum percentage of common stock ownership to exercise warrant	9.99%

Stockholders' Equity - Schedule of Warrants to Purchase Shares of Common Stock Issued and Outstanding (Detail)	12 Months Ended
	Dec. 31, 2020 $ / shares shares
Equity [Abstract]
Issue Date	Dec. 28, 2020
Expiration Date	The first date on which no shares of common stock are issuable upon a cash exercise of the Warrant
Warrant exercise price \| $ / shares	$ 3.34
Number of Warrants Outstanding \| shares	27,480,719

Stock Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for future issuance	3,108,412	2,851,883
Weighted-average grant-date fair value granted employee stock options granted	$ 5.39	$ 8.08	$ 6.59
Intrinsic value of stock option awards exercised	$ 0.9	$ 3.0	$ 0.8
Total cost related to non-vested employee and consultant options expected to be recognized	$ 11.4
Total cost related to non-vested employee and consultant options unrecognized weighted average period	2 years 4 months 24 days
Fair value of shares vested	$ 6.8
2018 Award Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for future issuance				92,815
Number of shares available for issuance				2,690,000
Increase in shares available for issuance	4.00%
Maximum number of shares authorized	45,000,000
Minimum percentage of outstanding shares held by individual	10.00%
Minimum percentage fair market value of share price	110.00%
Option vesting period	4 years
Share based compensation by shared based payment award, description	If an individual owns stock representing 10% or more of the outstanding shares, the price of each share shall be at least 110% of the fair market value, as determined by the board of directors. Options granted have a term of up to 10 years and generally vest over a 4-year period with a straight-line vesting.
2018 Award Incentive Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option granted expiration period	10 years
2018 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for issuance				282,334
Increase in shares available for issuance	1.00%
Maximum number of shares authorized	5,000,000
Maximum contribution of employee's eligible compensation	15.00%

Stock-Based Compensation - Schedule of Fair Value of Stock Purchase Rights Granted Under 2018 Employee Stock Purchase Plan (Detail) - 2018 Employee Stock Purchase Plan [Member]	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%
Expected term	5 months 23 days	6 months
Risk-free interest rate	0.80%	2.20%
Expected volatility	81.00%	74.00%

Stock-Based Compensation - Schedule of Weighted-Average Assumptions Used in Black-Scholes Option-Pricing Model to Estimate Fair Value of Employee Stock Options at Grant Date (Detail) - Employee or Director [Member]	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%	0.00%
Expected term	6 years 3 days	6 years 3 days	6 years 14 days
Risk-free interest rate	1.10%	2.30%	2.80%
Expected volatility	73.00%	83.00%	88.00%

Stock-Based Compensation - Schedule of Stock Options Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Number of Shares Available for Issuance
Number of options beginning balance	2,851,883
Authorized	1,454,553
Granted	(1,822,300)
Exercised	0
Cancelled	624,276
Number of options ending balance	3,108,412	2,851,883
Number of Shares
Outstanding at beginning of period	3,165,961
Granted	1,822,300
Exercised	(197,544)
Cancelled	(624,276)
Outstanding at end of period	4,166,441	3,165,961
Options Outstanding, Number of Shares, Vested and exercisable	1,941,879
Options Outstanding, Number of Shares, Vested and expected to vest	3,894,070
Weighted-Average Exercise Price
Beginning Balance	$ 7.83
Granted	8.41
Exercised	1.06
Cancelled	9.66
Ending Balance	8.13	$ 7.83
Weighted-Average Exercise Price, Vested and exercisable	6.99
Weighted-Average Exercise Price, Vested and expected to vest	$ 8.06
Options Outstanding, Weighted-Average Remaining Contractual Term (in years)	8 years 1 month 24 days	8 years 5 months 12 days
Options Outstanding, Weighted-Average Remaining Contractual Term, Vested and exercisable (in years)	7 years 6 months 7 days
Options Outstanding, Weighted-Average Remaining Contractual Term, Vested and expected to vest (in years)	8 years 1 month 6 days
Options Outstanding, Aggregate Intrinsic Value	$ 2,092	$ 8,875
Options Outstanding, Aggregate Intrinsic Value, Vested and exercisable	1,874
Options Outstanding, Aggregate Intrinsic Value, Vested and expected to vest	$ 2,078

Stock-Based Compensation - Schedule of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock Based Compensation Expense	$ 7,110	$ 5,299	$ 2,995
Research and Development Expense [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock Based Compensation Expense	4,453	3,437	2,081
General and Administrative Expense [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock Based Compensation Expense	$ 2,657	$ 1,862	$ 914

Income Taxes - Schedule of Reconciliation of the Statutory Federal Income Tax Rate to the Company's Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Statutory federal income tax rate	21.00%	21.00%	21.00%
State tax, net of federal benefit	7.90%	4.20%	4.30%
Permanent differences	(0.60%)	(0.80%)	(1.00%)
Research and development tax credits	2.80%	2.60%	4.40%
Other	(0.20%)	(0.80%)	(0.20%)
Change in state tax apportionment	9.40%
Change in valuation allowance	(40.30%)	(26.30%)	(28.50%)
Total provision for income taxes	0.00%	0.00%	0.00%

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018
Income Taxes [Line Items]
Increase in valuation allowance	$ 42,400,000	$ 24,900,000
Changes in ownership over period	3 years
Unrecognized tax benefits that would impact effective tax rate	$ 0	0
Accrued interest and penalties related to unrecognized tax benefits	$ 0	$ 0	$ 0
Minimum [Member]
Income Taxes [Line Items]
Cumulative changes in ownership interest of shareholders	50.00%
Research and Development \| California
Income Taxes [Line Items]
Business incentive tax credits to offset against taxes	$ 5,100,000
Research and Development \| Massachusetts
Income Taxes [Line Items]
Business incentive tax credits to offset against taxes	$ 700,000
Income tax credit carryforwards, expiration year	2035
Federal
Income Taxes [Line Items]
Net operating loss carryforwards	$ 277,100,000			$ 226,500,000
Net operating loss carryforwards, expiration year	2035
Business incentive tax credits to offset against taxes	$ 8,252,000
Federal \| Research and Development
Income Taxes [Line Items]
Business incentive tax credits to offset against taxes	$ 8,300,000
Income tax credit carryforwards, expiration year	2035
State
Income Taxes [Line Items]
Net operating loss carryforwards	$ 242,922,000
Net operating loss carryforwards, expiration year	2035
Business incentive tax credits to offset against taxes	$ 5,772,000

Income Taxes - Components of the Net Deferred Tax Assets/Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Federal and state net operating loss carryforwards	$ 74,278	$ 45,228
Research and development tax credits	9,247	6,205
Lease liabilities	7,327	5,920
Accruals and other	3,354	2,172
Amortization	11,155	2,700
Deferred revenue	3,414	3,659
Deferred tax liabilities:
Other depreciation	(763)	(638)
Operating lease right-of-use assets	(6,231)	(5,904)
Total net deferred tax assets	101,781	59,342
Less valuation allowance	(101,781)	(59,342)
Deferred tax assets, net of allowance	$ 0	$ 0

Income Taxes - Schedule of Net Operating Loss and Tax Credit Carryforwards (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Jan. 01, 2018
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 277,100	$ 226,500
Business incentive tax credits to offset against taxes	$ 8,252
Tax credits, Expiration year	2035 - 2040
Federal \| Post December312017
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 226,510
Net operating losses, Expiration year	Do not expire
Federal \| Pre January12018
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 50,587
Net operating losses, Expiration year	2035 - 2037
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 242,922
Business incentive tax credits to offset against taxes	$ 5,772
Net operating losses, Expiration year	2035 - 2040
Tax credits, Expiration year	CA: credits do not expire MA: 2035 - 2040

Income Taxes - Schedule of Activity Related Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Beginning of year—unrecognized tax benefits	$ 2,610	$ 2,077	$ 1,089
Decrease for tax positions taken during prior periods		(711)	(453)
Increases for tax positions taken during current period	1,415	1,244	1,441
End of year—unrecognized tax benefits	$ 4,025	$ 2,610	$ 2,077

Net Loss Per Common Share - Computation of Basic and Diluted Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss	$ (27,167)	$ (26,073)	$ (25,868)	$ (26,206)	$ (27,704)	$ (27,548)	$ (21,172)	$ (18,009)	$ (105,314)	$ (94,433)	$ (64,775)
Denominator:
Weighted-average common shares outstanding, basic and diluted									37,792,365	33,554,823	8,919,281
Net loss per share, basic and diluted	$ (0.69)	$ (0.69)	$ (0.69)	$ (0.71)	$ (0.77)	$ (0.77)	$ (0.63)	$ (0.62)	$ (2.79)	$ (2.81)	$ (7.26)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

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+ Details

Net Loss Per Common Share - Additional Information (Detail) - Common Stock and Pre-funded Warrants [Member]	Dec. 31, 2020 $ / shares shares
Earnings Per Share Basic [Line Items]
Common stock called by warrants \| shares	27,480,719
Class of warrant nominal exercise price \| $ / shares	$ 0.01

Net Loss Per Common Share - Computation of Potentially Anti-dilutive Securities (Detail) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities that were not included in the diluted per share calculations	4,213,738	3,209,915	2,656,826
Options Issued and Outstanding and ESPP Shares Issuable and Outstanding [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities that were not included in the diluted per share calculations	4,213,738	3,179,041	2,429,859
Early Exercised Common Stock Subject To Future Vesting [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities that were not included in the diluted per share calculations		30,874	226,967

Related-Party Transactions - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Common stock, shares issued	47,552,693	36,332,956
Proceeds from Issuance of Common Stock	$ 10,879	$ 75,202	$ 92,586
Certain Stockholders Related Parties [Member]
Related Party Transaction [Line Items]
Common stock, shares issued			333,333
Proceeds from Issuance of Common Stock			$ 5,000

Defined Contribution Plan - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution, percent of employees' gross pay	4.00%
Defined Contribution Plan, Tax Status [Extensible List]	us-gaap:QualifiedPlanMember
Maximum [Member]
Defined Contribution Plan Disclosure [Line Items]
Employer's matching contribution percentage	50.00%

Subsequent Event - Additional Information (Detail) - Subsequent Event [Member] $ / shares in Units, $ in Millions	1 Months Ended
	Jan. 31, 2021 USD ($) $ / shares shares
Genevant [Member] \| Non-Exclusive License and Development Agreement [Member] \| Maximum [Member]
Subsequent Event [Line Items]
Upfront payments	$ 1.5
Contingent milestone payments	191.0
Gilead [Member] \| Collaboration, Option and License Agreement [Member]
Subsequent Event [Line Items]
Upfront cash payment	30.0
Aggregate contingent milestone payment	$ 725.0
Number of common shares sold \| shares	1,169,591
Purchase price per share \| $ / shares	$ 25.65
Proceeds from equity investment	$ 30.0

Selected Quarterly Financial Data (Unaudited) - Summary of Company's Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 1,348	$ 939	$ 488	$ 1,262	$ 884	$ 984	$ 1,150	$ 1,347	$ 4,037	$ 4,365	$ 1,187
Loss from operations	(27,147)	(26,142)	(26,057)	(26,671)	(28,313)	(28,484)	(22,214)	(18,929)	(106,017)	(97,940)	(65,584)
Net loss	$ (27,167)	$ (26,073)	$ (25,868)	$ (26,206)	$ (27,704)	$ (27,548)	$ (21,172)	$ (18,009)	$ (105,314)	$ (94,433)	$ (64,775)
Net loss per share, basic and diluted	$ (0.69)	$ (0.69)	$ (0.69)	$ (0.71)	$ (0.77)	$ (0.77)	$ (0.63)	$ (0.62)	$ (2.79)	$ (2.81)	$ (7.26)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

+ Details

- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (a)
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+ Details

Fair Value Measurements - Financial Assets and Liabilities Subject To Fair Value Measurements on a Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	$ 37,803	$ 116,750
Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	36,801	46,382
Cash Equivalents [Member] \| Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	36,801	42,133
Cash Equivalents [Member] \| Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value		1,749
Cash Equivalents [Member] \| Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value		2,500
Short-Term Marketable Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	1,002	70,368
Short-Term Marketable Securities [Member] \| U.S. Treasuries [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	1,002
Short-Term Marketable Securities [Member] \| Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value		31,491
Short-Term Marketable Securities [Member] \| Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value		38,246
Short-Term Marketable Securities [Member] \| Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value		631
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	37,803	42,133
Fair Value, Inputs, Level 1 [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	36,801	42,133
Fair Value, Inputs, Level 1 [Member] \| Cash Equivalents [Member] \| Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	36,801	42,133
Fair Value, Inputs, Level 1 [Member] \| Short-Term Marketable Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	1,002
Fair Value, Inputs, Level 1 [Member] \| Short-Term Marketable Securities [Member] \| U.S. Treasuries [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	$ 1,002
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value		74,617
Fair Value, Inputs, Level 2 [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value		4,249
Fair Value, Inputs, Level 2 [Member] \| Cash Equivalents [Member] \| Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value		1,749
Fair Value, Inputs, Level 2 [Member] \| Cash Equivalents [Member] \| Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value		2,500
Fair Value, Inputs, Level 2 [Member] \| Short-Term Marketable Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value		70,368
Fair Value, Inputs, Level 2 [Member] \| Short-Term Marketable Securities [Member] \| Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value		31,491
Fair Value, Inputs, Level 2 [Member] \| Short-Term Marketable Securities [Member] \| Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value		38,246
Fair Value, Inputs, Level 2 [Member] \| Short-Term Marketable Securities [Member] \| Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value		$ 631