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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2018	Mar. 22, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	GRTS
Entity Registrant Name	Gritstone Oncology, Inc.
Entity Central Index Key	0001656634
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		29,193,819
Entity Public Float			$ 0
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 52,183	$ 39,007
Marketable securities	100,927	46,946
Prepaid expenses and other current assets	4,526	2,526
Total current assets	157,636	88,479
Property and equipment, net	29,494	27,211
Deposits and other long-term assets	2,428	1,610
Total assets	189,558	117,300
Current liabilities:
Accounts payable	4,825	3,935
Accrued compensation	3,951	2,227
Accrued liabilities	992	1,490
Deferred revenue, current portion	5,340
Total current liabilities	15,108	7,652
Deferred rent, net of current portion	1,353	1,749
Other non-current liabilities	12	96
Lease financing obligation, net of current portion	10,490	10,521
Deferred revenue, net of current portion	13,473
Total liabilities	40,436	20,018
Commitments and contingencies (Notes 6 and 7)
Stockholders’ equity:
Convertible preferred stock, $0.0001 par value; 10,000,000 and 125,362,551 shares authorized at December 31, 2018 and 2017, respectively; none and 17,797,529 shares issued and outstanding at December 31, 2018 and 2017, respectively; aggregate liquidation preference of $157,268 at December 31, 2017		156,937
Common stock, $0.0001 par value; 300,000,000 and 160,000,000 shares authorized at December 31, 2018 and 2017, respectively; 28,823,130 and 2,152,525 shares issued and outstanding at December 31, 2018 and 2017, respectively	16	1
Additional paid-in capital	275,593	2,045
Accumulated other comprehensive loss	(85)	(74)
Accumulated deficit	(126,402)	(61,627)
Total stockholders’ equity	149,122	97,282
Total liabilities and stockholders’ equity	$ 189,558	$ 117,300

Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	125,362,551
Preferred stock, shares issued	0	17,797,529
Preferred stock, shares outstanding	0	17,797,529
Preferred stock, aggregate liquidation preference		$ 157,268
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	160,000,000
Common stock, shares issued	28,823,130	2,152,525
Common stock, shares outstanding	28,823,130	2,152,525

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Collaboration revenue	$ 1,187
Operating expenses:
Research and development	54,965	$ 35,691	$ 13,916
General and administrative	11,806	6,072	5,064
Total operating expenses	66,771	41,763	18,980
Loss from operations	(65,584)	(41,763)	(18,980)
Interest income, net	809	386	230
Net loss	(64,775)	(41,377)	(18,750)
Other comprehensive loss:
Unrealized loss on marketable securities, net of tax	(11)	(71)	(3)
Net and comprehensive loss	$ (64,786)	$ (41,448)	$ (18,753)
Net loss per share, basic and diluted	$ (7.26)	$ (20.70)	$ (11.21)
Weighted-average number of shares used in computing net loss per share, basic and diluted	8,919,281	1,999,044	1,672,545

Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	IPO [Member]	Convertible Preferred Stock [Member]	Series A Convertible Preferred Stock [Member]	Series A, B, and C Convertible Preferred Stock [Member]	Series B Convertible Preferred Stock [Member]	Series C Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member] IPO [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] IPO [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2015	$ 23,987		$ 25,425					$ 1		$ 61			$ (1,500)
Balance, shares at Dec. 31, 2015			3,699,259					1,657,580
Issuance of stock, net of issuance costs	35,714			$ 35,714
Issuance of stock, net of issuance costs, shares				5,178,968
Unrealized loss on marketable securities, net of tax	(3)											$ (3)
Lapse of repurchase rights related to common stock issued pursuant to early exercises	53									53
Lapse of repurchase rights related to common stock issued pursuant to early exercises, shares								154,210
Stock-based compensation	597									597
Issuance of common stock warrants for license	91									91
Net loss	(18,750)												(18,750)
Balance at Dec. 31, 2016	41,689		$ 61,139					$ 1		802		(3)	(20,250)
Balance, shares at Dec. 31, 2016			8,878,227					1,811,790
Issuance of stock, net of issuance costs	95,798					$ 95,798
Issuance of stock, net of issuance costs, shares						8,919,302
Unrealized loss on marketable securities, net of tax	(71)											(71)
Lapse of repurchase rights related to common stock issued pursuant to early exercises	$ 117									117
Lapse of repurchase rights related to common stock issued pursuant to early exercises, shares								338,924
Issuance of common stock upon exercise of stock options, shares	41,665							1,811
Stock-based compensation	$ 1,126									1,126
Net loss	(41,377)												(41,377)
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
Balance, shares at Dec. 31, 2017	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 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							80,463
Issuance of common stock for consulting services	$ 36									36
Issuance of common stock for consulting services, shares								4,347
Stock-based compensation	2,959									2,959
Exercise of common stock warrants	13									13
Exercise of common stock warrants, shares								40,257
Net loss	(64,775)												(64,775)
Balance at Dec. 31, 2018	$ 149,122							$ 16		$ 275,593		$ (85)	$ (126,402)
Balance, shares at Dec. 31, 2018								28,823,130
Balance, shares at Dec. 31, 2018	28,823,130

Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands		1 Months Ended		12 Months Ended
	Oct. 20, 2017	Jun. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Oct. 31, 2018	Aug. 31, 2018	Jul. 31, 2018
IPO [Member]
Payment for issuance cost				$ 10,276
Shares issued price per share				$ 15.00			$ 15.00
Series A Preferred Stock [Member]
Payment for issuance cost						$ 21
Shares issued price per share						$ 6.90
Series B Preferred Stock [Member]
Payment for issuance cost	$ 200		$ 200		$ 210
Shares issued price per share	$ 10.76		$ 10.76		$ 10.76
Series C Preferred Stock [Member]
Payment for issuance cost		$ 100		$ 81
Shares issued price per share		$ 13.04		$ 13.04				$ 13.04	$ 13.04

Condensed Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
Condensed Statements of Cash Flows - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (64,775)	$ (41,377)	$ (18,750)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	3,961	1,970	823
Net amortization of premiums and discounts on marketable securities	(552)	(158)	43
Stock-based compensation	2,995	1,126	597
Warrant issuance in conjunction with license agreement			91
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,781)	(416)	(459)
Deposits and other long-term assets	(818)	(78)	(487)
Accounts payable	3,249	2,273	(760)
Accrued compensation	1,724	1,190	1,018
Accrued and other non-current liabilities	(582)	810	532
Deferred rent	(396)	(311)	2,060
Deferred revenue	18,813
Net cash used in operating activities	(38,162)	(34,971)	(15,292)
Investing activities
Purchase of marketable securities	(102,160)	(63,228)	(47,993)
Maturities of marketable securities	48,720	41,467	22,850
Purchase of property and equipment	(5,663)	(11,522)	(6,984)
Disposition of property and equipment		31
Net cash used in investing activities	(59,103)	(33,252)	(32,127)
Financing activities
Proceeds from issuance of common stock, net of issuance costs	92,586	14	232
Payments of deferred IPO costs	(3,080)
Proceeds from issuance of convertible preferred stock, net of issuance costs	20,935	95,798	35,714
Net cash provided by financing activities	110,441	95,812	35,946
Net increase (decrease) in cash, cash equivalents and restricted cash	13,176	27,589	(11,473)
Cash, cash equivalents and restricted cash at beginning of period	39,999	12,410	23,883
Cash, cash equivalents and restricted cash at end of period	53,175	39,999	12,410
Supplemental disclosures of non-cash investing and financing information
Property and equipment purchases accrued but not yet paid	$ 1,482	900	$ 61
Assets acquired under leasing obligations		9,300
Receivable from lessor funded financing		$ 1,226

Organization

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization

1.	Organization

Description of Business

Gritstone Oncology, Inc. (“Gritstone” or “the Company”) is an immuno-oncology company developing personalized cancer immunotherapies to fight multiple cancer types. 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.

Initial Public Offering

In October 2018, the Company closed its initial public offering (“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 $92.6 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 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.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2.	Summary of Significant Accounting Policies

Basis of Presentation

The accompanying financial statements have been prepared in accordance with United States generally accepted accounting principles (“U.S. GAAP”) and the rules and regulations of Securities and Exchange Commission (“SEC”) for reporting.

Reverse Stock Split

On September 20, 2018, the Company amended and restated its amended and restated certificate of incorporation to effect a 1-for-6.9 reverse split (“Reverse Split”) of shares of the Company’s common and convertible preferred stock. The par value and authorized shares of common stock and convertible preferred stock were not adjusted as a result of the Reverse Split. All of the share and per share information included in the accompanying financial statements has been adjusted to reflect the Reverse Split.

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 $126.4 million and $61.6 million as of December 31, 2018 and 2017, respectively. The Company had net losses of $64.8 million, $41.4 million, and $18.8 million for the years ended December 31, 2018, 2017, and 2016, respectively, and net cash used in operating activities of $38.2 million and $35.0 million, and $15.3 million for years ended December 31, 2018, 2017, and 2016, 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 contracts with customers. 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 financial statements are issued. Management expects operating losses to continue for the foreseeable future. As a result, the Company will need to raise additional capital. If sufficient funds on acceptable terms are not available when needed, the Company could be required to significantly reduce its operating expenses and delay, reduce the scope of, or eliminate one or more of its development programs. Failure to manage discretionary spending or raise additional financing, as needed, may adversely impact the Company’s ability to achieve its intended business objectives.

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 study trial accruals, fair value of assets and liabilities, the fair value of leased buildings and other assumptions associated with lease financing transactions, and the fair value of common stock and stock-based compensation. 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

Accounting Standards Codification (ASC) Topic 820, Fair Value Measurement, 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.

ASC 820 identifies fair value 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, ASC 820 establishes a three-tier fair value hierarchy that 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 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 maturities of three 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 balance sheet 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 statements of cash flows (in thousands).

	December 31,
	2018		2017
Cash and cash equivalents	$	52,183	$	39,007
Restricted cash		992		992
Total cash, cash equivalents and restricted cash	$	53,175	$	39,999

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 original maturities of greater than 90 days from the date of purchase but less than one year from the balance sheet date are classified as short-term, while marketable securities with maturities in one year or beyond one year from the balances 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 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 and other income (expense), 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.

Concentrations of Credit Risk and Other Risks and Uncertainties

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 balance sheets. Through December 31, 2018, the Company has no off-balance sheet concentrations of credit risk.

The Company is subject to a number of risks similar to those of other preclinical-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.

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

Property and equipment includes a leased building which did not meet the sale-leaseback criteria and was recorded at its fair value plus the cost of improvements made during the constriction period. The leased building is being depreciated over the lease term to a residual value that will approximate the remaining lease financing obligation at the end of the lease (see Note 6).

Impairment of Long-Lived Assets

The Company evaluates long-lived assets, including property and equipment, 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. No impairment losses have been recorded for the periods presented.

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 balance sheets. If the related performance obligation is expected to be satisfied within the next twelve months this will be classified in current liabilities. Amounts recognized as revenue prior to receipt are recorded as contract assets in the Company’s balance sheets. If the Company expects to have an unconditional right to receive consideration in the next twelve 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.

Stock-Based Compensation

The Company measures and recognizes compensation expense for all stock-based awards made to employees and directors 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 valuation model used for calculating the fair value of awards for stock compensation expense is 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. 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.

Management recognizes stock-based compensation expense for stock options granted to non-employees based on the estimated fair value of the award on the measurement dates using the Black-Scholes model. The estimated fair value of options granted to non-employees is re-measured at each reporting period using the Black-Scholes model until the awards vest and the resulting change in value, if any, is recognized in the statement of operations and comprehensive loss.

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 stock-based compensation, consulting fees, laboratory supplies, and facility costs, as well as external research and development expenses incurred under arrangements with third parties and fees paid to other entities that conduct certain research and development activities on behalf of the Company. 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 rendered. 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 assess whether such contingent consideration meets the definition of a derivative and to date the Company has determined that such contingent consideration are not derivatives.

Pre-clinical costs are a component of research and development expense. The Company accrues and expenses 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 and Deferred Rent and Lease Financing Obligation

The Company rents its office space and facilities under non-cancelable operating lease agreements and recognize related rent expense on a straight-line basis over the term of the lease. The Company’s lease agreements contain rent holidays, scheduled rent increases, and renewal options. Rent holidays and scheduled rent increases are included in the determination of rent expense to be recorded ratably over the lease term. The Company does not assume renewals in its determination of the lease term unless they are deemed to be reasonably assured at the inception of the lease. The Company begins recognizing rent expense on the date that it obtains the legal right to use and control the leased space. Deferred rent consists of the difference between cash payments and the recognition of rent expense on a straight-line basis for the buildings the Company occupies.

Funding of leasehold improvements by the Company’s landlord is 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 is involved in the construction of improvements to buildings it is leasing. To the extent the Company is involved with the structural improvements of the construction project or takes construction risk, the Company is considered to be the owner of the building and related improvements for accounting purposes during the construction period. The Company records the fair value of the building and related improvements subject to the lease within property and equipment on the balance sheet. The Company also records a corresponding lease financing obligation on its balance sheet representing the amounts financed by the lessor for the building and lessor financed improvements. Lessor financed improvement incentives due but not yet received of $1.2 million at December 31, 2017 were recorded as prepaid expense and other current assets on the balance sheet. Such amounts were fully collected in April 2018. Once a construction project is complete, the Company considers 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 remain on the Company’s balance sheet and are subject to depreciation and assessment of impairment.

For such arrangements, at both pre and post the construction period, the Company bifurcates its lease payments into a portion allocated to the building and a portion allocated to the parcel of land on which the building has been built. The portion of the lease payments allocated to the land is treated for accounting purposes as operating lease payments, and therefore is recorded as rent expense in the statements of operations and comprehensive loss. The portion of the lease payments allocated to the building is 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.

Income Taxes

Management 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, 2018, the Company had not accrued interest or penalties related to uncertain tax positions.

Comprehensive Loss

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

Net Loss Per Share

Basic net loss per share is calculated by dividing the net loss be 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 Issued Accounting Pronouncements

In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update No. 2016-02, Leases (Topic 842) (“ASU No. 2016-02”). ASU No. 2016-02 amends a number of aspects of lease accounting, including requiring lessees to recognize almost all leases with a term greater than one year as a right-of-use asset and corresponding liability, measured at the present value of the lease payments. Subsequent measurement, including the presentation of expenses and cash flows, will depend on the classification of the lease as either a finance or an operating lease. Initial costs directly attributable to negotiating and arranging the lease will be included in the asset. For public entities, this standard is effective for annual reporting periods beginning after December 31, 2018, including interim periods within that reporting period. Originally, entities were required to adopt ASU No. 2016-02 using a modified retrospective approach, which required prior periods to be presented under this new standard with various practical expedients allowed. In July 2018, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2018-11, Leases (Topic 842): Targeted Improvements, which now allow entities the option of recognizing the cumulative effect of applying the new standard as an adjustment to the opening balance of retained earnings in the year of adoption (January 1, 2019) while continuing to present all prior periods under previous lease accounting guidance. The Company will adopt this standard on January 1, 2019 using the modified retrospective approach with a cumulative effect adjustment to accumulated deficit at the beginning of the period of adoption. The Company will also adopt certain practical expedients provided by ASU 2018-11. The adoption of Topic 842 will have a material impact on the Company’s financial statements and related notes due to the recognition of right of use (“ROU”) assets and lease liabilities on the Company’s balance sheets, but it will not have a material impact on the Company’s operations. Adoption of the standard will result in the recognition of additional ROU assets and lease liabilities of $14.2 million and $11.2 million, respectively, and the derecognition of the deferred rent balance of $1.8 million as of January 1, 2019. The Company will also record an insignificant reduction to opening accumulated deficit as of January 1, 2019 and will derecognize the $14.5 million building and related accumulated depreciation and $10.5 million financing lease obligation, due to the cumulative impact of adopting this standard, with the impact relating to a change in the classification of our Pleasanton Lease from a financing lease transaction to an operating lease. For a description of the Pleasanton Lease, see Note 6.

In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting (“ASU No. 2018-07”). ASU No. 2018-07 is intended to reduce cost and complexity and to improve financial reporting for share-based payments issued to non-employees (for example, service providers, external legal counsel, suppliers, etc.). The ASU expands the scope of Topic 718, Compensation—Stock Compensation, which currently only includes share-based payments issued to employees, to also include share-based payments issued to non-employees for goods and services. Consequently, the accounting for share-based payments to non-employees and employees will be substantially aligned. ASU No. 2018-07 is effective for annual and interim periods beginning after December 15, 2018. Early adoption of the standard is permitted. The standard will be applied in a retrospective approach for each period presented. The Company is currently evaluating the timing and impact of adopting this accounting standard update on its financial statements and related disclosures.

In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement. ASU No. 2018-13 eliminates, adds and modifies certain disclosure requirements for fair value measurements and requires companies to disclose certain information. The Company is currently evaluating the impact of adopting this accounting update on its financial statements and related disclosures.

In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606, which clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under Topic 606 when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in Topic 606 should be applied, including recognition, measurement, presentation, and disclosure requirements. The standard adds unit-of-account guidance in Topic 808 to align with the guidance in Topic 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of Topic 606, and requires that in a transaction with a collaborative arrangement participant that is not directly related to sales to third parties, presenting the transaction together with revenue recognized under Topic 606 is precluded if the collaborative arrangement participant is not a customer. The standard is effective for interim and annual periods beginning after December 15, 2019, with early adoption permitted, including adoption in any interim period for public business entities for periods in which financial statements have not been issued. Amendments in the standard should be applied retrospectively to the date of initial application of Topic 606, but entities may elect to apply the amendments in this Update retrospectively either to all contracts or only to contracts that are not completed at the date of initial application of Topic 606, and should disclose the election. An entity may also elect to apply the practical expedient for contract modifications that is permitted for entities using the modified retrospective transition method in Topic 606. The Company is currently assessing the impact of this standard on its financial statements.

In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses: Measurement of Credit Losses on Financial Instruments (“ASU No. 2016-13”), which requires that expected credit losses relating to financial assets measured on an amortized cost basis and available-for-sale debt securities be recorded through an allowance for credit losses. ASU No. 2016-13 limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which carrying value exceeds fair value and also requires the reversal of previously recognized credit losses if fair value increases. The new standard will be effective on January 1, 2020. Early adoption will be available on January 1, 2019. The Company is currently evaluating the effect that the updated standard will have on its financial statements and related disclosures.

In August 2018, the FASB issued ASU No. 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 the Company 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 is currently evaluating the impact of adoption on its financial statements.

Recently Adopted Accounting Pronouncements

In January 2017, the FASB issued ASU No. 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business (“ASU No. 2017-01”). This ASU clarifies the definition of a business when evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The Company adopted ASU No. 2017-01 effective January 1, 2018. The adoption of this accounting standards update did not have a material impact on the Company’s financial statements.

In May 2017, the FASB issued ASU No. 2017-09, Compensation – Stock Compensation (Topic 718): Scope of Modification Accounting , which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in the terms or conditions. This standard was adopted as of January 1, 2018 and was applied prospectively to any award modified after the adoption date, and the adoption of this standard did not have a material impact on the Company’s financial statements.

In August 2018, the SEC adopted amendments to certain disclosure requirements in Securities Act Release No. 33-10532, Disclosure Update and Simplification . These amendments eliminate, modify, or integrate into other SEC requirements certain disclosure rules. Among the amendments is the requirement to present an analysis of changes in stockholders’ equity in the interim financial statements included in quarterly reports on Form 10-Q. The analysis, which can be presented as a footnote or separate statement, is required for the current and comparative quarter and year-to-date interim periods. The amendments are effective for all filings made on or after November 5, 2018. In light of the anticipated timing of effectiveness of the amendments and expected proximity of effectiveness to the filing date for most filers’ quarterly reports, the SEC’s Division of Corporate Finance issued a Compliance and Disclosure Interpretation related to Exchange Act Forms, or CDI – Question 105.09, that provides transition guidance related to this disclosure requirement. CDI – Question 105.09 states that the SEC would not object if the filer’s first presentation of the changes in shareholders’ equity is included in its Form 10-Q for the quarter that begins after the effective date of the amendments. As such, the Company adopted these SEC amendments on November 5, 2018 and will present the analysis of changes in stockholders’ equity in its interim financial statements in its March 31, 2019 Form 10-Q. The Company does not anticipate that the adoption of these SEC amendments will have a material effect on the Company’s financial position, results of operations, cash flows or stockholders’ equity.

Cash Equivalents and Marketable Securities

12 Months Ended

Dec. 31, 2018

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, 2018
Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Money market funds	$	36,148	$	—	$	—		$	36,148
Commercial paper		45,244		—		(40	)		45,204
Corporate debt securities		67,815		1		(46	)		67,770
	$	149,207	$	1	$	(86	)	$	149,122
Classified as:
Cash equivalents								$	48,195
Marketable securities									100,927
Total								$	149,122

	December 31, 2017
Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Money market funds	$	27,711	$	—	$	—		$	27,711
Commercial paper		32,257		—		(48	)		32,209
Corporate debt securities		19,930		—		(26	)		19,904
	$	79,898	$	—	$	(74	)	$	79,824
Classified as:
Cash equivalents								$	32,878
Marketable securities									46,946
Total								$	79,824

As of December 31, 2018 and 2017, the Company had a total of $153.1 million and $85.9 million in cash, cash equivalents and marketable securities, which includes $52.2 million and $39.0 million in cash and cash equivalents and $100.9 million and $46.9 million in marketable securities, respectively.

All marketable securities held as of December 31, 2018, had contractual maturities of less than one year. There have been no realized gains or losses on marketable securities for the periods presented. None of the Company’s investments in marketable securities has been in an unrealized loss position for more than one year. The Company determined that it did have 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, 2018, 2017, or 2016.

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

Fair Value Measurements

12 Months Ended

Dec. 31, 2018

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, 2018
Description	Total		Level 1		Level 2		Level 3
Money market funds	$	36,148	$	36,148	$	—	$	—
Commercial paper		45,204		—		45,204		—
Corporate debt securities		67,770		—		67,770		—
Total	$	149,122	$	36,148	$	112,974	$	—

	December 31, 2017
Description	Total		Level 1		Level 2		Level 3
Money market funds	$	27,711	$	27,711	$	—	$	—
Commercial paper		32,209		—		32,209		—
Corporate debt securities		19,904		—		19,904		—
Total	$	79,824	$	27,711	$	52,113	$	—

The Company measures the fair value of money market funds based on quoted prices in active markets for identical securities. Commercial paper 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, 2018

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,
	2018			2017
Computer equipment and software	$	470		$	353
Furniture and fixtures		935			785
Laboratory equipment		16,406			10,515
Leasehold improvements		3,063			2,977
Buildings and related improvements capitalized under a lease financing transaction		15,371			15,371
		36,245			30,001
Less accumulated depreciation and amortization		(6,751	)		(2,790	)
Total property and equipment, net	$	29,494		$	27,211

Depreciation and amortization expense was $4.0 million, $2.0 million, and $0.8 million for the periods ended December 31, 2018, 2017, and 2016, respectively.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

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. 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 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. 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 renovations. The scope of these tenant improvements were considered to be “normal tenant improvements” under the lease accounting guidance. The Company recorded the tenant allowance received as leasehold improvements under the property and equipment account and deferred rent liability on the accompanying balance sheets.

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, 2018, 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 scope of the tenant improvements did not qualify under the lease accounting guidance as “normal tenant improvements” and the Company was deemed owner of the leased building during the construction period for accounting purposes. The Company has capitalized the $9.3 million fair value of the leased building within property and equipment, net, and recognized a corresponding non-current lease financing obligation. The fair value of the leased building was estimated using a market approach that utilized comparable observable sales for similar assets (Level 2 inputs). The Company has also recognized building improvements totaling $6.1 million for additions to the leased building incurred by the Company during the construction period, of which $1.2 million were due but had not yet been received from the landlord as of December 31, 2017 and were recorded as an increase to the lease financing obligation and prepaid and other current assets on the balance sheet. Such amounts were subsequently reimbursed by the landlord in April 2018. In November 2017, construction on the Pleasanton Facility was substantially completed and the leased property was placed into service. The Company determined the completed construction project did not qualify for sale-leaseback accounting due to the collateral held by the landlord in the form of a letter of credit and instead has been accounted for as a financing lease transaction. The leased building for the Pleasanton Facility and related improvements remain on the Company’s balance sheet as of December 31, 2018 and rental payments associated with the Pleasanton Lease have been allocated to operating lease expense for the ground underlying the leased building and principal and interest payments on the lease financing obligation.

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 is included in prepaid expenses and other current assets on the balance sheet as of December 31, 2018 and a cash security deposit of $0.3 million recorded in deposits and other long-term assets. The lease agreement includes an escalation clause for increased rent.

Rent expense recorded by the Company associated with the ground lease was not material for any period presented. No interest expense was recognized for the lease financing obligation while the leased building was being constructed during the year ended December 31, 2017. Total interest, which represents the cost of the lease financing obligation under the Pleasanton Lease agreement, was approximately $0.8 million for the year ended December 31, 2018, which was recognized within the statement of operations and comprehensive loss. The allocation of the Pleasanton Lease payment to ground lease rent expense and principal and interest expense on the lease financing obligation was estimated using income and market approaches that utilized comparable observable sales for similar assets, land capitalization rates and an estimate of the Company’s incremental borrowing rate (Level 2 and Level 3 inputs).

As of December 31, 2018, minimum annual payments under the Company’s non-cancelable lease agreements and lease financing obligation are as follows (in thousands):

	Lease Financing Obligation			Operating Lease
Year ending December 31:
2019	$	794		$	2,329
2020		818			2,674
2021		843			1,803
2022		868			1,119
2023		894			281
Thereafter		842			114
Total minimum payments		5,059		$	8,320
Less: Amount representing interest expense		(4,435	)
		624
Residual value of lease financing obligation		9,896
		10,520
Less: Lease financing obligation, short-term		(30	)
Lease financing obligation, long-term	$	10,490

Rent expense was $1.8 million, $1.2 million, and $2.4 million for the years ended December 31, 2018, 2017, and 2016, respectively.

Agreement with CRO

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 personalized immunotherapies in the treatment of cancer. Under the agreement, the Company paid an upfront payment of $0.5 million to the CRO. The upfront payment has been capitalized and will be recognized as research and development expense using the straight-line method over the term of the agreement, which is one year. The Company is also obligated to pay up to $0.9 million to the CRO upon the completion of certain phases of the research services. These costs will be recorded to research and development expense over the expected period of each phase of the research services. The Company is also 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, 2018. During the year ended December 31, 2018, the Company recognized a total of $1.1 million of research and development expense under the agreement. During 2017, the Company recognized a total of $0.1 million of research and development expense under the agreement.

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, 2018

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 (“Collaboration Agreement”) with bluebird bio, Inc. (“bluebird”). Under the terms of the 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 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 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 Collaboration Agreement. None of these events had occurred as of December 31, 2018 and no royalties were due from the sale of licensed products. bluebird may terminate the 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 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 fifty percent 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 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. As such, 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 which 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 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, 2018. 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 year ended December 31, 2018, the Company recognized $1.2 million 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 $18.8 million is recorded on the balance sheet in both current and long-term liabilities as of December 31, 2018, 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, 2018 are as follows (in thousands):

	Deferred Revenue
Balance at January 1, 2018	$	—
Additions		20,000
Deductions		(1,187	)
Balance at December 31, 2018	$	18,813

There were no receivables or net contract assets recorded as of December 31, 2018 associated with the Collaboration Agreement.

The Company expensed all incremental costs of obtaining Collaboration Agreement as such amounts were insignificant.

Arbutus Biopharma Corporation

In October 2017, the Company entered into an Exclusive License Agreement with Arbutus Biopharma Corporation (Arbutus). Certain terms of the agreement were modified by amendment in July 2018. Under the license agreement, the Company has a worldwide, exclusive license to certain technology of Arbutus, including Arbutus’ portfolio of proprietary and clinically validated lipid nanoparticle products and associated intellectual property, as well as technology transfer of Arbutus’ manufacturing know-how. Under this license agreement, the Company paid an upfront payment of $5.0 million which was included in research and development expenses during 2017. The Company also reimbursed Arbutus for materials and personnel costs totaling $0.2 million, which were included in research and development expenses during 2017. During 2018, 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 low single-digit royalties on sales of its licensed products. Following the acceptance of our investigational new drug application for GRANITE-001 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. None of the other events had occurred as of December 31 2018, 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. None of these events had occurred as of December 31, 2018 and no royalties were due from the sales of licensed products. The Company also issued a ten-year warrant to the cancer center for the right to purchase 40,257 shares of its common stock at $0.35 per share. The estimated fair value of the warrant was not significant and was included in research and development expense and additional paid-in capital. The warrant was exercised in full in January 2018. In December 2018 the Company made an insignificant milestone payment to the non-profit hospital cancer center.

Balance Sheet Components

12 Months Ended

Dec. 31, 2018

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,
	2018		2017
Receivable from landlord	$	—	$	1,226
Prepaid rent		860		93
Prepaid insurance		966		—
Interest and other receivables		462		484
Prepaid research and development-related expenses		1,789		628
Other		449		95
Total prepaid expenses and other current assets	$	4,526	$	2,526

Deposits and Other Long-Term Assets

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

	December 31,
	2018		2017
Lease security deposit	$	632	$	368
Prepaid research and development-related expenses		554		—
Restricted cash		992		992
Other		250		250
Total deposits and other long-term assets	$	2,428	$	1,610

Accrued Liabilities

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

	December 31,
	2018		2017
Deferred rent	$	445	$	381
Research and development-related expenses		252		683
Other		295		426
Total accrued current liabilities	$	992	$	1,490

Convertible Preferred Stock and Common Stock

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Convertible Preferred Stock and Common Stock

9.	Convertible Preferred Stock and Common Stock

Convertible Preferred Stock

Series A Equity Financing

The Company entered into a Series A preferred stock purchase agreement with certain investors in September 2015, and upon approval by the Company’s Board of Directors, the Company completed a Series A convertible preferred stock financing (“Series A—First Tranche”) at a price per share of $6.90. The net cash proceeds from this round of financing totaled $25.4 million, and 3,699,259 shares of Series A convertible preferred stock were issued. Issuance costs totaled $0.1 million and were recorded as a reduction of the proceeds.

In April 2016, and upon approval by the Company’s Board of Directors, the Company completed a Series A convertible preferred stock financing (“Series A—Second Tranche”) at a price per share of $6.90. The net cash proceeds from this round of financing totaled $35.7 million, and 5,178,968 shares of Series A convertible preferred stock were issued. Issuance costs totaled $0.02 million and were recorded as a reduction of the proceeds.

Upon approval by the Company’s Board of Directors and a majority of the holders of the Series A convertible preferred stock, the Company could proceed with the third closing of the Series A convertible preferred stock for a total of 5,918,840 shares at a purchase price of $6.90 per share (“Series A—Third Tranche”). However, for a period of 90 days following such approval, the Company may solicit alternative financing at financially superior terms to those of the Series A—Third Tranche, including a purchase price greater than $6.90 per share (the “Superior Financing Transaction”). If approved by the Board of Directors, the Company’s obligation to complete the Series A—Third Tranche shall terminate and the Superior Financing Transaction would proceed. Each Series A convertible preferred stockholder will have the right to purchase at least 50% of its original Series A—Third Tranche amount in the Superior Financing Transaction. The Series A—Second Tranche and Series A—Third Tranche rights are considered to be mutual options as neither the purchasers nor the Company have a commitment or obligation to purchase or sell additional shares. As such, these rights are not accounted for separately. In connection with the Company’s Series B Equity Financing the Company’s Board of Directors and investors terminated the ability to complete the Series A—Third Tranche.

Series B Equity Financing

The Company entered into a Series B preferred stock purchase agreement with certain investors in September 2017 and October 20, 2017, and upon approval by the Company’s Board of Directors, the Company completed a Series B convertible preferred stock financing (Series B) at a price per share of $10.76. The net cash proceeds totaled $95.8 million and 8,919,302 shares of Series B convertible preferred stock were issued. Issuance costs totaled $0.2 million and were recorded as a reduction of the proceeds.

Series C Equity Financing

The Company entered into a Series C preferred stock purchase agreement (“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 (“Series C”) 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 Collaboration Agreement entered into with bluebird, 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.

The preferred stock has various features, including convertibility and non-cumulative dividends. The Company determined that none of the features required bifurcation from the underlying shares, either because they are clearly and closely related to the underlying shares or because they do not meet the definition of a derivative. The Series A and Series B convertible preferred stock are considered permanent equity and have not been accreted up to their redemption value. The Second and Third Tranche rights are considered to be mutual options as neither the purchasers nor the Company have a commitment or obligation to purchase or sell additional shares. As such, these rights are not accounted for separately. Moreover, in any such redemption (i.e. deemed liquidation) all equity holders (common and preferred) will receive the same form of consideration. The preferred stockholders cannot contractually redeem their shares, or redeem their shares through separate negotiation, without the Company’s common stockholders being able to also redeem their shares for the same form of consideration.

At December 31, 2017, convertible preferred stock consisted of the following (in thousands, except share and per share amounts):

	Shares Authorized		Shares Issued and Outstanding		Issuance Price Per Share		Carrying Value		Liquidation Preference
Series B		64,102,551		8,919,302	$	10.76	$	95,798	$	96,008
Series A—First Tranche		61,260,000		3,699,259	$	6.90		25,425		25,525
Series A—Second Tranche		—		5,178,968	$	6.90		35,714		35,735
Total convertible preferred stock		125,362,551		17,797,529			$	156,937	$	157,268

At December 31, 2018, there were 10,000,000 shares of convertible preferred stock authorized and no shares were outstanding.

Prior to the conversion of the convertible preferred stock upon closing of the IPO, the rights, preferences, and privileges of the convertible preferred stock were as follows:

Redemption Rights

The preferred stock is not redeemable by holders unless a redemption event occurs. A redemption event will only occur upon the liquidation or winding up of the Company, a greater than 50% change in control, or the sale of substantially all of the assets of the Company. Management has also elected not to adjust the carrying values of the Series A, Series B and Series C convertible preferred stock to the redemption value of such shares, since it is uncertain whether or when a redemption event will occur. Subsequent adjustments to increase the carrying value to the redemption values will be made when it becomes probable that such a redemption will occur.

Dividends Rights

The holders of Series A, Series B, and Series C convertible preferred stock are entitled to receive dividends, from any assets legally available, prior and in preference to any declaration or payment of any dividend to the common stockholders, at the rate of 8% of the original issue price (as determined on a per annum basis and on an as-converted basis). Such dividends are payable if and when declared by the Board of Directors and are not cumulative. After payment of such dividends, any additional dividends shall be distributed among the holders of the Series A, Series B, and Series C convertible preferred stock and common stock pro rata based on the number of shares of common stock then held by each holder (assuming conversion of all such preferred stock into common stock). As of December 31, 2018 and 2017, no such dividends had been declared or accrued.

Liquidation Rights

In the event of any liquidation, dissolution, or winding up of the Company, whether voluntary or involuntary (Liquidation Event), the holders of Series C convertible preferred stock are entitled to receive, prior and in preference to any distribution of any of the assets of the Company to the holders of the Series A and B convertible preferred stock, $13.04 per share (as adjusted for any stock splits, combinations, reorganizations, or similar transactions, plus any declared and unpaid dividends). The holders of Series B convertible preferred stock are entitled to receive, prior and in preference to any distribution of any of the assets of the Company to the holders of the Series A convertible preferred stock, $10.76 per share (as adjusted for any stock splits, combinations, reorganizations, or similar transactions, plus any declared and unpaid dividends). After payment of the above, the holders of Series A convertible preferred stock are entitled to receive, prior and in preference to any distribution of any of the assets of the Company to the holders of common stock, $6.90 per share (as adjusted for any stock splits, combinations, reorganizations, or similar transactions, plus any declared and unpaid dividends).

If, upon the occurrence of such an event, the proceeds to be distributed are insufficient to permit the payment to such holders of the full preferential amounts, then the entire amount legally available for distribution shall be distributed among the holders of the Series A, Series B, and Series C preferred stock in proportion to the full preferential amount that each such holder is otherwise entitled to receive had such proceeds been available.

After liquidation preference payments have been made to the holders of the convertible preferred stock as described above, all of the remaining assets and funds of the Company are to be distributed ratably among the holders of the preferred and common stock, as if the preferred stock had been converted to common stock. However, Series C holders are limited to the greater of (1) $65.21 per share (as adjusted for any stock splits, combinations, reorganizations or similar transactions) and (2) the amount the holder would have received if all shares of Series C convertible preferred stock had been converted to common stock prior to such liquidation, dissolution, or winding up of the Company. Series B preferred stockholders are limited to the greater of (1) $53.82 per share (as adjusted for any stock splits, combinations, reorganizations or similar transactions) and (2) the amount the holder would have received if all shares of Series B convertible preferred stock had been converted to common stock prior to such liquidation, dissolution, or winding up of the Company. Series A holders are limited to the greater of (1) $34.50 per share (as adjusted for any stock splits, combinations, reorganizations or similar transactions) and (2) the amount the holder would have received if all shares of Series A convertible preferred stock had been converted to common stock prior to such liquidation, dissolution, or winding up of the Company.

Voting Rights

Except as otherwise required by law, the holders of common and Series A, Series B, and Series C convertible preferred stock vote together as a single class. The holders of the convertible preferred stock are entitled to the number of votes equal to the number of shares of common stock into which the convertible preferred stock could be converted on the record date for the vote, or upon the written consent of the stockholders.

The holders of the Series A convertible preferred stock are entitled to elect three directors of the Company, the holders of the Series B convertible preferred stock are entitled to elect one director of the Company, and the holders of common stock shall be entitled to elect one director of the Company.

Conversion Rights

Each share of Series A, Series B and Series C convertible preferred stock, at the option of the holder and at any time after the date of issuance, is convertible into the number of shares of common stock determined by dividing the respective original issue price by the conversion price (the Conversion Price). The Company’s Series A, Series B, and Series C Conversion Prices were $6.90, $10.76, and $13.04, respectively, and were subject to certain future adjustments.

As part of the Company’s Series C convertible preferred stock financing, the Company’s certificate of incorporation was amended to reduce the public offering automatic conversion price for the Series A and B convertible preferred stock from $21.53 to $15.66 per share. The Company accounted for this as a modification of an instrument akin to equity that resulted in no incremental fair value being attributed to the Series A and Series B convertible preferred stock.

In October 2018, upon closing of the IPO, all outstanding shares of convertible preferred stock converted into 19,409,132 shares of common stock. Per the terms of the Convertible Preferred Stock Agreement, the shares of outstanding convertible preferred stock converted at the election of the holders of at least a majority of the then outstanding shares of convertible preferred stock, voting together as a single class and on an as-converted to common stock basis.

Common Stock

The Company is authorized to issue 300,000,000 shares of common stock. Holders of common stock are generally entitled to one vote per share on all matters to be voted upon by the stockholders of the Company.

Subject to the preferences that may be applicable to any outstanding shares of preferred stock, the holders of common stock are entitled to receive ratably such dividends, if any, as may be declared by the Board of Directors. No dividends have been declared to date.

Stock-Based Compensation

12 Months Ended

Dec. 31, 2018

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”). The Company permitted early exercise of certain stock options prior to vesting to certain directors, officers, and employees. Any shares issued pursuant to unvested options are restricted and subject to repurchase by the Company until the conditions for vesting are met. The amounts paid for shares purchased under an early exercise of stock options and subject to repurchase by the Company are reported as a liability, then in stockholders’ equity once those shares vest. Upon termination of employment of an option holder, the Company has the right to repurchase, at the original purchase price, any unvested options. The shares issued pursuant to unvested options have not been included in shares issued and outstanding on the balance sheet and statement of stockholders’ equity as such shares are not considered outstanding for accounting purposes. As of December 31, 2018 and 2017, the Company recorded $0.08 million and $0.2 million, respectively, as short- term and long-term liabilities associated with shares issued subject to repurchase rights.

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. The grant date fair value of the Company’s common stock was determined using valuation methodologies which utilizes certain assumptions including probability weighting of events, volatility, time to liquidation, a risk-free interest rate and an assumption for a discount for lack of marketability (Level 3 inputs). In determining the fair value of the Company’s common stock, the methodologies used to estimate the enterprise value of the Company were performed using methodologies, approaches, and assumptions consistent with the American Institute of Certified Public Accountants Accounting and Valuation Guide, Valuation of Privately-Held-Company Equity Securities Issued as Compensation (“AICPA Accounting and Valuation Guide”). For stock awards granted during 2016 and 2017, deemed fair values of $2.21 and $3.17 per common share were used in calculating stock-based compensation expense, as determined by management using hindsight.

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.

In February 2018, the Company’s Board of Directors approved a 507,246 share increase in the number of shares to be reserved under the Company’s 2015 Equity Incentive 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. The 2015 Plan will continue to govern all outstanding awards by their existing terms.

In September 2018, the Company’s Board of Directors approved the 2018 Plan. Under the 2018 Plan, the Company may grant stock options, stock appreciation rights, restricted stock, restricted stock units and other certain awards to individuals who are employees, officers, directors or consultants of the Company. A total of 2,690,000 shares of our common stock are initially reserved for issuance pursuant to a variety of stock-based compensation awards, including stock options, stock appreciation rights, or SARs, restricted stock awards, restricted stock unit awards and other stock-based awards, 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 ISOs under the 2018 Plan is 45,000,000.

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”). 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. Our initial purchase period commences in 2019.

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,
	2018			2017			2016
Expected dividend yield		—			—			—
Expected term	6.04 years			6.04 years			6.05 years
Risk-free interest rate		2.80	%		1.97	%		1.43	%
Expected volatility		88	%		94	%		103	%

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 $6.59, $2.00, and $2.19 per share during the years ended December 31, 2018, 2017, and 2016, 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, 2016		1,148,227			509,189		$	0.41		9.63	$	178
Authorized		362,318			—		$	—
Granted		(900,257	)		900,257		$	1.13
Exercised					(41,665	)	$	0.35
Cancelled		15,941			(15,941	)	$	0.46
Balance at December 31, 2017		626,229			1,351,840		$	0.89		9.13	$	3,087
Authorized		3,197,246
Granted		(1,344,973	)		1,344,973		$	8.92
Exercised					(80,463	)	$	0.58
Cancelled		186,491			(186,491	)	$	1.31
Repurchased		30,117					$	0.35
Balance at December 31, 2018		2,695,110			2,429,859		$	5.31		8.86	$	25,646
Vested and exercisable – December 31, 2018					582,436		$	1.24		8.12	$	8,274
Vested and expected to vest – December 31, 2018					2,521,869		$	4.75		8.69	$	27,896

For the years ended December 31, 2018, 2017, and 2016, the total intrinsic value of stock option awards exercised was $0.75 million, $0.08 million, and $1.3 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, 2018, $8.9 million of total unrecognized compensation cost related to non-vested employee and consultant options is expected to be recognized over a weighted-average period of 3.18 years. The total fair value of shares vested during the year ended December 31, 2018 was $1.8 million.

Stock-based compensation expense and awards granted to non-employees was insignificant for the years ended December 31, 2018, December 31, 2017, and December 31, 2016.

Stock-Based Compensation Expense

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

	Year Ended December 31,
	2018		2017		2016
Research and development expenses	$	2,081	$	888	$	385
General and administrative expenses		914		238		212
Total	$	2,995	$	1,126	$	597

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

11.	Income Taxes

The reconciliation of the statutory federal income tax rate to the Company’s effective tax rate is as follows:

	Year Ended December 31,
	2018			2017			2016
Statutory federal income tax rate		21.0	%		34.0	%		34.0	%
State tax, net of federal benefit		4.3			2.2			3.3
Permanent differences		(1.0	)		(2.3	)		(0.8	)
Effective change in enacted tax rate		—			(17.3	)		—
Research and development tax credits		4.4			4.1			0.8
Other		(0.2	)		—			—
Change in valuation allowance		(28.5	)		(20.7	)		(37.3	)
Effective income tax rate		—	%		—	%		—	%

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, 2018 and 2017. The valuation allowance increased by $18.4 million during 2018 and increased by $8.5 million during 2017.

The U.S. Tax Cuts and Jobs Act (“Tax Act”) was enacted on December 22, 2017 and introduces significant changes to U.S. income tax law, including the remeasurement of deferred taxes. Effective in 2018, the Tax Act reduces the U.S. federal statutory corporate tax rate from 35% to 21% for years after 2017. The Company remeasured its deferred taxes as of December 31, 2017 to reflect the reduced U.S. tax rate of the Tax Act and recognized a reduction to the deferred tax assets of $7.1 million, which was off-set by a corresponding reduction in the valuation allowance. The Act did not have a material impact on the Company's financial statements as of December 31, 2018.

In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”) to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Tax Act. The Company was able to provide a reasonable estimate for the revaluation of deferred taxes at December 31, 2017. The Company completed its review in December 2018 with no material adjustments to the provisional amount previously recorded.

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

	December 31,
	2018			2017
Deferred tax assets:
Net operating loss carryforwards	$	27,311		$	12,251
Research and development tax credits		4,503			1,990
Lease financing obligation		2,658			2,490
Accruals and other		1,387			1,124
Amortization		1,726			1,222
Deferred tax liabilities:
Other depreciation		(655	)		(536	)
Leased building depreciation		(2,446	)		(2,475	)
Total net deferred tax assets		34,484			16,066
Less valuation allowance		(34,484	)		(16,066	)
Deferred tax assets, net of allowance	$	—		$	—

At December 31, 2018, the Company’s federal and state income tax net operating loss carryforwards were approximately $109.8 million and $65.7 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 incurred in 2018 and in future years may be carried forward indefinitely, but the deductibility of such federal net operating losses is limited. In addition, the Company has federal and certain California and Massachusetts research and development income tax credit carryforwards of $4.1 million, $2.3 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 2032.

The net operating loss (“NOL”) 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 Sections 382 and 383 of the Internal Revenue Code (“IRC”) of 1986. 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, 2018 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.

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

	December 31,
	2018			2017			2016
Beginning of year—unrecognized tax benefits	$	1,089		$	230		$	7
Decrease for tax positions taken during prior periods		(453	)		(47	)		—
Increases for tax positions taken during current period		1,441			906			223
End of year—unrecognized tax benefits	$	2,077		$	1,089		$	230

The Company does not expect any material changes to the estimated amount of liability associated with its uncertain tax positions within the next 12 months.

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.

Net Loss Per Common Share

12 Months Ended

Dec. 31, 2018

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,
	2018			2017			2016
Numerator:
Net loss	$	(64,775	)	$	(41,377	)	$	(18,750	)
Denominator:
Weighted-average common shares outstanding, basic and diluted		8,919,281			1,999,044			1,672,545
Net loss per share, basic and diluted	$	(7.26	)	$	(20.70	)	$	(11.21	)

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,
	2018		2017		2016
Convertible preferred stock		—		17,797,529		8,878,227
Options issued and outstanding		2,429,859		1,351,840		509,189
Early exercised common stock subject to future vesting		226,967		539,289		838,359
Warrants to purchase common stock		—		40,257		40,257
Total		2,656,826		19,728,915		10,266,032

Related-Party Transactions

12 Months Ended

Dec. 31, 2018

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. During the year ended December 31, 2017, the Company issued 2,560,342 additional shares of Series B convertible preferred stock for total proceeds of $27.6 million to these related parties. During the year ended December 31, 2016, the Company issued 4,236,261 shares of Series A convertible preferred stock for total proceeds of $29.2 million to certain stockholders, a member of the Company’s Board of Directors and immediate family members of certain executive officers of the Company, that are in each case considered to be related parties.

Defined Contribution Plan

12 Months Ended

Dec. 31, 2018

Postemployment Benefits [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 year ended December 31, 2018 and 2017, expenses recognized for the 401(k) Plan were insignificant.

Subsequent Event

12 Months Ended

Dec. 31, 2018

Subsequent Events [Abstract]

Subsequent Event

15.	Subsequent Event

On January 31, 2019, the Company entered into a new operating lease effective as of January 28, 2019, for approximately 34,469 square feet of office and laboratory space located in Emeryville, California for the Company’s new principal executive offices. The new lease provides for annual base rent of approximately $1.7 million, which increases on a yearly basis up to approximately $2.6 million for the final 12 months of the initial Lease Term. In connection with the new lease agreement, the Company also entered into a Lease Termination Agreement to early terminate the Company’s existing lease, dated November 23, 2015, for its current premises. The current lease will terminate effective no later than 60 days after the rent commencement date under the new lease.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2018

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 2018 and 2017 and has been prepared in accordance with GAAP for interim financial reporting (in thousands, except per share amounts):

	Quarter Ended
	December 31, 2018			September 30, 2018			June 30, 2018			March 31, 2018
Collaboration revenue	$	1,091		$	96		$	—		$	—
Loss from operations	$	(18,027	)	$	(18,614	)	$	(15,504	)	$	(13,439	)
Net loss	$	(17,338	)	$	(18,588	)	$	(15,473	)	$	(13,376	)
Net loss per share, basic and diluted	$	0.61		$	(7.60	)	$	(6.57	)	$	(6.03	)

	Quarter Ended
	December 31, 2017			September 30, 2017			June 30, 2017			March 31, 2017
Collaboration revenue	$	—		$	—		$	—		$	—
Loss from operations	$	(17,580	)	$	(9,488	)	$	(7,851	)	$	(6,844	)
Net loss	$	(17,368	)	$	(9,451	)	$	(7,790	)	$	(6,768	)
Net loss per share, basic and diluted	$	(8.18	)	$	(4.59	)	$	(3.95	)	$	(3.68	)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation

Reverse Stock Split

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,
	2018		2017
Cash and cash equivalents	$	52,183	$	39,007
Restricted cash		992		992
Total cash, cash equivalents and restricted cash	$	53,175	$	39,999

Marketable Securities

Concentrations of Credit Risk and 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

Impairment of Long-Lived Assets

Revenue Recognition

Share-Based Compensation

Stock-Based Compensation

Research and Development Expenses

Leases and Deferred Rent and Lease Financing Obligation

Income Taxes

Comprehensive Loss

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

Net Loss per Share

Net Loss Per Share

Recently Issued Accounting Pronouncements

Recently Adopted Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

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 statements of cash flows (in thousands).

	December 31,
	2018		2017
Cash and cash equivalents	$	52,183	$	39,007
Restricted cash		992		992
Total cash, cash equivalents and restricted cash	$	53,175	$	39,999

Schedule of Straight Line Method Over Estimated Useful Lives of Respective 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

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

	December 31,
	2018			2017
Computer equipment and software	$	470		$	353
Furniture and fixtures		935			785
Laboratory equipment		16,406			10,515
Leasehold improvements		3,063			2,977
Buildings and related improvements capitalized under a lease financing transaction		15,371			15,371
		36,245			30,001
Less accumulated depreciation and amortization		(6,751	)		(2,790	)
Total property and equipment, net	$	29,494		$	27,211

Cash Equivalents and Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2018

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, 2018
Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Money market funds	$	36,148	$	—	$	—		$	36,148
Commercial paper		45,244		—		(40	)		45,204
Corporate debt securities		67,815		1		(46	)		67,770
	$	149,207	$	1	$	(86	)	$	149,122
Classified as:
Cash equivalents								$	48,195
Marketable securities									100,927
Total								$	149,122

	December 31, 2017
Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Money market funds	$	27,711	$	—	$	—		$	27,711
Commercial paper		32,257		—		(48	)		32,209
Corporate debt securities		19,930		—		(26	)		19,904
	$	79,898	$	—	$	(74	)	$	79,824
Classified as:
Cash equivalents								$	32,878
Marketable securities									46,946
Total								$	79,824

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

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, 2018
Description	Total		Level 1		Level 2		Level 3
Money market funds	$	36,148	$	36,148	$	—	$	—
Commercial paper		45,204		—		45,204		—
Corporate debt securities		67,770		—		67,770		—
Total	$	149,122	$	36,148	$	112,974	$	—

	December 31, 2017
Description	Total		Level 1		Level 2		Level 3
Money market funds	$	27,711	$	27,711	$	—	$	—
Commercial paper		32,209		—		32,209		—
Corporate debt securities		19,904		—		19,904		—
Total	$	79,824	$	27,711	$	52,113	$	—

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Schedule of Straight Line Method Over Estimated Useful Lives of Respective 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

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

	December 31,
	2018			2017
Computer equipment and software	$	470		$	353
Furniture and fixtures		935			785
Laboratory equipment		16,406			10,515
Leasehold improvements		3,063			2,977
Buildings and related improvements capitalized under a lease financing transaction		15,371			15,371
		36,245			30,001
Less accumulated depreciation and amortization		(6,751	)		(2,790	)
Total property and equipment, net	$	29,494		$	27,211

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Minimum Annual Payments under Non-Cancelable Lease Agreements and Lease Financing Obligation

As of December 31, 2018, minimum annual payments under the Company’s non-cancelable lease agreements and lease financing obligation are as follows (in thousands):

	Lease Financing Obligation			Operating Lease
Year ending December 31:
2019	$	794		$	2,329
2020		818			2,674
2021		843			1,803
2022		868			1,119
2023		894			281
Thereafter		842			114
Total minimum payments		5,059		$	8,320
Less: Amount representing interest expense		(4,435	)
		624
Residual value of lease financing obligation		9,896
		10,520
Less: Lease financing obligation, short-term		(30	)
Lease financing obligation, long-term	$	10,490

Collaboration and License Agreements (Tables)

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Schedule of Changes in Deferred Revenue Balance

Changes in the deferred revenue balance during the year ended December 31, 2018 are as follows (in thousands):

	Deferred Revenue
Balance at January 1, 2018	$	—
Additions		20,000
Deductions		(1,187	)
Balance at December 31, 2018	$	18,813

Balance Sheet Components (Tables)

12 Months Ended

Dec. 31, 2018

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,
	2018		2017
Receivable from landlord	$	—	$	1,226
Prepaid rent		860		93
Prepaid insurance		966		—
Interest and other receivables		462		484
Prepaid research and development-related expenses		1,789		628
Other		449		95
Total prepaid expenses and other current assets	$	4,526	$	2,526

Schedule of Deposits and Other Long-Term Assets

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

	December 31,
	2018		2017
Lease security deposit	$	632	$	368
Prepaid research and development-related expenses		554		—
Restricted cash		992		992
Other		250		250
Total deposits and other long-term assets	$	2,428	$	1,610

Schedule of Accrued Current Liabilities

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

	December 31,
	2018		2017
Deferred rent	$	445	$	381
Research and development-related expenses		252		683
Other		295		426
Total accrued current liabilities	$	992	$	1,490

Convertible Preferred Stock and Common Stock (Tables)

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Schedule of Convertible Preferred Stock

At December 31, 2017, convertible preferred stock consisted of the following (in thousands, except share and per share amounts):

	Shares Authorized		Shares Issued and Outstanding		Issuance Price Per Share		Carrying Value		Liquidation Preference
Series B		64,102,551		8,919,302	$	10.76	$	95,798	$	96,008
Series A—First Tranche		61,260,000		3,699,259	$	6.90		25,425		25,525
Series A—Second Tranche		—		5,178,968	$	6.90		35,714		35,735
Total convertible preferred stock		125,362,551		17,797,529			$	156,937	$	157,268

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Fair Value of Stock Options Granted Estimated using Weighted Average Assumptions

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,
	2018			2017			2016
Expected dividend yield		—			—			—
Expected term	6.04 years			6.04 years			6.05 years
Risk-free interest rate		2.80	%		1.97	%		1.43	%
Expected volatility		88	%		94	%		103	%

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, 2016		1,148,227			509,189		$	0.41		9.63	$	178
Authorized		362,318			—		$	—
Granted		(900,257	)		900,257		$	1.13
Exercised					(41,665	)	$	0.35
Cancelled		15,941			(15,941	)	$	0.46
Balance at December 31, 2017		626,229			1,351,840		$	0.89		9.13	$	3,087
Authorized		3,197,246
Granted		(1,344,973	)		1,344,973		$	8.92
Exercised					(80,463	)	$	0.58
Cancelled		186,491			(186,491	)	$	1.31
Repurchased		30,117					$	0.35
Balance at December 31, 2018		2,695,110			2,429,859		$	5.31		8.86	$	25,646
Vested and exercisable – December 31, 2018					582,436		$	1.24		8.12	$	8,274
Vested and expected to vest – December 31, 2018					2,521,869		$	4.75		8.69	$	27,896

Schedule of Stock Based Compensation Expense

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

	Year Ended December 31,
	2018		2017		2016
Research and development expenses	$	2,081	$	888	$	385
General and administrative expenses		914		238		212
Total	$	2,995	$	1,126	$	597

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

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

The reconciliation of the statutory federal income tax rate to the Company’s effective tax rate is as follows:

	Year Ended December 31,
	2018			2017			2016
Statutory federal income tax rate		21.0	%		34.0	%		34.0	%
State tax, net of federal benefit		4.3			2.2			3.3
Permanent differences		(1.0	)		(2.3	)		(0.8	)
Effective change in enacted tax rate		—			(17.3	)		—
Research and development tax credits		4.4			4.1			0.8
Other		(0.2	)		—			—
Change in valuation allowance		(28.5	)		(20.7	)		(37.3	)
Effective income tax rate		—	%		—	%		—	%

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,
	2018			2017
Deferred tax assets:
Net operating loss carryforwards	$	27,311		$	12,251
Research and development tax credits		4,503			1,990
Lease financing obligation		2,658			2,490
Accruals and other		1,387			1,124
Amortization		1,726			1,222
Deferred tax liabilities:
Other depreciation		(655	)		(536	)
Leased building depreciation		(2,446	)		(2,475	)
Total net deferred tax assets		34,484			16,066
Less valuation allowance		(34,484	)		(16,066	)
Deferred tax assets, net of allowance	$	—		$	—

Schedule of Activity Related Unrecognized Tax Benefits

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

	December 31,
	2018			2017			2016
Beginning of year—unrecognized tax benefits	$	1,089		$	230		$	7
Decrease for tax positions taken during prior periods		(453	)		(47	)		—
Increases for tax positions taken during current period		1,441			906			223
End of year—unrecognized tax benefits	$	2,077		$	1,089		$	230

Net Loss Per Common Share (Tables)

12 Months Ended

Dec. 31, 2018

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,
	2018			2017			2016
Numerator:
Net loss	$	(64,775	)	$	(41,377	)	$	(18,750	)
Denominator:
Weighted-average common shares outstanding, basic and diluted		8,919,281			1,999,044			1,672,545
Net loss per share, basic and diluted	$	(7.26	)	$	(20.70	)	$	(11.21	)

Computation of Potentially Anti-Dilutive Securities

	December 31,
	2018		2017		2016
Convertible preferred stock		—		17,797,529		8,878,227
Options issued and outstanding		2,429,859		1,351,840		509,189
Early exercised common stock subject to future vesting		226,967		539,289		838,359
Warrants to purchase common stock		—		40,257		40,257
Total		2,656,826		19,728,915		10,266,032

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Summary of Company’s Quarterly Financial Information

	Quarter Ended
	December 31, 2018			September 30, 2018			June 30, 2018			March 31, 2018
Collaboration revenue	$	1,091		$	96		$	—		$	—
Loss from operations	$	(18,027	)	$	(18,614	)	$	(15,504	)	$	(13,439	)
Net loss	$	(17,338	)	$	(18,588	)	$	(15,473	)	$	(13,376	)
Net loss per share, basic and diluted	$	0.61		$	(7.60	)	$	(6.57	)	$	(6.03	)

	Quarter Ended
	December 31, 2017			September 30, 2017			June 30, 2017			March 31, 2017
Collaboration revenue	$	—		$	—		$	—		$	—
Loss from operations	$	(17,580	)	$	(9,488	)	$	(7,851	)	$	(6,844	)
Net loss	$	(17,368	)	$	(9,451	)	$	(7,790	)	$	(6,768	)
Net loss per share, basic and diluted	$	(8.18	)	$	(4.59	)	$	(3.95	)	$	(3.68	)

Organization - Additional Information (Detail) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) Segment $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($)	Oct. 02, 2018 $ / shares shares
Organization And Nature Of Operations [Line Items]
Number of operating segments \| Segment		1
Net proceeds from issuance of stock \| $		$ 92,586	$ 14	$ 232
Common stock, shares authorized		300,000,000	160,000,000
Common stock, par value \| $ / shares		$ 0.0001	$ 0.0001
Preferred stock, shares authorized		10,000,000	125,362,551
Preferred stock, par value \| $ / shares		$ 0.0001	$ 0.0001
IPO [Member]
Organization And Nature Of Operations [Line Items]
Number of common shares sold	6,854,202
Stock issued, price per share \| $ / shares	$ 15.00	$ 15.00
Net proceeds from issuance of stock \| $	$ 92,600
Conversion of preferred stock shares into common stock shares	19,409,132
Reclassified common stock and additional paid in capital \| $	$ 177,900
Over-Allotment Option [Member]
Organization And Nature Of Operations [Line Items]
Number of common shares sold	187,535
Amended and Restated Certificate of Incorporation [Member]
Organization And Nature Of Operations [Line Items]
Common stock, shares authorized					300,000,000
Common stock, par value \| $ / shares					$ 0.0001
Preferred stock, shares authorized					10,000,000
Preferred stock, par value \| $ / shares					$ 0.0001

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands		3 Months Ended								12 Months Ended
	Sep. 20, 2018	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2019 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Reverse stock split	0.1449
Accumulated deficit		$ 126,402				$ 61,627				$ 126,402	$ 61,627
Net loss		$ 17,338	$ 18,588	$ 15,473	$ 13,376	17,368	$ 9,451	$ 7,790	$ 6,768	64,775	41,377	$ 18,750
Net cash used in operating activities										38,162	34,971	15,292
Impairment losses										$ 0	0	$ 0
Lessor financed improvement incentives due but not yet received						$ 1,200					$ 1,200
Percentage of realized upon ultimate settlement										50.00%
Accounting Standards Update 2016-02 [Member] \| Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Lease, right-of-use asset													$ 14,200
Lease liability													11,200
Derecognition of deferred rent													1,800
Derecognition of building and related accumulated depreciation													14,500
Derecognition of financing lease obligation													$ 10,500

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 52,183	$ 39,007
Restricted cash	992	992
Total cash, cash equivalents and restricted cash	$ 53,175	$ 39,999

Summary of Significant Accounting Policies - Schedule of Straight Line Method Over Estimated Useful Lives of Respective Assets (Detail)	12 Months Ended
	Dec. 31, 2018
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful life	5 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful lives	Shorter of useful life or lease term
Minimum [Member] \| Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful life	3 years
Minimum [Member] \| Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful life	5 years
Maximum [Member] \| Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful life	5 years
Maximum [Member] \| Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful life	7 years

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, 2018	Dec. 31, 2017
Cash, cash equivalents and marketable securities [Line Items]
Amortized Cost	$ 149,207	$ 79,898
Unrealized Gains	1	0
Unrealized Losses	(86)	(74)
Fair Value	149,122	79,824
Cash equivalents	48,195	32,878
Marketable securities	100,927	46,946
Money Market Funds [Member]
Cash, cash equivalents and marketable securities [Line Items]
Amortized Cost	36,148	27,711
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	36,148	27,711
Commercial Paper [Member]
Cash, cash equivalents and marketable securities [Line Items]
Amortized Cost	45,244	32,257
Unrealized Gains	0	0
Unrealized Losses	(40)	(48)
Fair Value	45,204	32,209
Corporate Debt Securities [Member]
Cash, cash equivalents and marketable securities [Line Items]
Amortized Cost	67,815	19,930
Unrealized Gains	1	0
Unrealized Losses	(46)	(26)
Fair Value	$ 67,770	$ 19,904

Cash Equivalents and Marketable Securities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash, cash equivalents and marketable securities [Line Items]
Cash and cash equivalents and marketable securities	$ 153,100,000	$ 85,900,000
Cash and cash equivalents	52,183,000	39,007,000
Marketable securities	100,927,000	46,946,000
Realized gains or losses on marketable securities	0
Marketable securities unrealized loss	0
Marketable securities recognition of other-than-temporary impairment	$ 0	$ 0	$ 0
Maximum [Member]
Cash, cash equivalents and marketable securities [Line Items]
Marketable securities contractual maturities period	1 year

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, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	$ 149,122	$ 79,824
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	36,148	27,711
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	112,974	52,113
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	36,148	27,711
Money Market Funds [Member] \| Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	36,148	27,711
Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	45,204	32,209
Commercial Paper [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	45,204	32,209
Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	67,770	19,904
Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	$ 67,770	$ 19,904

Fair Value Measurements - Additional Information (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
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, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 36,245	$ 30,001
Less accumulated depreciation and amortization	(6,751)	(2,790)
Total property and equipment, net	29,494	27,211
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	470	353
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	935	785
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	16,406	10,515
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	3,063	2,977
Buildings and improvements capitalized under a lease financing transaction [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 15,371	$ 15,371

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 3,961	$ 1,970	$ 823

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended			12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($) ft²	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Feb. 28, 2016 USD ($)	Nov. 30, 2015
Commitments and Contingencies Disclosure [Line Items]
Operating lease cash security deposit				$ 632,000	$ 368,000
Leased building improvements incurred but not yet received from landlord					1,200,000
Prepaid rent				860,000	93,000
Rent expense				1,800,000	1,200,000	$ 2,400,000
Research and Development Expense				54,965,000	35,691,000	$ 13,916,000
Contract Research and Development Agreement [Member] \| Contract Research Organization [Member]
Commitments and Contingencies Disclosure [Line Items]
Upfront payment		$ 500,000
Term of agreement		1 year
Maximum payment to CRO upon the completion of certain phases of research services		$ 900,000
Maximum milestone payments		$ 36,400,000
Research and Development Expense				1,100,000	100,000
Emeryville California [Member]
Commitments and Contingencies Disclosure [Line Items]
Operating leasing term								84 months
Operating leasing renewal option to extend lease								3 years
Operating lease for additional temporary space	12 months
Temporary space lease termination notice period	30 days
Cambridge Massachusetts [Member]
Commitments and Contingencies Disclosure [Line Items]
Operating leasing term							67 months
Operating leasing renewal option to extend lease							3 years
Operating lease cash security deposit							$ 300,000
Tenant improvement allowance							$ 2,100,000
Pleasanton [Member]
Commitments and Contingencies Disclosure [Line Items]
Tenant improvement allowance				1,200,000
Net rentable area \| ft²			42,620
Lease expiration date			Nov. 30, 2024
Finance lease renewal option to extend lease			5 years
Letters of credit outstanding, amount			$ 1,000,000
Withdrawal from irrevocable letters of credit				0
Property plant and equipment additions					9,300,000
Building improvements					6,100,000
Leased building improvements incurred but not yet received from landlord					1,200,000
Total interest expense for the cost of the lease financing obligation				800,000	$ 0
Cambridge [Member]
Commitments and Contingencies Disclosure [Line Items]
Operating leasing term	24 months
Operating lease cash security deposit				300,000
Base rent paid for the year				1,300,000
Prepaid rent				$ 900,000

Commitments and Contingencies - Minimum Annual Payments under Non-Cancelable Lease Agreements and Lease Financing Obligation (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Lease Financing Obligation
2019	$ 794
2020	818
2021	843
2022	868
2023	894
Thereafter	842
Total minimum payments	5,059
Less: Amount representing interest expense	(4,435)
Total lease payment	624
Residual value of lease financing obligation	9,896
Capital lease	10,520
Less: Lease financing obligation, short-term	(30)
Lease financing obligation, long-term	10,490	$ 10,521
Operating Leases
2019	2,329
2020	2,674
2021	1,803
2022	1,119
2023	281
Thereafter	114
Total minimum payments	$ 8,320

Collaboration and License Agreements - Additional Information (Detail) - USD ($)	1 Months Ended				12 Months Ended
	Oct. 31, 2018	Aug. 31, 2018	Jul. 31, 2018	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Preferred stock shares issued					0	17,797,529
Additional preferred stock purchase					$ 20,935,000	$ 95,798,000	$ 35,714,000
Collaboration and license revenue					(1,187,000)
Deferred revenue					$ 18,813,000	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 fifty percent for the remainder of the agreement term.
Bluebird Bio Inc [Member] \| Collaboration Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from non refundable upfront payment		$ 20,000,000
Series C preferred stock equity investment received as consideration		$ 10,000,000
Offering price per share	$ 15.00
Additional preferred stock purchase	$ 10,000,000
Number of common shares sold	666,667
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					$ 1,200,000
Deferred revenue					18,800,000
Receivables					0
Contract assets					0
Bluebird Bio Inc [Member] \| Series C [Member] \| Collaboration Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Preferred stock shares issued		768,115
Offering price per share		$ 13.04
Arbutus Biopharma Corporation and Protiva Biotherapeutics Inc [Member] \| Collaboration Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment						5,000,000
Reimbursed Expense						$ 200,000
Maximum milestone consideration payable					123,500,000
Development milestone payment					2,500,000
Reimbursements expense materials and personnel costs					$ 400,000
Non Profit Hospital Cancer Center [Member] \| Collaboration Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock issued under warrant					40,257
Common stock issued under warrant exercise price					$ 0.35

Collaboration and License Agreements - Schedule of Changes in Deferred Revenue Balance (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Revenue From Contract With Customer [Abstract]
Balance at January 1, 2018	$ 0
Additions	20,000
Deductions	(1,187)
Balance at December 31, 2018	$ 18,813

Balance Sheet Components - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Prepaid Expense And Other Assets Current [Abstract]
Receivable from landlord		$ 1,226
Prepaid rent	$ 860	93
Prepaid insurance	966
Interest and other receivables	462	484
Prepaid research and development-related expenses	1,789	628
Other	449	95
Total prepaid expenses and other current assets	$ 4,526	$ 2,526

Balance Sheet Components - Schedule of Deposits and Other Long-Term Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Other Assets Unclassified [Abstract]
Lease security deposit	$ 632	$ 368
Prepaid research and development-related expenses	554
Restricted cash	992	992
Other	250	250
Total deposits and other long-term assets	$ 2,428	$ 1,610

Balance Sheet Components - Schedule of Accrued Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Deferred rent	$ 445	$ 381
Research and development-related expenses	252	683
Other	295	426
Total accrued current liabilities	$ 992	$ 1,490

Convertible Preferred Stock and Common Stock - Additional Information (Detail)		1 Months Ended							12 Months Ended
	Oct. 20, 2017 USD ($) $ / shares shares	Oct. 31, 2018 $ / shares shares	Aug. 31, 2018 USD ($) $ / shares shares	Jul. 31, 2018 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Apr. 30, 2016 USD ($) $ / shares shares	Sep. 30, 2015 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) Director $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares
Class of Stock [Line Items]
Preferred stock, shares authorized \| shares									10,000,000	125,362,551
Preferred stock, shares outstanding \| shares									0	17,797,529
Preferred stock redemption terms									A redemption event will only occur upon the liquidation or winding up of the Company, a greater than 50% change in control, or the sale of substantially all of the assets of the Company.
Cumulative dividends rate									8.00%
Dividend declared or accrued \| $										$ 0
Common stock, shares authorized \| shares									300,000,000	160,000,000
Common stock voting rights									Holders of common stock are generally entitled to one vote per share on all matters to be voted upon by the stockholders of the Company.
Dividends declared \| $									$ 0
IPO [Member]
Class of Stock [Line Items]
Stock issued, price per share		$ 15.00							$ 15.00
Stock issued, number of shares \| shares		6,854,202
Payment for issuance cost \| $									$ 10,276,000
Conversion of preferred stock shares into common stock shares \| shares		19,409,132
Common Stock [Member]
Class of Stock [Line Items]
Number of directors to be elected by shareholders \| Director									1
Common Stock [Member] \| IPO [Member]
Class of Stock [Line Items]
Stock issued, number of shares \| shares									6,854,202
Minimum [Member]
Class of Stock [Line Items]
Percentage of change in control, basis for redemption event									50.00%
Series A First Tranche [Member]
Class of Stock [Line Items]
Stock issued, price per share								$ 6.90		$ 6.90
Proceed from issuance of stock \| $								$ 25,400,000
Stock issued, number of shares \| shares								3,699,259
Payment for issuance cost \| $								$ 100,000
Preferred stock, shares authorized \| shares										61,260,000
Preferred stock, shares outstanding \| shares										3,699,259
Series A Second Tranche [Member]
Class of Stock [Line Items]
Stock issued, price per share							$ 6.90			$ 6.90
Proceed from issuance of stock \| $							$ 35,700,000
Stock issued, number of shares \| shares							5,178,968
Payment for issuance cost \| $							$ 20,000
Preferred stock, shares outstanding \| shares										5,178,968
Series A Third Tranche [Member]
Class of Stock [Line Items]
Stock issued, price per share									$ 6.90
Stock issued, number of shares \| shares									5,918,840
Convertible preferred stock term									90 days
Superior Financing Transaction [Member]
Class of Stock [Line Items]
Minimum percentage of right to purchase original Series A-Third Tranche amount									50.00%
Superior Financing Transaction [Member] \| Minimum [Member]
Class of Stock [Line Items]
Stock issued, price per share									$ 6.90
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Stock issued, price per share	$ 10.76					$ 10.76				$ 10.76
Proceed from issuance of stock \| $	$ 95,800,000					$ 95,800,000
Stock issued, number of shares \| shares	8,919,302					8,919,302
Payment for issuance cost \| $	$ 200,000					$ 200,000				$ 210,000
Preferred stock, shares authorized \| shares										64,102,551
Preferred stock, shares outstanding \| shares										8,919,302
Preferred stock liquidation preference									10.76
Preferred stock redemption price per share									$ 53.82
Number of directors to be elected by shareholders \| Director									1
Preferred stock conversion price									$ 10.76
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Stock issued, price per share			$ 13.04	$ 13.04	$ 13.04				$ 13.04
Proceed from issuance of stock \| $			$ 10,000,000	$ 2,000,000	$ 8,900,000
Stock issued, number of shares \| shares			768,115	153,360	690,128
Payment for issuance cost \| $					$ 100,000				$ 81,000
Preferred stock liquidation preference									$ 13.04
Preferred stock redemption price per share									65.21
Preferred stock conversion price									13.04
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Stock issued, price per share											$ 6.90
Payment for issuance cost \| $											$ 21,000
Preferred stock liquidation preference									6.90
Preferred stock redemption price per share									$ 34.50
Number of directors to be elected by shareholders \| Director									3
Preferred stock conversion price									$ 6.90
Series A B And C Preferred Stock [Member] \| Minimum [Member]
Class of Stock [Line Items]
Preferred stock conversion price after amendment									15.66
Series A B And C Preferred Stock [Member] \| Maximum [Member]
Class of Stock [Line Items]
Preferred stock conversion price before amendment									$ 21.53

Convertible Preferred Stock and Common Stock - Schedule of Convertible Preferred Stock (Detail) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Oct. 20, 2017	Sep. 30, 2017	Apr. 30, 2016	Sep. 30, 2015
Class of Stock [Line Items]
Shares Authorized	10,000,000	125,362,551
Preferred stock shares issued	0	17,797,529
Shares Outstanding	0	17,797,529
Carrying Value		$ 156,937
Preferred Stock Liquidation Preference		$ 157,268
Series B [Member]
Class of Stock [Line Items]
Shares Authorized		64,102,551
Preferred stock shares issued		8,919,302
Shares Outstanding		8,919,302
Stock issued, price per share		$ 10.76	$ 10.76	$ 10.76
Carrying Value		$ 95,798
Preferred Stock Liquidation Preference		$ 96,008
Series A First Tranche [Member]
Class of Stock [Line Items]
Shares Authorized		61,260,000
Preferred stock shares issued		3,699,259
Shares Outstanding		3,699,259
Stock issued, price per share		$ 6.90				$ 6.90
Carrying Value		$ 25,425
Preferred Stock Liquidation Preference		$ 25,525
Series A Second Tranche [Member]
Class of Stock [Line Items]
Preferred stock shares issued		5,178,968
Shares Outstanding		5,178,968
Stock issued, price per share		$ 6.90			$ 6.90
Carrying Value		$ 35,714
Preferred Stock Liquidation Preference		$ 35,735

Stock Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Sep. 30, 2018	Feb. 28, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Short- term and long-term liabilities associated with shares issued subject to repurchase rights			$ 80	$ 200
Number of shares available for future issuance			2,695,110	626,229	1,148,227
Weighted-average grant-date fair value granted employee stock options granted			$ 6.59	$ 2.00	$ 2.19
Intrinsic value of stock option awards exercised			$ 750	$ 80	$ 1,300
Unrecognized compensation cost related to non-vested employee and consultant options			$ 8,900
Unrecognized compensation cost related to non-vested employee and consultant			3 years 2 months 4 days
Fair value of shares vested			$ 1,800
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%
Share-based Compensation Arrangement by Share-based Payment Award, 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
2015 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards granted, deemed fair values per common share				$ 3.17	$ 2.21
Number of shares to be reserved		507,246
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%
Share-based Compensation Arrangement by Share-based Payment Award, maximum number of shares authorized	5,000,000

Stock-Based Compensation - Schedule of Fair Value of Stock Options Granted Estimated using Weighted Average Assumptions (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Expected term	6 years 14 days	6 years 14 days	6 years 18 days
Risk-free interest rate	2.80%	1.97%	1.43%
Expected volatility	88.00%	94.00%	103.00%

Stock-Based Compensation - Schedule of Stock Options Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Options Available for Grant
Number of options beginning balance	626,229	1,148,227
Authorized	3,197,246	362,318
Granted	(1,344,973)	(900,257)
Cancelled	186,491	15,941
Repurchased	30,117
Number of options ending balance	2,695,110	626,229	1,148,227
Outstanding Options
Outstanding at beginning of period	1,351,840	509,189
Granted	1,344,973	900,257
Exercised	(80,463)	(41,665)
Cancelled	(186,491)	(15,941)
Outstanding at end of period	2,429,859	1,351,840	509,189
Options Outstanding, Number of Shares, Vested and exercisable	582,436
Options Outstanding, Number of Shares, Vested and expected to vest	2,521,869
Weighted Average Exercise Price
Beginning Balance	$ 0.89	$ 0.41
Granted	8.92	1.13
Exercised	0.58	0.35
Cancelled	1.31	0.46
Repurchased	0.35
Ending Balance	5.31	$ 0.89	$ 0.41
Weighted-Average Exercise Price, Vested and exercisable	1.24
Weighted-Average Exercise Price, Vested and expected to vest	$ 4.75
Options Outstanding, Weighted-Average Remaining Contractual Term (in years)	8 years 10 months 9 days	9 years 1 month 17 days	9 years 7 months 17 days
Options Outstanding, Weighted-Average Remaining Contractual Term, Vested and exercisable (in years)	8 years 1 month 13 days
Options Outstanding, Weighted-Average Remaining Contractual Term, Vested and expected to vest (in years)	8 years 8 months 8 days
Options Outstanding, Aggregate Intrinsic Value	$ 25,646	$ 3,087	$ 178
Options Outstanding, Aggregate Intrinsic Value, Vested and exercisable	8,274
Options Outstanding, Aggregate Intrinsic Value, Vested and expected to vest	$ 27,896

Stock-Based Compensation - Schedule of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Based Compensation Expense	$ 2,995	$ 1,126	$ 597
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Based Compensation Expense	2,081	888	385
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Based Compensation Expense	$ 914	$ 238	$ 212

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Statutory federal income tax rate	21.00%	34.00%	34.00%
State tax, net of federal benefit	4.30%	2.20%	3.30%
Permanent differences	(1.00%)	(2.30%)	(0.80%)
Effective change in enacted tax rate		(17.30%)
Research and development tax credits	4.40%	4.10%	0.80%
Other	(0.20%)
Change in valuation allowance	(28.50%)	(20.70%)	(37.30%)

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Increase in valuation allowance	$ 18.4	$ 8.5
U.S. federal statutory corporate tax rate	21.00%	34.00%	34.00%
Reduction to the deferred tax assets	$ 7.1
Changes in ownership interest rate period	3 years
Minimum [Member]
Income Taxes [Line Items]
Cumulative changes in ownership interest of shareholders	50.00%
Research and Development \| California
Income Taxes [Line Items]
Income tax credit carryforwards	$ 2.3
Research and Development \| Massachusetts
Income Taxes [Line Items]
Income tax credit carryforwards	$ 0.7
Income tax credit carryforwards, expiration year	2032
Federal
Income Taxes [Line Items]
Net operating loss carryforwards	$ 109.8
Net operating loss carryforwards, expiration year	2035
Federal \| Research and Development
Income Taxes [Line Items]
Income tax credit carryforwards	$ 4.1
Income tax credit carryforwards, expiration year	2035
State
Income Taxes [Line Items]
Net operating loss carryforwards	$ 65.7
Net operating loss carryforwards, expiration year	2035

Income Taxes - Components of the Net Deferred Tax Assets/Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 27,311	$ 12,251
Research and development tax credits	4,503	1,990
Lease financing obligation	2,658	2,490
Accruals and other	1,387	1,124
Amortization	1,726	1,222
Deferred tax liabilities:
Other depreciation	(655)	(536)
Leased building depreciation	(2,446)	(2,475)
Total net deferred tax assets	34,484	16,066
Less valuation allowance	$ (34,484)	$ (16,066)

Income Taxes - Schedule of Activity Related Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Beginning of year—unrecognized tax benefits	$ 1,089	$ 230	$ 7
Decrease for tax positions taken during prior periods	(453)	(47)
Increases for tax positions taken during current period	1,441	906	223
End of year—unrecognized tax benefits	$ 2,077	$ 1,089	$ 230

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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ (17,338)	$ (18,588)	$ (15,473)	$ (13,376)	$ (17,368)	$ (9,451)	$ (7,790)	$ (6,768)	$ (64,775)	$ (41,377)	$ (18,750)
Denominator:
Weighted-average common shares outstanding, basic and diluted									8,919,281	1,999,044	1,672,545
Net loss per share, basic and diluted	$ 0.61	$ (7.60)	$ (6.57)	$ (6.03)	$ (8.18)	$ (4.59)	$ (3.95)	$ (3.68)	$ (7.26)	$ (20.70)	$ (11.21)

- Definition

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

+ References + Details

Net Loss Per Common Share - Computation of Potentially Anti-dilutive Securities (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities that were not included in the diluted per share calculations	2,656,826	19,728,915	10,266,032
Convertible Preferred Stock [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		17,797,529	8,878,227
Options Issued 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	2,429,859	1,351,840	509,189
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	226,967	539,289	838,359
Warrant [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		40,257	40,257

Related-Party Transactions - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Common stock, shares issued	28,823,130	2,152,525
Proceeds from Issuance of Common Stock	$ 92,586	$ 14	$ 232
Preferred stock, shares issued	0	17,797,529
Proceeds from issuance of convertible preferred stock	$ 20,935	$ 95,798	$ 35,714
Certain Stockholders Related Parties [Member]
Class of Stock [Line Items]
Common stock, shares issued	333,333
Proceeds from Issuance of Common Stock	$ 5,000
Series A Convertible Preferred Stock [Member] \| Certain Stockholders Related Parties [Member]
Class of Stock [Line Items]
Preferred stock, shares issued			4,236,261
Proceeds from issuance of convertible preferred stock			$ 29,200
Series B Convertible Preferred Stock [Member] \| Certain Stockholders Related Parties [Member]
Class of Stock [Line Items]
Preferred stock, shares issued		2,560,342
Proceeds from issuance of convertible preferred stock		$ 27,600

Defined Contribution Plan - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018
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] $ in Millions	1 Months Ended
	Jan. 31, 2019 USD ($) ft²
Subsequent Event [Line Items]
Area of land, operating lease \| ft²	34,469
Operating lease, annual base rent	$ 1.7
Lease termination description	In connection with the new lease agreement, the Company also entered into a Lease Termination Agreement to early terminate the Company’s existing lease, dated November 23, 2015, for its current premises. The current lease will terminate effective no later than 60 days after the rent commencement date under the new lease
Lease execution date	Nov. 23, 2015
Lease termination period	60 days
Maximum [Member]
Subsequent Event [Line Items]
Operating lease, annual base rent	$ 2.6

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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue	$ 1,091	$ 96							$ 1,187
Loss from operations	(18,027)	(18,614)	$ (15,504)	$ (13,439)	$ (17,580)	$ (9,488)	$ (7,851)	$ (6,844)	(65,584)	$ (41,763)	$ (18,980)
Net loss	$ (17,338)	$ (18,588)	$ (15,473)	$ (13,376)	$ (17,368)	$ (9,451)	$ (7,790)	$ (6,768)	$ (64,775)	$ (41,377)	$ (18,750)
Net loss per share, basic and diluted	$ 0.61	$ (7.60)	$ (6.57)	$ (6.03)	$ (8.18)	$ (4.59)	$ (3.95)	$ (3.68)	$ (7.26)	$ (20.70)	$ (11.21)

- Definition

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

+ References + Details