Gritstone bio (GRTSQ) 10-K 11 Mar 20 2019 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Mar. 06, 2020	Jun. 28, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Registrant Name	Gritstone Oncology, Inc.
Entity Central Index Key	0001656634
Current Fiscal Year End Date	--12-31
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Trading Symbol	GRTS
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		36,976,352
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Public Float			$ 277.1
Entity File Number	001-38663
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-4859534
Entity Address, Address Line One	5959 Horton Street
Entity Address, Address Line Two	Suite 300
Entity Address, City or Town	Emeryville
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94608
City Area Code	510
Local Phone Number	871-6100
Document Annual Report	true
Document Transition Report	false
Documents Incorporated by Reference	Portions of the Registrant’s Definitive Proxy Statement relating to the Annual Meeting of Shareholders, scheduled to be held on June 18, 2020, are incorporated by reference into Part III of this Annual Report on Form 10-K where indicated. Such Definitive Proxy Statement will be filed with the Securities and Exchange Commission within 120 days after the fiscal year to which this report relates.

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 57,408	$ 52,183
Marketable securities	70,368	100,927
Prepaid expenses and other current assets	3,497	4,526
Total current assets	131,273	157,636
Property and equipment, net	26,911	29,494
Operating lease right-of-use assets	23,427	0
Deposits and other long-term assets	2,778	2,428
Total assets	184,389	189,558
Current liabilities:
Accounts payable	4,621	4,825
Accrued compensation	4,598	3,951
Accrued liabilities	1,041	740
Accrued research and development expenses	1,779	252
Lease liabilities, current portion	2,505	0
Deferred revenue, current portion	4,956	5,340
Total current liabilities	19,500	15,108
Deferred rent, net of current portion	0	1,353
Other non-current liabilities	0	12
Lease financing obligation, net of current portion	0	10,490
Lease liabilities, net of current portion	20,985	0
Deferred revenue, net of current portion	9,560	13,473
Total liabilities	50,045	40,436
Commitments and contingencies (Notes 6 and 7)
Stockholders’ equity:
Convertible preferred stock, $0.0001 par value; 10,000,000 shares authorized; no shares issued and outstanding	0	0
Common stock, $0.0001 par value; 300,000,000 shares authorized at December 31, 2019 and 2018; 36,332,956 and 28,823,130 shares issued and outstanding at December 31, 2019 and 2018, respectively	17	16
Additional paid-in capital	355,291	275,593
Accumulated other comprehensive gain (loss)	24	(85)
Accumulated deficit	(220,988)	(126,402)
Total stockholders’ equity	134,344	149,122
Total liabilities and stockholders’ equity	$ 184,389	$ 189,558

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	36,332,956	28,823,130
Common stock, shares outstanding	36,332,956	28,823,130

Statements of Operations and Co

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Collaboration revenue	$ 4,365	$ 1,187
Type of Revenue [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Operating expenses:
Research and development	$ 82,896	$ 54,965	$ 35,691
General and administrative	19,409	11,806	6,072
Total operating expenses	102,305	66,771	41,763
Loss from operations	(97,940)	(65,584)	(41,763)
Interest income, net	3,507	809	386
Net loss	(94,433)	(64,775)	(41,377)
Other comprehensive gain (loss):
Unrealized gain (loss) on marketable securities, net of tax	109	(11)	(71)
Net and comprehensive loss	$ (94,324)	$ (64,786)	$ (41,448)
Net loss per share, basic and diluted	$ (2.81)	$ (7.26)	$ (20.70)
Weighted-average number of shares used in computing net loss per share, basic and diluted	33,554,823	8,919,281	1,999,044

Statements of Stockholders' Equ

Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	IPO [Member]	Follow-on Public Offering [Member]	ATM Equity Offering Program [Member]	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]	Common Stock [Member]Follow-on Public Offering [Member]	Common Stock [Member]ATM Equity Offering Program [Member]	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member]IPO [Member]	Additional Paid-In Capital [Member]Follow-on Public Offering [Member]	Additional Paid-In Capital [Member]ATM Equity Offering Program [Member]	Accumulated Other Comprehensive Gain/(Loss) [Member]	Accumulated Deficit [Member]
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 gain/(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									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
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 gain/(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
Issuance of common stock for consulting services	36												36
Issuance of common stock for consulting services, shares									4,347
Exercise of common stock warrants	13												13
Exercise of common stock warrants, shares									40,257
Stock-based compensation	2,959												2,959
Net loss	(64,775)																	(64,775)
Balance at Dec. 31, 2018	149,122								$ 16				275,593				(85)	(126,402)
Balance, shares at Dec. 31, 2018									28,823,130
Cumulative effect of adoption of Topic 842 | ASU 2016-02 [Member]	(153)																	(153)
Issuance of stock, net of issuance costs			$ 69,709	$ 3,763							$ 1				$ 69,708	$ 3,763
Issuance of stock, net of issuance costs, shares				490,880							6,500,000	490,880
Issuance of common stock under employee stock purchase plan (“ESPP”)	413												413
Issuance of common stock under employee stock purchase plan (“ESPP”), shares									55,727
Unrealized gain/(loss) on marketable securities, net of tax	109																109
Lapse of repurchase rights related to common stock issued pursuant to early exercises	63												63
Lapse of repurchase rights related to common stock issued pursuant to early exercises, shares									182,195
Issuance of common stock upon exercise of stock options	$ 452												452
Issuance of common stock upon exercise of stock options, shares	281,024								281,024
Stock-based compensation	$ 5,299												5,299
Net loss	(94,433)																	(94,433)
Balance at Dec. 31, 2019	$ 134,344								$ 17				$ 355,291				$ 24	$ (220,988)
Balance, shares at Dec. 31, 2019									36,332,956

Statements of Stockholders' E_2

Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Common Stock [Member] | IPO [Member]
Payment for issuance cost		$ 10,276
Shares issued price per share		$ 15
Common Stock [Member] | Follow-on Public Offering [Member]
Payment for issuance cost	$ 556
Shares issued price per share	$ 11.50
Common Stock [Member] | ATM Equity Offering Program [Member]
Payment for issuance cost	$ 301
Series C Convertible Preferred Stock [Member]
Payment for issuance cost		$ 81
Shares issued price per share		$ 13.04

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net loss	$ (94,433)	$ (64,775)	$ (41,377)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	4,745	3,961	1,970
Net amortization of premiums and discounts on marketable securities	(1,349)	(552)	(158)
Stock-based compensation	5,299	2,995	1,126
Non-cash operating lease expense	6,382	0	0
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	37	(1,781)	(416)
Operating lease right-of-use assets	(3,328)	0	0
Deposits and other long-term assets	(351)	(818)	(78)
Accounts payable	45	3,249	2,273
Accrued compensation	650	1,724	1,190
Accrued and other non-current liabilities	9	(582)	810
Accrued research and development expenses	1,526	0	0
Deferred rent	0	(396)	(311)
Lease liability	54	0	0
Deferred revenue	(4,297)	18,813	0
Net cash used in operating activities	(85,011)	(38,162)	(34,971)
Investing activities
Purchase of marketable securities	(80,979)	(102,160)	(63,228)
Maturities of marketable securities	112,993	48,720	41,467
Purchase of property and equipment	(16,173)	(5,663)	(11,522)
Disposition of property and equipment	0	0	31
Net cash provided by (used in) investing activities	15,841	(59,103)	(33,252)
Financing activities
Proceeds from issuance of common stock, net of issuance costs	75,202	92,586	14
Payments of deferred financing costs	(807)	(3,080)	0
Proceeds from issuance of convertible preferred stock, net of issuance costs	0	20,935	95,798
Net cash provided by financing activities	74,395	110,441	95,812
Net increase in cash, cash equivalents and restricted cash	5,225	13,176	27,589
Cash, cash equivalents and restricted cash at beginning of period	53,175	39,999	12,410
Cash, cash equivalents and restricted cash at end of period	58,400	53,175	39,999
Supplemental disclosures of non-cash investing and financing information
Property and equipment purchases accrued but not yet paid	1,232	1,482	900
Remeasurement of operating lease right-of-use asset for lease modification	1,878	0	0
Assets acquired under leasing obligations	0	0	9,300
Receivable from lessor funded financing	$ 0	$ 0	$ 1,226

Organization

Organization	12 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	1. 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.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
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. 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 $221.0 million and $126.4 million as of December 31, 2019 and 2018, respectively. The Company had net losses of $94.4 million, $64.8 million, and $41.4 million for the years ended December 31, 2019, 2018, and 2017, respectively, and net cash used in operating activities of $85.0 million, $38.2 million, and $35.0 million for years ended December 31, 2019, 2018, and 2017, respectively. To date, none of the Company’s drug candidates have been approved for sale. 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 and clinical study trial accruals, fair value of assets and liabilities, the fair value of right-of-use assets (“ROU Assets”) and lease liabilities, revenue recognition, and the fair value of 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 U.S. GAAP establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances. Fair value is established as the exchange price, or exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, an established three-tier fair value hierarchy distinguishes between the following: • Level 1 inputs are quoted prices in active markets that are accessible at the market date for identical assets or liabilities. • Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. • Level 3 inputs are unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the assets or liability. Financial assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value instrument. The carrying amounts reflected on the balance sheets for cash and cash equivalents, prepaid expenses and other current assets, accounts payable, accrued compensation and accrued liabilities approximate their fair values due to their short-term nature. Cash, Cash Equivalents and Restricted Cash Cash equivalents, which consist primarily of highly liquid investments with original maturities of three 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, 2019 2018 Cash and cash equivalents $ 57,408 $ 52,183 Restricted cash 992 992 Total cash, cash equivalents and restricted cash $ 58,400 $ 53,175 Marketable Securities The Company invests its excess cash in investment grade short-term fixed income securities. Such investments in marketable securities are considered available for sale, and reported at fair value with unrealized gains and losses included as a component of accumulated other comprehensive income (loss). Marketable securities with maturities of greater than three months 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 income, net. The cost of securities sold is determined using specific identification method. The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other than temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Factors considered include quoted market prices, recent financial results and operating trends, implied values from any recent transactions or offers of investee securities, credit quality of debt instrument issuers, other publicly available information that may affect the value of the marketable security, duration and severity of the decline in value, and the Company’s strategy and intentions for holding the marketable security. To date the Company has not recorded any impairment charges on its marketable securities related to other-than-temporary declines in market value. 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, 2019, 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 clinical-stage immunotherapy companies, including dependence on key individuals; the need to develop commercially viable therapeutics; competition from other companies, many of which are larger and better capitalized; and the need to obtain adequate additional financing to fund the development of its products. The Company currently depends on third-party suppliers for key materials and services used in its research and development manufacturing process, and is subject to certain risks related to the loss of these third-party suppliers or their inability to supply the Company with adequate materials and services. 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 for 2018 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 was being depreciated over the lease term to a residual value that will approximate the remaining lease financing obligation at the end of the lease, and was derecognized on January 1, 2019 upon the Company’s adoption of Topic 842 (see Note 6). Long-Lived Assets The Company evaluates long-lived assets, including property and equipment and right-of-use operating lease assets, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the asset may not be fully recoverable. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of the asset and its eventual disposition are less than its carrying amount. Impairment, if any, is measured as the amount by which the carrying amount of a long-lived asset exceeds its fair value. There were no indicators of impairment of long-lived assets and no impairment losses have been recorded 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. Management may be required to exercise considerable judgment in estimating revenue to be recognized. Judgment is required in identifying performance obligations, estimating the transaction price, estimating the stand-alone selling prices of identified performance obligations, which may include forecasted revenue, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success, and estimating the progress towards satisfaction of performance obligations. Stock-Based Compensation The Company measures and recognizes compensation expense for all stock-based awards made to employees, directors, and non-employees based on the grant date estimated fair value of each award. Such expense is recognized on a straight-line basis over the requisite service period which is generally the vesting period for the entire award. Expense is adjusted for estimated forfeitures. Forfeitures of awards are estimated based on historical forfeiture experience and the experience of other companies in the same industry. The estimate of forfeitures will be adjusted over the service period to the extent that actual forfeitures differ, or are expected to differ, from prior estimates. The Company estimates the fair value of stock option grants using the Black-Scholes option-pricing model (“the Black-Scholes model”). The Black-Scholes model requires management to make assumptions and judgments about the variables used in the calculation, including the expected term (weighted-average period of time that the options granted are expected to be outstanding), the expected volatility of common stock, an assumed risk-free interest rate, and expected dividends the Company may pay. Management uses the simplified calculation of the expected term. Volatility is based on an average of the historical volatilities of the common stock of entities with characteristics similar to the Company’s. The risk-free rate is based on the U.S. Treasury yield curve in effect at the time of grant for periods corresponding with the expected life of the option. The Company uses an assumed dividend yield of zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock . Research and Development Expenses All research and development costs, including work performed by third parties, are expensed as incurred. Research and development costs consist of salaries and other personnel-related expenses, including associated non-cash stock-based compensation, consulting fees, laboratory supplies, and facility costs, as well as external research and development expenses incurred under arrangements with third parties, fees paid to other entities that conduct certain research and development activities on behalf of the Company, and costs incurred related to our Collaboration Agreement. Costs to develop the Company’s technologies are recorded as research and development expense unless certain costs which meet the criteria to be capitalized as internal-use software costs is met. Payments made prior to the receipt of goods or services to be used in research and development are capitalized until the goods are received or services are realized or consumed. Such payments are evaluated for current or long-term classification based on when they will be realized. The Company has and may continue to enter into license agreements to access and utilize certain technology. In each case, the Company evaluates of the license agreement results in the acquisition of an asset or a business. To date, none of the Company’s license agreements have been considered to be acquisitions of businesses. For asset acquisitions, the upfront payments to acquire such licenses, as well as any future milestone payments, are immediately recognized as research and development expense when paid, provided that there is no alternative future use of the rights in other research and development projects. These license agreements may also include contingent consideration in the form of cash payments to be made for future milestone events. The Company 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. Clinical and pre-clinical costs are a component of research and development expense. The Company accrues and expenses clinical and pre-clinical trial activities performed by third parties based upon actual work completed in accordance with agreements established with its service providers. The Company determines the actual costs through discussions with internal personnel and external service providers as to the progress or stage of completion of services and the agreed-upon fee to be paid for such services. Leases Prior to January 1, 2019, the Company rented its office space and facilities under non-cancelable operating lease agreements and recognizes related rent expense on a straight-line basis over the term of the lease. The Company’s lease agreements contained rent holidays, scheduled rent increases, and renewal options. Rent holidays and scheduled rent increases were included in the determination of rent expense to be recorded ratably over the lease term. The Company did not assume renewals in its determination of the lease term unless they were deemed to be reasonably assured at the inception of the lease. The Company began recognizing rent expense on the date that it obtained the legal right to use and control the leased space. Deferred rent consisted of the difference between cash payments and the recognition of rent expense on a straight-line basis for the buildings the Company occupied. Funding of leasehold improvements by the Company’s landlord was accounted for as a tenant improvement allowance and recorded as current and non-current deferred rent liabilities and amortized on a straight-line basis as a reduction of rent expense over the term of the lease. In certain arrangements, the Company was involved in the construction of improvements to buildings it is leasing. To the extent the Company was involved with the structural improvements of the construction project or takes construction risk, the Company was considered to be the owner of the building and related improvements for accounting purposes during the construction period. The Company recorded the fair value of the building and related improvements subject to the lease within property and equipment on the balance sheet. The Company also recorded 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 was complete, the Company considered the requirements for sale-leaseback accounting treatment. If the Company concludes the arrangement does not qualify for sale-leaseback accounting treatment, the building and related improvements remained on the Company’s balance sheet and were subject to depreciation and assessment of impairment. For such arrangements, at both pre and post the construction period, the Company bifurcated its lease payments into a portion allocated to the building and a portion allocated to the parcel of land on which the building had been built. The portion of the lease payments allocated to the land was treated for accounting purposes as operating lease payments, and therefore was recorded as rent expense in the statements of operations and comprehensive loss. The portion of the lease payments allocated to the building was further bifurcated into a portion allocated to interest expense and a portion allocated to reduce the lease financing obligation. The interest rate used for the lease financing obligation represents the Company’s estimated incremental borrowing rate at the inception of the lease, adjusted to reduce any built in loss. Subsequent to January 1, 2019, the Company determines whether the arrangement is or contains a lease at the inception of the arrangement and if such a lease is classified as a financing lease or operating lease. All of the Company’s leases are classified as operating leases. Leases with a term greater than one year are included in operating lease ROU assets, lease liabilities, current portion, and lease liabilities, net of current portion in our balance sheet at December 31, 2019. The Company has elected not to recognize on the balance sheet leases with terms of one year or less. Lease liabilities and their corresponding ROU assets are recorded based on the present value of lease payments over the expected lease term. In determining the net present value of lease payments, the interest rate implicit in lease contracts is typically not readily determinable. As such, the Company utilizes the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment. Certain adjustments to the right-of-use asset may be required for items such as initial direct costs paid or incentives received and impairment charges if we determine the ROU asset is impaired. The Company considers a lease term to be the noncancelable period that it has the right to use the underlying asset, including any periods where it is reasonably assured the Company will exercise the option to extend the contract. Periods covered by an option to extend are included in the lease term if the lessor controls the exercise of that option. The Company recognizes lease expense on a straight-line basis over the expected lease term. The Company has elected to not separate lease and non-lease components for its leased assets and accounts for all lease and non-lease components of its agreements as a single lease component. The lease components resulting in a ROU asset have been recorded on the balance sheet and amortized as lease expense on a straight-line basis over the lease term. Income Taxes The Company accounts for income taxes under the liability method. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates that will be in effect when the differences are expected to reverse. Valuation allowances are established when necessary to reduce deferred tax assets to the amounts expected to be realized. The Company recognizes and measures uncertain tax positions using a two—step approach set forth in authoritative guidance. The first step is to evaluate the tax position taken or expected to be taken by determining whether the weight of available evidence indicates that it is more likely than not that the tax position will be sustained in an audit, including resolution of any related appeals or litigation processes. The second step is to measure the tax benefit as the largest amount that is more than 50% likely to be realized upon ultimate settlement. Significant judgment is required to evaluate uncertain tax positions. The Company evaluates uncertain tax positions on a regular basis. The evaluations are based on a number of factors, including changes in facts and circumstances, changes in tax law, correspondence with tax authorities during the course of the audit, and effective settlement of audit issues. The provision for income taxes includes the effects of any accruals that the Company believes are appropriate. It is the Company’s policy to recognize interest and penalties related to income tax matters in income tax expense. Through December 31, 2019, 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 by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is the same as basic net loss per share, since the effects of potentially dilutive securities are antidilutive given the net loss for each period presented. Recently Issued Accounting Pronouncements In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses: Measurement of Credit Losses on Financial Instruments The standard is effective for interim and annual periods beginning after December 15, 2019. 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 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. In August 2018, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement . The new standard will be effective for fiscal years, and interim periods within those year, beginning after December 15, 2019. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes Recently Adopted Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842). Leases (Topic 842) Targeted Improvements “Topic 842”). The amendment has the same effective date and transition requirements as the new lease standard. The Company adopted this standard on January 1, 2019 using the modified retrospective approach and elected the pac

Cash Equivalents and Marketable

Cash Equivalents and Marketable Securities	12 Months Ended
	Dec. 31, 2019
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, 2019 Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents: Money market funds $ 42,133 $ — $ — $ 42,133 Commercial paper 1,749 — — 1,749 Corporate debt securities 2,500 — — 2,500 Total cash equivalents 46,382 — — 46,382 Short-term marketable securities: Certificates of deposit 631 — — 631 Commercial paper 31,476 15 — 31,491 Corporate debt securities 38,237 15 (6 ) 38,246 Total short-term marketable securities 70,344 30 (6 ) 70,368 Total $ 116,726 $ 30 $ (6 ) $ 116,750 December 31, 2018 Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents: Money market funds $ 36,148 $ — $ — $ 36,148 Commercial paper — — — — Corporate debt securities 12,047 — — 12,047 Total cash equivalents 48,195 — — 48,195 Short-term marketable securities: Commercial paper 45,244 — (40 ) 45,204 Corporate debt securities 55,768 1 (46 ) 55,723 Total short-term marketable securities 101,012 1 (86 ) 100,927 Total $ 149,207 $ 1 $ (86 ) $ 149,122 As of December 31, 2019 and 2018, the Company had a total of $127.8 million and $153.1 million in cash, cash equivalents and marketable securities, which includes $57.4 million and $52.2 million in cash and cash equivalents and $70.4 million and $100.9 million in marketable securities, respectively. All marketable securities held as of December 31, 2019, 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, 2019, 2018, or 2017. See Note 4 for further information regarding the fair value of the Company's financial instruments.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2019
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, 2019 Description Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 42,133 $ 42,133 $ — $ — Commercial paper 1,749 — 1,749 — Corporate debt securities 2,500 — 2,500 — Total cash equivalents 46,382 42,133 4,249 — Short-term marketable securities: Certificates of deposit 631 — 631 — Commercial paper 31,491 — 31,491 — Corporate debt securities 38,246 — 38,246 — Total short-term marketable securities 70,368 — 70,368 — Total $ 116,750 $ 42,133 $ 74,617 $ — December 31, 2018 Description Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 36,148 $ 36,148 $ — $ — Commercial paper — — — — Corporate debt securities 12,047 — 12,047 — Total cash equivalents 48,195 36,148 12,047 — Short-term marketable securities: Commercial paper 45,204 — 45,204 — Corporate debt securities 55,723 — 55,723 — Total short-term marketable securities 100,927 — 100,927 — Total $ 149,122 $ 36,148 $ 112,974 $ — The Company measures the fair value of money market funds based on quoted prices in active markets for identical securities. Commercial paper, certificates of deposit, and corporate debt securities are valued taking into consideration valuations obtained from third-party pricing services. The pricing services utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include reported trades of and broker/dealer quotes on the same or similar securities, issuer credit spreads; benchmark securities; prepayment/default projections based on historical data; and other observable inputs. There were no transfers between Level 1 and Level 2 during the periods presented.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 Computer equipment and software $ 863 $ 470 Furniture and fixtures 2,007 935 Laboratory equipment 21,025 16,406 Leasehold improvements 11,102 3,063 Construction-in-progress 2,224 — Buildings and related improvements capitalized under a lease financing transaction — 15,371 37,221 36,245 Less accumulated depreciation and amortization (10,310 ) (6,751 ) Total property and equipment, net $ 26,911 $ 29,494 Depreciation and amortization expense was $4.7 million, $4.0 million, and $2.0 million for the periods ended December 31, 2019, 2018, and 2017, respectively.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies Leases In November 2015, the Company entered into an 84-month non-cancelable operating lease, effective March 2016, for a new facility in Emeryville, California, with laboratory and office space. In conjunction with signing the lease, the Company paid a cash security deposit of $50,000. In January 2019, the Company entered into a 120-month operating lease for a new facility in Emeryville, California with office and laboratory space for the Company’s new principal executive offices. In conjunction with signing the lease, the Company paid a cash security deposit of $0.6 million, which is recorded as a deposit on the Company’s balance sheet as of December 31, 2019. The lease agreement includes a free rent period, an escalation clause for increased rent and a renewal provision allowing the Company to extend this lease for an additional two five-year periods at the then market rental rate. The lessor provided the Company a tenant improvement allowance for a total of $4.0 million to complete the laboratory and office renovation. The Company’s obligation to pay rent commenced on November 1, 2019. The Company has determined the tenant improvements to be lessee owned and therefore has recorded a $9.8 million ROU Asset and a $13.9 million lease liability on the balance sheet as of December 31, 2019. In connection with the new lease agreement, the Company also entered into an agreement (the “Lease Termination Agreement”) to early terminate the Company’s existing lease dated November 2015, for its current premises. The current lease will terminate effective no later than 60 days after the rent commencement date under the new lease, which is October 2019. The Company accounted for the Lease Termination Agreement as a separate contract and recorded an adjustment of $1.8 million, which is included within the December 31, 2019 balance sheet, to the ROU Asset and lease liability to reflect the remaining term of the modified agreement through October 2019. In February 2016, the Company entered into a 67-month non-cancellable operating lease effective October 2016 for a new facility in Cambridge, Massachusetts, with laboratory and office space. In conjunction with signing the lease, the Company paid a cash security deposit of $0.3 million. 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. Upon adoption of Topic 842, the deferred rent liability was reclassified against the ROU Asset on the balance sheet as of January 1, 2019 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, 2019, 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 had therefore 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 in the balance sheet as of December 31, 2018 Upon adoption of Topic 842, the Company analyzed the Pleasanton lease under the new guidance and determined that the lease would be classified as an operating lease under legacy GAAP. Additionally, given the Company had previously recognized the building and financing lease obligation solely as a result of the transactions build to suit designation under legacy GAAP, the Company derecognized the $14.5 million leased building and $10.5 million lease financing obligation from the balance sheet on January 1, 2019. The unamortized tenant improvement allowance of $4.0 million and was recognized as a component of ROU Assets on January 1, 2019. The Company also recorded a $0.2 million reduction to opening accumulated deficit as of January 1, 2019. In September 2018, the Company entered into a 24-month non-cancellable operating lease for an additional facility in Cambridge, Massachusetts with laboratory and office space. In conjunction with signing the lease, the Company prepaid the first twelve months base rent in the amount of $1.3 million of which $0.9 million as of January 1, 2019 was reclassified to the ROU Asset on the balance sheet upon adoption of Topic 842. The Company also paid a cash security deposit of $0.3 million, which included $0.1 million for the last month’s rent and was reclassified to the ROU Assets on the balance sheet upon adoption of Topic 842 on January 1, 2019. The remaining security deposit is recorded in deposits and other long-term assets on the Company’s balance sheet as of December 31, 2019 In July 2019, the Company amended its Cambridge, Massachusetts laboratory and office space facility lease. The amendment extended the original 24-month lease term ending in August 2020 by another 12 months through August 2021 and added additional leased laboratory and office space. Upon six months written notice, the Company has the right to terminate the amended lease agreement. The amendment provides for annual base rent of approximately $3.4 million, effective July 2019. In conjunction with signing the lease amendment, the Company prepaid an additional twelve months base rent for both the original leased space and the additional leased space in the amount of $3.2 million, which was reclassified to the ROU Asset on the balance sheet. The Company also paid a cash security deposit of $0.3 million, which included $0.1 million for the last month’s rent and was reclassified to the ROU Assets on the balance sheet. The remaining security deposit is recorded in deposits and other long-term assets on the Company’s balance sheet as of December 31, 2019. In May 2019, the Company entered into a 64-month non-cancellable operating lease for additional office space in Pleasanton, California. The lessor provided the Company a tenant improvement allowance for a total of $0.1 million to complete the office renovation. The Company’s obligation to pay rent commenced on August 1, 2019. The Company has determined the tenant improvements to be lessee owned and therefore has recorded a $0.3 million ROU Asset and a $0.5 million lease liability on the balance sheet as of December 31, 2019. The Company’s operating leases include various covenants, indemnities, defaults, termination rights, security deposits and other provisions customary for lease transactions of this nature. The components of lease costs, which were included in our statements of operations and comprehensive loss, were as follows (in thousands): Year ended December 31, 2019 Lease cost Operating lease cost $ 6,382 Short-term lease cost 271 Total lease cost $ 6,653 Supplemental information related to leases was as follows (in thousands): Year ended December 31, 2019 Cash paid for amounts included in the measurement of lease liabilities (in thousands): Operating cash flows from operating leases $ 2,495 New right-of-use assets obtained in exchange for lease obligations (in thousands): Operating leases $ 12,331 Weighted average remaining lease term (years): Operating leases 7.20 Weighted average discount rate: Operating leases 9.0 % As of December 31, 2019, minimum annual payments under the Company’s operating lease agreements are as follows (in thousands): Lease Financing Obligation Year ending December 31, 2020 $ 5,366 2021 6,359 2022 3,742 2023 3,459 2024 3,449 Thereafter 12,530 Total minimum payments 34,905 Less: Amounts representing interest expense (9,827 ) Less: Amounts representing tenant improvement allowance (1,588 ) Present value of future minimum lease payments 23,490 Less: Current portion of lease liability (2,505 ) Noncurrent portion of lease liability $ 20,985 The amounts representing the tenant improvement allowance are expected to be received by the Company in early 2020. Rent expense was $6.7 million, $1.8 million, and $1.2 million for the years ended December 31, 2019, 2018, and 2017, respectively. Agreements with CROs In September 2017, the Company entered into a contract research and development agreement with a third-party contract research organization (“CRO”) to provide research, analysis and antibody samples to further the Company’s development of its drug candidates In May 2019, the Company entered into a contract research and testing agreement with a third-party contract research organization to provide antibody discovery related services. The Company is obligated to pay the CRO certain milestone payments of up to $34.8 million on achievement of specified events. None of these events had occurred as of December 31, 2019. The Company recognized a total of $1.0 million in research and development expense under the agreement during the year ended December 31, 2019. Guarantees and Indemnifications The Company, as permitted under Delaware law and in accordance with its certification of incorporation and bylaws, and pursuant to indemnification agreements with certain of its officers and directors, indemnifies its officers and directors for certain events or occurrences, subject to certain limits, which the officer or director is or was serving at the Company’s request in such capacity. The term of the indemnification period lasts as long as an officer or director may be subject to any proceeding arising out of acts or omissions of such officer or director in such capacity. The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director and officer liability insurance. This insurance limits the Company’s exposure and may enable it to recover a portion of any future amounts paid. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, the Company has not recognized any liabilities relating to these obligations for any period presented.

Collaboration and License Agree

Collaboration and License Agreements	12 Months Ended
	Dec. 31, 2019
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, 2019 and no royalties were due from the sale of licensed products. In August 2019, the Company entered into a First Amendment to the Research Collaboration and License Agreement which extended the timeline for the Company and bluebird to execute a Patient Selection Services Agreement from within one year to within two years after the Effective Date of the Research Collaboration and License Agreement. The amendment was entered into for administrative purposes and the Company determined the amendment was not a modification of contract under ASC 606. 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, 2019. 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, 2019, the Company recognized $4.3 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 $14.5 million is recorded on the balance sheet in both current and long-term liabilities as of December 31, 2019, 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, 2019 are as follows (in thousands): Deferred Revenue Balance at December 31, 2018 $ 18,813 Additions — Deductions (4,297 ) Balance at December 31, 2019 $ 14,516 There were no receivables or net contract assets recorded as of December 31, 2019 associated with the Collaboration Agreement. The Company expensed all incremental costs of obtaining the Collaboration Agreement in 2018 as such amounts were insignificant. Arbutus Biopharma Corporation In October 2017, the Company entered into an Exclusive License Agreement with Arbutus Biopharma Corporation (“Arbutus”) and Protiva Biotherapeutics Inc. a wholly owned subsidiary of Arbutus and royalties on sales of its licensed products In August 2019, a milestone was met following the initial patient treatment of SLATE in the Company’s GO-005 clinical trial. In 2019, the Company recorded $3.0 million as research and development expense in connection with the milestone. Non-Profit Hospital Cancer Center In January 2016, the Company entered into an Exclusive License Agreement with a non-profit hospital cancer center. Under the license agreement, the Company has an exclusive license to utilize certain patents and know-how relating to immunotherapy for an insignificant upfront payment, cash milestone payments on achievement of specified events, and a low single digit royalty on sales of licensed products. The achievement of the milestones and payment of royalties is dependent upon obtaining regulatory approval. Upon achievement of a milestone related to the Company’s Phase 1 clinical trial for GRANITE, GO-004, in December 2018 the Company recorded an insignificant amount to research and development expense for amounts owed to the Hospital Cancer Center, which was paid to the hospital in February 2019.

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 Prepaid insurance $ 1,252 $ 966 Interest and other receivables 482 462 Prepaid research and development-related expenses 1,473 1,789 Prepaid rent — 860 Other 290 449 Total prepaid expenses and other current assets $ 3,497 $ 4,526 Deposits and Other Long-Term Assets Deposits and other long-term assets consist of the following (in thousands): December 31, 2019 2018 Lease security deposit $ 1,201 $ 632 Prepaid research and development-related expenses 585 554 Restricted cash 992 992 Other — 250 Total deposits and other long-term assets $ 2,778 $ 2,428 Accrued Liabilities Accrued current liabilities consist of the following (in thousands): December 31, 2019 2018 Deferred rent $ — $ 445 Property and equipment 761 — Other 280 295 Total accrued current liabilities $ 1,041 $ 740

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Stockholders' Equity	9 . Stockholders’ Equity In connection with the completion of its IPO, on October 2, 2018, the Company’s certificate of incorporation was amended and restated to provide for 300,000,000 authorized shares of common stock with a par value of $0.0001 per share and 10,000,000 authorized shares of preferred stock with a par value of $0.0001 per share. The Company has 10,000,000 shares of preferred stock authorized for issuance, par value of $0.0001 per share. As of December 31, 2019 and 2018, no shares of preferred stock were issued and outstanding. The Company has 300,000,000 shares of common stock authorized for issuance, par value of $0.0001 per share. Holders of the Company’s common stock are entitled to one vote per share. As of December 31, 2019 and 2018, there were 36,332,956 and 28,823,130 shares of common stock issued and outstanding. Sale of Common and Preferred Stock 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. The Company entered into a Series C preferred stock purchase agreement, with certain investors in June 2018, and upon approval by the Company’s Board of Directors, the Company completed a Series C convertible preferred stock financing (“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. 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 $92.5 million, after deducting underwriting discounts and commissions and offering costs. In connection with the IPO, all of the Company’s outstanding shares of convertible preferred stock were automatically converted into 19,409,132 shares of common stock. The related carrying value of $177.9 million was reclassified to common stock and additional paid-in capital. In April 2019, the Company completed an underwritten public offering and sold and issued an aggregate of 6,500,000 shares of common stock at a price to the public of $11.50 per share. The Company received aggregate net proceeds from the offering of approximately $69.7 million, after deducting underwriting discounts and commissions and offering costs. In October 2019, the Company filed a Registration Statement on Form S-3 (the “Shelf Registration Statement”) with the SEC, covering the offering of up to $250.0 million of common stock, preferred stock, debt securities, warrants and units. The Shelf Registration Statement included a prospectus covering the offering, issuance and sale of up to $75.0 million of the Company’s common stock from time to time through an at the market offering under the Securities Act of 1933, as amended (the “ATM Offering Program”). The SEC declared the Shelf Registration Statement effective on November 8, 2019. In October 2019, the Company also entered into a sales agreement (the “Sales Agreement”) with Cowen and Company, LLC (“Cowen”) to sell shares of the Company’s common stock, from time to time, with aggregate gross sales proceeds of up to $75.0 million, through an at the market offering under which Cowen will act as its sales agent Cowen is entitled to compensation for its services equal to up to 3.0% of the gross proceeds of any shares of common stock sold through Cowen under the Sales Agreement. In addition, the Company has agreed to reimburse a portion of the expenses of Cowen in connection with the offering up to a maximum of $50,000. During the year ended December 31, 2019, the Company issued and sold 490,880 shares of its common stock through its at the market offering and received net proceeds of approximately $3.8 million, after deducting commissions of $0.1 million and other offering expenses of $0.3 million.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	10 . Stock-Based Compensation Award Incentive Plans In August 2015, the Board of Directors approved the 2015 Equity Incentive Plan (“2015 Plan”). In 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. Prior to the Company’s IPO, the grant date fair value of the Company’s common stock was determined by the Company’s Board of Directors with the assistance of management and an independent third-party valuation specialist. Subsequent to the Company’s IPO, the grant date fair value of each share of common stock underlying stock option awards is based on the closing price of our common stock as reported by the Nasdaq Select Global Market on the date of grant of the award. The Company’s Board of Directors has the authority to determine to whom options will be granted, the number of shares, the term, and the exercise price. If an individual owns stock representing 10% or more of the outstanding shares, the price of each share shall be at least 110% of the fair market value, as determined by the board of directors. Options granted have a term of up to 10 years and generally vest over a 4-year period with a straight-line vesting. 2018 Employee Stock Purchase Plan In September 2018, the Company’s Board of Directors approved the 2018 Employee Stock Purchase Plan (“2018 ESPP”). The 2018 ESPP also became effective in September 2018. The offering periods are scheduled to start on the first trading day on or after June 1 or December 1 of each year. Contributions under the 2018 ESPP are limited to a maximum of 15% of an employee’s eligible compensation. The estimated fair value of stock purchase rights granted under the ESPP were calculated using the Black-Scholes option-pricing model using the following assumptions: Year ended December 31, 2019 Expected dividend yield — % Expected term 0.50 year Risk-free interest rate 2.2 % Expected volatility 74.0 % Valuation of Stock Options The fair value of each stock option granted to an employee or a director was estimated as of the date of grant using the Black-Scholes model with the following weighted-average assumptions: Year Ended December 31, 2019 2018 2017 Expected dividend yield — % — % — % Expected term 6.01 years 6.04 years 6.04 years Risk-free interest rate 2.3 % 2.8 % 2.0 % Expected volatility 83.0 % 88.0 % 94.0 % Management’s calculations are based on a grant date valuation approach. Using the Black-Scholes model, the weighted-average grant-date fair value of employee stock options granted was $8.08, $6.59, and $2.00 per share during the years ended December 31, 2019, 2018, and 2017, 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, 2018 2,695,110 2,429,859 $ 5.31 8.86 $ 25,646 Authorized 1,160,000 Granted (1,312,610 ) 1,312,610 $ 11.30 Exercised — (281,024 ) $ 1.61 Cancelled 295,484 (295,484 ) $ 8.46 Repurchased 13,899 — $ 0.35 Balance at December 31, 2019 2,851,883 3,165,961 $ 7.83 8.45 $ 8,875 Vested and exercisable – December 31, 2019 1,124,222 $ 5.16 7.88 $ 5,433 Vested and expected to vest – December 31, 2019 2,939,089 $ 7.54 8.38 $ 8,864 For the years ended December 31, 2019, 2018, and 2017, the total intrinsic value of stock option awards exercised was $2.97 million, $0.75 million, and $0.08 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, 2019, $11.5 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 2.76 Stock-based compensation expense and awards granted to non-employees was insignificant for the years ended December 31, 2019, 2018, and December 31, 2017. Stock-Based Compensation Expense Total stock-based compensation for all awards granted to employees and consultants and our 2018 ESPP Plan Year Ended December 31, 2019 2018 2017 Research and development expenses $ 3,437 $ 2,081 $ 888 General and administrative expenses 1,862 914 238 Total $ 5,299 $ 2,995 $ 1,126

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	1 1 . Income Taxes The effective tax rate for the years ended December 31, 2019, 2018 and 2017 is different from the federal statutory rate primarily due to the valuation allowance against deferred tax assets as a result of insufficient sources of income. The effective tax rate of the of the Company’s provision for income taxes differs from the federal statutory rate as follows: Year Ended December 31, 2019 2018 2017 Statutory federal income tax rate 21.0 % 21.0 % 34.0 % State tax, net of federal benefit 4.2 4.3 2.2 Permanent differences (0.8 ) (1.0 ) (2.3 ) Effective change in enacted tax rate — — (17.3 ) Research and development tax credits 2.6 4.4 4.1 Other (0.8 ) (0.2 ) — Change in valuation allowance (26.3 ) (28.5 ) (20.7 ) Total provision for income taxes — % — % — % On December 22, 2017, the U.S. federal government enacted the Tax Cuts and Jobs Act (the “Tax Act”). The Tax Act contains, among other things, significant changes to corporate taxation, including reduction of the corporate tax rate from a top marginal rate of 35% to a flat rate of 21% for tax years beginning after December 31, 2017, limitation of the deduction for net operating losses to 80% of current year taxable income and elimination of net operating loss carrybacks, implementing a territorial tax system. Pursuant to SAB 118, an entity may select between one of three scenarios to determine a reasonable estimate arising from the Tax Act. The scenarios are (i) a final estimate which effectively closes the measurement window; (ii) a reasonable estimate leaving the measurement window open for future revisions; and (iii) no estimate as the law is still being analyzed. The Company was able to provide a reasonable estimate for the revaluation of deferred taxes. As such, the Company recorded a $7.1 million reduction in deferred tax assets for the revaluation of deferred taxes in 2017 which was offset by a corresponding decrease to the Company’s full valuation allowance. The ultimate impact of the Act did not differ materially from provision amounts recorded. Adjustments, if any, would not have impacted the statement of operations and comprehensive loss due to the full valuation allowance on the Company’s deferred tax assets. 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, 2019 and 2018. The deferred tax assets were primarily comprised of federal and state tax net operating losses and tax credit carryforwards. Utilization of the net operating loss and tax credit carryforwards may be subject to an annual limitation due to historical or future ownership percentage change rules provided by the Internal Review Code of 1986, and similar state provisions. The annual limitation may result in the expiration of certain net operating loss and tax credit carryforwards before their utilization. The valuation allowance increased by $24.9 million during 2019 and increased by $18.4 million during 2018. The components of the net deferred tax assets/liabilities are as follows (in thousands): December 31, 2019 2018 Deferred tax assets: Federal and state net operating loss carryforwards $ 45,228 $ 27,311 Research and development tax credits 6,205 4,503 Lease financing obligation — 2,658 Lease liabilities 5,920 — Accruals and other 2,172 1,387 Amortization 2,700 1,726 Deferred Revenue 3,659 — Deferred tax liabilities: Other depreciation (638 ) (655 ) Leased building depreciation — (2,446 ) Operating lease right-of-use assets (5,904 ) — Total net deferred tax assets 59,342 34,484 Less valuation allowance (59,342 ) (34,484 ) Deferred tax assets, net of allowance $ — $ — At December 31, 2019, the Company’s federal and state income tax net operating loss carryforwards were approximately $180.8 million and $113.3 million, respectively, which may be subject to limitations as described below. If not utilized, the federal tax loss carryforwards will begin to expire in 2035 and the state tax loss carryforwards will begin to expire in 2035. Under the Tax Act, federal net operating losses generated after 2017 and in future years may be carried forward indefinitely, but the deductibility of such federal net operating losses is limited. The federal net operating losses generated after December 31, 2017 of $130.2 million are carried forward indefinitely. 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, 2019 under IRC Sections 382 and 383 to determine if the Company’s net operating loss carryforwards and research and development credits are limited due to a change in ownership. In connection with the Company’s IPO which closed in October 2018, the Company did experience an ownership change pursuant to Section 382. There was no reduction in federal or California net operating loss carryforwards or research and development income tax credits as a result of this ownership change. Other than the change identified in connection with the Company’s IPO, through December 31, 2019, the Company did not experience any other ownership change pursuant to Section 382. The following table summarizes the activity related to the Company’s unrecognized tax benefits (in thousands): December 31, 2019 2018 2017 Beginning of year—unrecognized tax benefits $ 2,077 $ 1,089 $ 230 Decrease for tax positions taken during prior periods (711 ) (453 ) (47 ) Increases for tax positions taken during current period 1,244 1,441 906 End of year—unrecognized tax benefits $ 2,610 $ 2,077 $ 1,089 If recognized, none of the unrecognized tax benefits as of December 31, 2019 and 2018 would reduce the annual effective tax rate, primarily due to corresponding adjustments to the valuation allowance. The Company does not expect any material changes to the estimated amount of liability associated with its uncertain tax positions within the next 12 months. During the years ended December 31, 2019, 2018, and 2017, the Company did not recognize accrued interest and penalties related to unrecognized tax benefits. The Company files income tax returns in the U.S. federal jurisdiction and various state jurisdictions. The Company is not currently under audit by the Internal Revenue Service or other similar state or local authorities. Due to the net operating loss carryforwards, all years remain open for income tax examination by tax authorities in the United States, various states and foreign tax jurisdictions in which the Company files tax returns.

Net Loss Per Common Share

Net Loss Per Common Share	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Net Loss Per Common Share	1 2 . Net Loss Per Common Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is the same as basic net loss per share, since the effects of potentially dilutive securities are antidilutive given the net loss for each period presented. 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, 2019 2018 2017 Numerator: Net loss $ (94,433 ) $ (64,775 ) $ (41,377 ) Denominator: Weighted-average common shares outstanding, basic and diluted 33,554,823 8,919,281 1,999,044 Net loss per share, basic and diluted $ (2.81 ) $ (7.26 ) $ (20.70 ) 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, 2019 2018 2017 Convertible preferred stock — — 17,797,529 Options issued and outstanding and ESPP shares issuable and outstanding 3,179,041 2,429,859 1,351,840 Early exercised common stock subject to future vesting 30,874 226,967 539,289 Warrants to purchase common stock — — 40,257 Total 3,209,915 2,656,826 19,728,915

Related-Party Transactions

Related-Party Transactions	12 Months Ended
	Dec. 31, 2019
Related Party Transactions [Abstract]
Related-Party Transactions	1 3 . Related-Party Transactions During the year ended December 31, 2018, the Company issued 333,333 shares of common stock for total net proceeds of $5.0 million to certain stockholders considered to be related parties.

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2019
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	1 4 . Defined Contribution Plan The Company began sponsoring a 401(k) Plan in 2017 which provides that eligible employees can elect to contribute to the 401(k) Plan, subject to certain limitations, on a pretax basis. The Company matches up to 50% of the first 4% of each employee’s contribution. During the years ended December 31, 2019 and 2018, expenses recognized for the 401(k) Plan were insignificant.

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2019
Subsequent Events [Abstract]
Subsequent Event	1 5 . Subsequent Event Subsequent to December 31, 2019, the Company issued an additional 568,369 shares of common stock through its ATM Offering Program resulting in net proceeds to the Company of approximately $5.6 million.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (Unaudited)	1 6 . 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 2019 and 2018 and has been prepared in accordance with GAAP for interim financial reporting (in thousands, except per share amounts). In the opinion of management, all adjustments (consisting of normal recurring accruals) considered necessary for a fair presentation have been included. Quarter Ended December 31, 2019 September 30, 2019 June 30, 2019 March 31, 2019 Collaboration revenue $ 884 $ 984 $ 1,150 $ 1,347 Loss from operations $ (28,313 ) $ (28,484 ) $ (22,214 ) $ (18,929 ) Net loss $ (27,704 ) $ (27,548 ) $ (21,172 ) $ (18,009 ) Net loss per share, basic and diluted $ (0.77 ) $ (0.77 ) $ (0.63 ) $ (0.62 ) 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 )

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	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.
Need for Additional Capital	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 $221.0 million and $126.4 million as of December 31, 2019 and 2018, respectively. The Company had net losses of $94.4 million, $64.8 million, and $41.4 million for the years ended December 31, 2019, 2018, and 2017, respectively, and net cash used in operating activities of $85.0 million, $38.2 million, and $35.0 million for years ended December 31, 2019, 2018, and 2017, respectively. To date, none of the Company’s drug candidates have been approved for sale. 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	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and judgments that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities as of the date of the financial statements and the reported amounts of expenses during the reporting period. On an ongoing basis, management evaluates its estimates, including those related to preclinical and clinical study trial accruals, fair value of assets and liabilities, the fair value of right-of-use assets (“ROU Assets”) and lease liabilities, revenue recognition, and the fair value of 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	Fair Value of Financial Instruments U.S. GAAP establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances. Fair value is established as the exchange price, or exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, an established three-tier fair value hierarchy distinguishes between the following: • Level 1 inputs are quoted prices in active markets that are accessible at the market date for identical assets or liabilities. • Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. • Level 3 inputs are unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the assets or liability. Financial assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value instrument. The carrying amounts reflected on the 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, Cash Equivalents and Restricted Cash Cash equivalents, which consist primarily of highly liquid investments with original 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, 2019 2018 Cash and cash equivalents $ 57,408 $ 52,183 Restricted cash 992 992 Total cash, cash equivalents and restricted cash $ 58,400 $ 53,175
Marketable Securities	Marketable Securities The Company invests its excess cash in investment grade short-term fixed income securities. Such investments in marketable securities are considered available for sale, and reported at fair value with unrealized gains and losses included as a component of accumulated other comprehensive income (loss). Marketable securities with maturities of greater than three months 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 income, net. The cost of securities sold is determined using specific identification method. The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other than temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Factors considered include quoted market prices, recent financial results and operating trends, implied values from any recent transactions or offers of investee securities, credit quality of debt instrument issuers, other publicly available information that may affect the value of the marketable security, duration and severity of the decline in value, and the Company’s strategy and intentions for holding the marketable security. To date the Company has not recorded any impairment charges on its marketable securities related to other-than-temporary declines in market value.
Concentrations of Credit Risk and Other Risks and Uncertainties	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, 2019, 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 clinical-stage immunotherapy companies, including dependence on key individuals; the need to develop commercially viable therapeutics; competition from other companies, many of which are larger and better capitalized; and the need to obtain adequate additional financing to fund the development of its products. The Company currently depends on third-party suppliers for key materials and services used in its research and development manufacturing process, and is subject to certain risks related to the loss of these third-party suppliers or their inability to supply the Company with adequate materials and services.
Property and Equipment, Net	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 for 2018 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 was being depreciated over the lease term to a residual value that will approximate the remaining lease financing obligation at the end of the lease, and was derecognized on January 1, 2019 upon the Company’s adoption of Topic 842 (see Note 6).
Long-Lived Assets	Long-Lived Assets The Company evaluates long-lived assets, including property and equipment and right-of-use operating lease assets, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the asset may not be fully recoverable. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of the asset and its eventual disposition are less than its carrying amount. Impairment, if any, is measured as the amount by which the carrying amount of a long-lived asset exceeds its fair value. There were no indicators of impairment of long-lived assets and no impairment losses have been recorded for the periods presented.
Revenue Recognition	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. Management may be required to exercise considerable judgment in estimating revenue to be recognized. Judgment is required in identifying performance obligations, estimating the transaction price, estimating the stand-alone selling prices of identified performance obligations, which may include forecasted revenue, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success, and estimating the progress towards satisfaction of performance obligations.
Share-Based Compensation	Stock-Based Compensation The Company measures and recognizes compensation expense for all stock-based awards made to employees, directors, and non-employees based on the grant date estimated fair value of each award. Such expense is recognized on a straight-line basis over the requisite service period which is generally the vesting period for the entire award. Expense is adjusted for estimated forfeitures. Forfeitures of awards are estimated based on historical forfeiture experience and the experience of other companies in the same industry. The estimate of forfeitures will be adjusted over the service period to the extent that actual forfeitures differ, or are expected to differ, from prior estimates. The Company estimates the fair value of stock option grants using the Black-Scholes option-pricing model (“the Black-Scholes model”). The Black-Scholes model requires management to make assumptions and judgments about the variables used in the calculation, including the expected term (weighted-average period of time that the options granted are expected to be outstanding), the expected volatility of common stock, an assumed risk-free interest rate, and expected dividends the Company may pay. Management uses the simplified calculation of the expected term. Volatility is based on an average of the historical volatilities of the common stock of entities with characteristics similar to the Company’s. The risk-free rate is based on the U.S. Treasury yield curve in effect at the time of grant for periods corresponding with the expected life of the option. The Company uses an assumed dividend yield of zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock .
Research and Development Expenses	Research and Development Expenses All research and development costs, including work performed by third parties, are expensed as incurred. Research and development costs consist of salaries and other personnel-related expenses, including associated non-cash stock-based compensation, consulting fees, laboratory supplies, and facility costs, as well as external research and development expenses incurred under arrangements with third parties, fees paid to other entities that conduct certain research and development activities on behalf of the Company, and costs incurred related to our Collaboration Agreement. Costs to develop the Company’s technologies are recorded as research and development expense unless certain costs which meet the criteria to be capitalized as internal-use software costs is met. Payments made prior to the receipt of goods or services to be used in research and development are capitalized until the goods are received or services are realized or consumed. Such payments are evaluated for current or long-term classification based on when they will be realized. The Company has and may continue to enter into license agreements to access and utilize certain technology. In each case, the Company evaluates of the license agreement results in the acquisition of an asset or a business. To date, none of the Company’s license agreements have been considered to be acquisitions of businesses. For asset acquisitions, the upfront payments to acquire such licenses, as well as any future milestone payments, are immediately recognized as research and development expense when paid, provided that there is no alternative future use of the rights in other research and development projects. These license agreements may also include contingent consideration in the form of cash payments to be made for future milestone events. The Company 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. Clinical and pre-clinical costs are a component of research and development expense. The Company accrues and expenses clinical and pre-clinical trial activities performed by third parties based upon actual work completed in accordance with agreements established with its service providers. The Company determines the actual costs through discussions with internal personnel and external service providers as to the progress or stage of completion of services and the agreed-upon fee to be paid for such services.
Leases	Leases Prior to January 1, 2019, the Company rented its office space and facilities under non-cancelable operating lease agreements and recognizes related rent expense on a straight-line basis over the term of the lease. The Company’s lease agreements contained rent holidays, scheduled rent increases, and renewal options. Rent holidays and scheduled rent increases were included in the determination of rent expense to be recorded ratably over the lease term. The Company did not assume renewals in its determination of the lease term unless they were deemed to be reasonably assured at the inception of the lease. The Company began recognizing rent expense on the date that it obtained the legal right to use and control the leased space. Deferred rent consisted of the difference between cash payments and the recognition of rent expense on a straight-line basis for the buildings the Company occupied. Funding of leasehold improvements by the Company’s landlord was accounted for as a tenant improvement allowance and recorded as current and non-current deferred rent liabilities and amortized on a straight-line basis as a reduction of rent expense over the term of the lease. In certain arrangements, the Company was involved in the construction of improvements to buildings it is leasing. To the extent the Company was involved with the structural improvements of the construction project or takes construction risk, the Company was considered to be the owner of the building and related improvements for accounting purposes during the construction period. The Company recorded the fair value of the building and related improvements subject to the lease within property and equipment on the balance sheet. The Company also recorded 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 was complete, the Company considered the requirements for sale-leaseback accounting treatment. If the Company concludes the arrangement does not qualify for sale-leaseback accounting treatment, the building and related improvements remained on the Company’s balance sheet and were subject to depreciation and assessment of impairment. For such arrangements, at both pre and post the construction period, the Company bifurcated its lease payments into a portion allocated to the building and a portion allocated to the parcel of land on which the building had been built. The portion of the lease payments allocated to the land was treated for accounting purposes as operating lease payments, and therefore was recorded as rent expense in the statements of operations and comprehensive loss. The portion of the lease payments allocated to the building was further bifurcated into a portion allocated to interest expense and a portion allocated to reduce the lease financing obligation. The interest rate used for the lease financing obligation represents the Company’s estimated incremental borrowing rate at the inception of the lease, adjusted to reduce any built in loss. Subsequent to January 1, 2019, the Company determines whether the arrangement is or contains a lease at the inception of the arrangement and if such a lease is classified as a financing lease or operating lease. All of the Company’s leases are classified as operating leases. Leases with a term greater than one year are included in operating lease ROU assets, lease liabilities, current portion, and lease liabilities, net of current portion in our balance sheet at December 31, 2019. The Company has elected not to recognize on the balance sheet leases with terms of one year or less. Lease liabilities and their corresponding ROU assets are recorded based on the present value of lease payments over the expected lease term. In determining the net present value of lease payments, the interest rate implicit in lease contracts is typically not readily determinable. As such, the Company utilizes the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment. Certain adjustments to the right-of-use asset may be required for items such as initial direct costs paid or incentives received and impairment charges if we determine the ROU asset is impaired. The Company considers a lease term to be the noncancelable period that it has the right to use the underlying asset, including any periods where it is reasonably assured the Company will exercise the option to extend the contract. Periods covered by an option to extend are included in the lease term if the lessor controls the exercise of that option. The Company recognizes lease expense on a straight-line basis over the expected lease term. The Company has elected to not separate lease and non-lease components for its leased assets and accounts for all lease and non-lease components of its agreements as a single lease component. The lease components resulting in a ROU asset have been recorded on the balance sheet and amortized as lease expense on a straight-line basis over the lease term.
Income Taxes	Income Taxes The Company accounts for income taxes under the liability method. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates that will be in effect when the differences are expected to reverse. Valuation allowances are established when necessary to reduce deferred tax assets to the amounts expected to be realized. The Company recognizes and measures uncertain tax positions using a two—step approach set forth in authoritative guidance. The first step is to evaluate the tax position taken or expected to be taken by determining whether the weight of available evidence indicates that it is more likely than not that the tax position will be sustained in an audit, including resolution of any related appeals or litigation processes. The second step is to measure the tax benefit as the largest amount that is more than 50% likely to be realized upon ultimate settlement. Significant judgment is required to evaluate uncertain tax positions. The Company evaluates uncertain tax positions on a regular basis. The evaluations are based on a number of factors, including changes in facts and circumstances, changes in tax law, correspondence with tax authorities during the course of the audit, and effective settlement of audit issues. The provision for income taxes includes the effects of any accruals that the Company believes are appropriate. It is the Company’s policy to recognize interest and penalties related to income tax matters in income tax expense. Through December 31, 2019, the Company had not accrued interest or penalties related to uncertain tax positions.
Comprehensive Loss	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 Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is the same as basic net loss per share, since the effects of potentially dilutive securities are antidilutive given the net loss for each period presented.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses: Measurement of Credit Losses on Financial Instruments The standard is effective for interim and annual periods beginning after December 15, 2019. 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 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. In August 2018, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement . The new standard will be effective for fiscal years, and interim periods within those year, beginning after December 15, 2019. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842). Leases (Topic 842) Targeted Improvements “Topic 842”). The amendment has the same effective date and transition requirements as the new lease standard. The Company adopted this standard on January 1, 2019 using the modified retrospective approach and elected the package of practical expedients permitted under transition guidance, which allowed the Company to carry forward its historical assessments of: 1) whether contracts are or contain leases, 2) lease classification and 3) initial direct costs, where applicable. The Company did not elect the practical expedient allowing the use-of-hindsight which would require the Company to reassess the lease term of its leases based on all facts and circumstances through the effective date and did not elect the practical expedient pertaining to land easements as this is not applicable to the current contract portfolio. The Company elected the post-transition practical expedient to not separate lease components from non-lease components for all existing lease classes. The Company also elected a policy of not recording leases on its balance sheets when the leases have a term of 12 months or less and the Company is not reasonably certain to elect an option to purchase the leased asset. The impact of the adoption of Topic 842 on the balance sheet as of January 1, 2019 was as follows (in thousands): December 31, 2018 Adjustments due to Adoption of Topic 842 January 1, 2019 Property and equipment, net $ 29,494 $ (14,524 ) $ 14,970 Operating lease right-of-use assets — 14,224 14,224 Operating liabilities: Lease liabilities, current portion — 2,200 2,200 Accrued liabilities 740 (475 ) 265 Deferred rent, net of current portion 1,353 (1,353 ) — Lease financing obligation, net of current portion 10,490 (10,490 ) — Lease liabilities, net of current portion — 8,980 8,980 Accumulated deficit (126,402 ) (153 ) (126,555 ) The adjustments due to the adoption of Topic 842 primarily related to the recognition of operating lease ROU assets and lease liabilities for the Company’s operating leases. In addition, the adoption of Topic 842 resulted in a change in classification of build-to-suit component of our lease in Pleasanton, California to an operating lease and resulted in the derecognition of the $15.4 million capitalized building and related accumulated depreciation of $0.9 million and $10.5 million financing lease obligation, as the Company had been deemed to own the building under legacy GAAP (Note 6). The Company also recorded an insignificant reduction to opening accumulated deficit as of January 1, 2019 as a result of the adoption of Topic 842. The impact of the adoption of Topic 842 on the statements of operations and comprehensive loss was not material. In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting Compensation—Stock Compensation

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 Cash and cash equivalents $ 57,408 $ 52,183 Restricted cash 992 992 Total cash, cash equivalents and restricted cash $ 58,400 $ 53,175
Estimated Useful Lives of Assets	Depreciation is calculated using the straight-line method over the estimated useful lives of the respective assets, which are as follows: Asset Estimated Useful Life Computer equipment and software 3 to 5 years Furniture and fixtures 5 years Laboratory equipment 5 to 7 years Leasehold improvements Shorter of useful life or lease term
Impact of Adoption of Topic 842 on the Balance Sheet	The impact of the adoption of Topic 842 on the balance sheet as of January 1, 2019 was as follows (in thousands): December 31, 2018 Adjustments due to Adoption of Topic 842 January 1, 2019 Property and equipment, net $ 29,494 $ (14,524 ) $ 14,970 Operating lease right-of-use assets — 14,224 14,224 Operating liabilities: Lease liabilities, current portion — 2,200 2,200 Accrued liabilities 740 (475 ) 265 Deferred rent, net of current portion 1,353 (1,353 ) — Lease financing obligation, net of current portion 10,490 (10,490 ) — Lease liabilities, net of current portion — 8,980 8,980 Accumulated deficit (126,402 ) (153 ) (126,555 )

Cash Equivalents and Marketab_2

Cash Equivalents and Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents: Money market funds $ 42,133 $ — $ — $ 42,133 Commercial paper 1,749 — — 1,749 Corporate debt securities 2,500 — — 2,500 Total cash equivalents 46,382 — — 46,382 Short-term marketable securities: Certificates of deposit 631 — — 631 Commercial paper 31,476 15 — 31,491 Corporate debt securities 38,237 15 (6 ) 38,246 Total short-term marketable securities 70,344 30 (6 ) 70,368 Total $ 116,726 $ 30 $ (6 ) $ 116,750 December 31, 2018 Description Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents: Money market funds $ 36,148 $ — $ — $ 36,148 Commercial paper — — — — Corporate debt securities 12,047 — — 12,047 Total cash equivalents 48,195 — — 48,195 Short-term marketable securities: Commercial paper 45,244 — (40 ) 45,204 Corporate debt securities 55,768 1 (46 ) 55,723 Total short-term marketable securities 101,012 1 (86 ) 100,927 Total $ 149,207 $ 1 $ (86 ) $ 149,122

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 Description Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 42,133 $ 42,133 $ — $ — Commercial paper 1,749 — 1,749 — Corporate debt securities 2,500 — 2,500 — Total cash equivalents 46,382 42,133 4,249 — Short-term marketable securities: Certificates of deposit 631 — 631 — Commercial paper 31,491 — 31,491 — Corporate debt securities 38,246 — 38,246 — Total short-term marketable securities 70,368 — 70,368 — Total $ 116,750 $ 42,133 $ 74,617 $ — December 31, 2018 Description Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 36,148 $ 36,148 $ — $ — Commercial paper — — — — Corporate debt securities 12,047 — 12,047 — Total cash equivalents 48,195 36,148 12,047 — Short-term marketable securities: Commercial paper 45,204 — 45,204 — Corporate debt securities 55,723 — 55,723 — Total short-term marketable securities 100,927 — 100,927 — Total $ 149,122 $ 36,148 $ 112,974 $ —

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment and related accumulated depreciation and amortization are as follows (in thousands): December 31, 2019 2018 Computer equipment and software $ 863 $ 470 Furniture and fixtures 2,007 935 Laboratory equipment 21,025 16,406 Leasehold improvements 11,102 3,063 Construction-in-progress 2,224 — Buildings and related improvements capitalized under a lease financing transaction — 15,371 37,221 36,245 Less accumulated depreciation and amortization (10,310 ) (6,751 ) Total property and equipment, net $ 26,911 $ 29,494

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Schedule of Components of Lease Costs	The components of lease costs, which were included in our statements of operations and comprehensive loss, were as follows (in thousands): Year ended December 31, 2019 Lease cost Operating lease cost $ 6,382 Short-term lease cost 271 Total lease cost $ 6,653
Schedule of Supplemental Information Related to Leases	Supplemental information related to leases was as follows (in thousands): Year ended December 31, 2019 Cash paid for amounts included in the measurement of lease liabilities (in thousands): Operating cash flows from operating leases $ 2,495 New right-of-use assets obtained in exchange for lease obligations (in thousands): Operating leases $ 12,331 Weighted average remaining lease term (years): Operating leases 7.20 Weighted average discount rate: Operating leases 9.0 %
Schedule of Minimum Annual Rental Payments Under Operating Lease Agreements	As of December 31, 2019, minimum annual payments under the Company’s operating lease agreements are as follows (in thousands): Lease Financing Obligation Year ending December 31, 2020 $ 5,366 2021 6,359 2022 3,742 2023 3,459 2024 3,449 Thereafter 12,530 Total minimum payments 34,905 Less: Amounts representing interest expense (9,827 ) Less: Amounts representing tenant improvement allowance (1,588 ) Present value of future minimum lease payments 23,490 Less: Current portion of lease liability (2,505 ) Noncurrent portion of lease liability $ 20,985

Collaboration and License Agr_2

Collaboration and License Agreements (Tables)	12 Months Ended
	Dec. 31, 2019
Collaborative And License Agreements [Abstract]
Schedule of Changes in Deferred Revenue Balance	Changes in the deferred revenue balance during the year ended December 31, 2019 are as follows (in thousands): Deferred Revenue Balance at December 31, 2018 $ 18,813 Additions — Deductions (4,297 ) Balance at December 31, 2019 $ 14,516

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 Prepaid insurance $ 1,252 $ 966 Interest and other receivables 482 462 Prepaid research and development-related expenses 1,473 1,789 Prepaid rent — 860 Other 290 449 Total prepaid expenses and other current assets $ 3,497 $ 4,526
Schedule of Deposits and Other Long-Term Assets	Deposits and other long-term assets consist of the following (in thousands): December 31, 2019 2018 Lease security deposit $ 1,201 $ 632 Prepaid research and development-related expenses 585 554 Restricted cash 992 992 Other — 250 Total deposits and other long-term assets $ 2,778 $ 2,428
Schedule of Accrued Current Liabilities	Accrued current liabilities consist of the following (in thousands): December 31, 2019 2018 Deferred rent $ — $ 445 Property and equipment 761 — Other 280 295 Total accrued current liabilities $ 1,041 $ 740

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Assumptions Used for Estimating the Fair Value of Stock Purchase Rights Granted Under Employee Stock Purchase Plan	The estimated fair value of stock purchase rights granted under the ESPP were calculated using the Black-Scholes option-pricing model using the following assumptions: Year ended December 31, 2019 Expected dividend yield — % Expected term 0.50 year Risk-free interest rate 2.2 % Expected volatility 74.0 %
Schedule of Weighted-Average Assumptions Used in Black-Scholes Option-Pricing Model to Estimate Fair Value of Employee Stock Options at Grant Date	The fair value of each stock option granted to an employee or a director was estimated as of the date of grant using the Black-Scholes model with the following weighted-average assumptions: Year Ended December 31, 2019 2018 2017 Expected dividend yield — % — % — % Expected term 6.01 years 6.04 years 6.04 years Risk-free interest rate 2.3 % 2.8 % 2.0 % Expected volatility 83.0 % 88.0 % 94.0 %
Schedule of Stock Options Activity	A summary of the 2015 Plan and 2018 Plan activity is as follows: Options Outstanding Number of Shares Available for Issuance Number of Shares Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (in years) Aggregate Intrinsic Value (in thousands) Balance at December 31, 2018 2,695,110 2,429,859 $ 5.31 8.86 $ 25,646 Authorized 1,160,000 Granted (1,312,610 ) 1,312,610 $ 11.30 Exercised — (281,024 ) $ 1.61 Cancelled 295,484 (295,484 ) $ 8.46 Repurchased 13,899 — $ 0.35 Balance at December 31, 2019 2,851,883 3,165,961 $ 7.83 8.45 $ 8,875 Vested and exercisable – December 31, 2019 1,124,222 $ 5.16 7.88 $ 5,433 Vested and expected to vest – December 31, 2019 2,939,089 $ 7.54 8.38 $ 8,864
Schedule of Stock Based Compensation Expense	Total stock-based compensation for all awards granted to employees and consultants and our 2018 ESPP Plan Year Ended December 31, 2019 2018 2017 Research and development expenses $ 3,437 $ 2,081 $ 888 General and administrative expenses 1,862 914 238 Total $ 5,299 $ 2,995 $ 1,126

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Schedule of Reconciliation of Statutory Federal Income Tax Rate to Company's Effective Tax Rate	The effective tax rate for the years ended December 31, 2019, 2018 and 2017 is different from the federal statutory rate primarily due to the valuation allowance against deferred tax assets as a result of insufficient sources of income. The effective tax rate of the of the Company’s provision for income taxes differs from the federal statutory rate as follows: Year Ended December 31, 2019 2018 2017 Statutory federal income tax rate 21.0 % 21.0 % 34.0 % State tax, net of federal benefit 4.2 4.3 2.2 Permanent differences (0.8 ) (1.0 ) (2.3 ) Effective change in enacted tax rate — — (17.3 ) Research and development tax credits 2.6 4.4 4.1 Other (0.8 ) (0.2 ) — Change in valuation allowance (26.3 ) (28.5 ) (20.7 ) Total provision for income taxes — % — % — %
Schedule of Components of Net Deferred Tax Assets/Liabilities	The components of the net deferred tax assets/liabilities are as follows (in thousands): December 31, 2019 2018 Deferred tax assets: Federal and state net operating loss carryforwards $ 45,228 $ 27,311 Research and development tax credits 6,205 4,503 Lease financing obligation — 2,658 Lease liabilities 5,920 — Accruals and other 2,172 1,387 Amortization 2,700 1,726 Deferred Revenue 3,659 — Deferred tax liabilities: Other depreciation (638 ) (655 ) Leased building depreciation — (2,446 ) Operating lease right-of-use assets (5,904 ) — Total net deferred tax assets 59,342 34,484 Less valuation allowance (59,342 ) (34,484 ) Deferred tax assets, net of allowance $ — $ —
Schedule of Unrecognized Tax Benefits	The following table summarizes the activity related to the Company’s unrecognized tax benefits (in thousands): December 31, 2019 2018 2017 Beginning of year—unrecognized tax benefits $ 2,077 $ 1,089 $ 230 Decrease for tax positions taken during prior periods (711 ) (453 ) (47 ) Increases for tax positions taken during current period 1,244 1,441 906 End of year—unrecognized tax benefits $ 2,610 $ 2,077 $ 1,089

Net Loss Per Common Share (Tabl

Net Loss Per Common Share (Tables)	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss Per Share	Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is the same as basic net loss per share, since the effects of potentially dilutive securities are antidilutive given the net loss for each period presented. 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, 2019 2018 2017 Numerator: Net loss $ (94,433 ) $ (64,775 ) $ (41,377 ) Denominator: Weighted-average common shares outstanding, basic and diluted 33,554,823 8,919,281 1,999,044 Net loss per share, basic and diluted $ (2.81 ) $ (7.26 ) $ (20.70 )
Computation of Potentially Anti-Dilutive Securities	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, 2019 2018 2017 Convertible preferred stock — — 17,797,529 Options issued and outstanding and ESPP shares issuable and outstanding 3,179,041 2,429,859 1,351,840 Early exercised common stock subject to future vesting 30,874 226,967 539,289 Warrants to purchase common stock — — 40,257 Total 3,209,915 2,656,826 19,728,915

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Summary of Company's Quarterly Financial Information	The following tables show a summary of the Company’s quarterly financial information for each of the four quarters of 2019 and 2018 and has been prepared in accordance with GAAP for interim financial reporting (in thousands, except per share amounts). In the opinion of management, all adjustments (consisting of normal recurring accruals) considered necessary for a fair presentation have been included. Quarter Ended December 31, 2019 September 30, 2019 June 30, 2019 March 31, 2019 Collaboration revenue $ 884 $ 984 $ 1,150 $ 1,347 Loss from operations $ (28,313 ) $ (28,484 ) $ (22,214 ) $ (18,929 ) Net loss $ (27,704 ) $ (27,548 ) $ (21,172 ) $ (18,009 ) Net loss per share, basic and diluted $ (0.77 ) $ (0.77 ) $ (0.63 ) $ (0.62 ) 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 )

Organization - Additional Infor

Organization - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019Segment
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Number of operating segments	1

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit	$ 220,988				$ 126,402				$ 220,988	$ 126,402
Net loss	$ 27,704	$ 27,548	$ 21,172	$ 18,009	$ 17,338	$ 18,588	$ 15,473	$ 13,376	94,433	64,775	$ 41,377
Net cash used in operating activities									85,011	38,162	34,971
Impairment losses									$ 0	$ 0	0
Lessor financed improvement incentives due but not yet received											$ 1,200
Percentage of realized upon ultimate settlement									50.00%
Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit												$ 126,555
Derecognized property, plant, and equipment												15,400
Derecognized property, plant, and equipment accumulated depreciation												900
Derecognized of financing lease obligation												$ 10,500

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 57,408	$ 52,183
Restricted cash	992	992
Total cash, cash equivalents and restricted cash	$ 58,400	$ 53,175

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Schedule of Straight Line Method Over Estimated Useful Lives of Respective Assets (Detail)	12 Months Ended
	Dec. 31, 2019
Computer Equipment and Software [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful life	3 years
Computer Equipment and Software [Member] | Maximum [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful life	5 years
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful life	5 years
Laboratory Equipment [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful life	5 years
Laboratory Equipment [Member] | Maximum [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful life	7 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property, and equipment, estimated useful lives	Shorter of useful life or lease term

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Schedule of Impact of Topic 842 on Balance Sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, net	$ 26,911		$ 29,494
Operating lease right-of-use assets	23,427		0
Operating liabilities:
Lease liabilities, current portion	2,505		0
Accrued liabilities	1,041		740
Deferred rent, net of current portion	0		1,353
Lease financing obligation, net of current portion	0		10,490
Lease liabilities, net of current portion	20,985		0
Accumulated deficit	$ (220,988)		$ (126,402)
Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, net		$ 14,970
Operating lease right-of-use assets		14,224
Operating liabilities:
Lease liabilities, current portion		2,200
Accrued liabilities		265
Deferred rent, net of current portion		0
Lease financing obligation, net of current portion		0
Lease liabilities, net of current portion		8,980
Accumulated deficit		(126,555)
Accounting Standards Update 2016-02 [Member] | Adjustments due to Adoption of Topic 842 [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, net		(14,524)
Operating lease right-of-use assets		14,224
Operating liabilities:
Lease liabilities, current portion		2,200
Accrued liabilities		(475)
Deferred rent, net of current portion		(1,353)
Lease financing obligation, net of current portion		(10,490)
Lease liabilities, net of current portion		8,980
Accumulated deficit		$ (153)

Cash Equivalents and Marketab_3

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, 2019	Dec. 31, 2018
Cash equivalents and marketable securities [Line Items]
Amortized Cost	$ 57,408	$ 52,183
Unrealized Losses	(6)	(86)
Amortized Cost	116,726	149,207
Unrealized Gains	30	1
Fair Value	116,750	149,122
Cash Equivalents [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost	46,382	48,195
Fair Value	46,382	48,195
Cash Equivalents [Member] | Money Market Funds [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost	42,133	36,148
Fair Value	42,133	36,148
Cash Equivalents [Member] | Commercial Paper [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost	1,749
Fair Value	1,749
Cash Equivalents [Member] | Corporate Debt Securities [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost	2,500	12,047
Fair Value	2,500	12,047
Short-Term Marketable Securities [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost	70,344	101,012
Unrealized Gains	30	1
Unrealized Losses	(6)	(86)
Fair Value	70,368	100,927
Short-Term Marketable Securities [Member] | Commercial Paper [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost	31,476	45,244
Unrealized Gains	15
Unrealized Losses		(40)
Fair Value	31,491	45,204
Short-Term Marketable Securities [Member] | Corporate Debt Securities [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost	38,237	55,768
Unrealized Gains	15	1
Unrealized Losses	(6)	(46)
Fair Value	38,246	$ 55,723
Short-Term Marketable Securities [Member] | Certificates of Deposit [Member]
Cash equivalents and marketable securities [Line Items]
Amortized Cost	631
Fair Value	$ 631

Cash Equivalents and Marketab_4

Cash Equivalents and Marketable Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019USD ($)Investment	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Cash equivalents and marketable securities [Line Items]
Cash and cash equivalents and marketable securities	$ 127,800,000	$ 153,100,000
Cash and cash equivalents	57,408,000	52,183,000
Marketable securities	70,368,000	100,927,000
Realized gains or losses on marketable securities	$ 0
Number of investments in marketable securities | Investment	0
Marketable securities recognition of other-than-temporary impairment	$ 0	$ 0	$ 0
Maximum [Member]
Cash equivalents and marketable securities [Line Items]
Marketable securities contractual maturities period	1 year

Fair Value Measurements - Finan

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, 2019	Dec. 31, 2018
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	$ 116,750	$ 149,122
Cash Equivalents [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	46,382	48,195
Cash Equivalents [Member] | Money Market Funds [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	42,133	36,148
Cash Equivalents [Member] | Commercial Paper [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	1,749
Cash Equivalents [Member] | Corporate Debt Securities [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	2,500	12,047
Short-Term Marketable Securities [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	70,368	100,927
Short-Term Marketable Securities [Member] | Commercial Paper [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	31,491	45,204
Short-Term Marketable Securities [Member] | Corporate Debt Securities [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	38,246	55,723
Short-Term Marketable Securities [Member] | Certificates of Deposit [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	631
Fair Value, Inputs, Level 1 [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	42,133	36,148
Fair Value, Inputs, Level 1 [Member] | Cash Equivalents [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	42,133	36,148
Fair Value, Inputs, Level 1 [Member] | Cash Equivalents [Member] | Money Market Funds [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	42,133	36,148
Fair Value, Inputs, Level 2 [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	74,617	112,974
Fair Value, Inputs, Level 2 [Member] | Cash Equivalents [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	4,249	12,047
Fair Value, Inputs, Level 2 [Member] | Cash Equivalents [Member] | Commercial Paper [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	1,749
Fair Value, Inputs, Level 2 [Member] | Cash Equivalents [Member] | Corporate Debt Securities [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	2,500	12,047
Fair Value, Inputs, Level 2 [Member] | Short-Term Marketable Securities [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	70,368	100,927
Fair Value, Inputs, Level 2 [Member] | Short-Term Marketable Securities [Member] | Commercial Paper [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	31,491	45,204
Fair Value, Inputs, Level 2 [Member] | Short-Term Marketable Securities [Member] | Corporate Debt Securities [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	38,246	$ 55,723
Fair Value, Inputs, Level 2 [Member] | Short-Term Marketable Securities [Member] | Certificates of Deposit [Member]
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	$ 631

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
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 - S

Property and Equipment, Net - Summary of Property and Equipment and Related Accumulated Depreciation and Amortization (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 37,221	$ 36,245
Less accumulated depreciation and amortization	(10,310)	(6,751)
Total property and equipment, net	26,911	29,494
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	863	470
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,007	935
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	21,025	16,406
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	11,102	3,063
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 2,224
Buildings and improvements capitalized under a lease financing transaction [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		$ 15,371

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 4,745	$ 3,961	$ 1,970

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)

Jan. 01, 2019USD ($)

Sep. 30, 2018USD ($)

Jul. 31, 2019USD ($)

Jan. 31, 2019USD ($)renewal_term

Sep. 30, 2018USD ($)

Mar. 31, 2017USD ($)ft²

Nov. 30, 2015USD ($)

Dec. 31, 2019USD ($)

Dec. 31, 2018USD ($)

Dec. 31, 2017USD ($)

May 31, 2019USD ($)

Sep. 30, 2017USD ($)

Feb. 28, 2016USD ($)

Commitments and Contingencies Disclosure [Line Items]

Operating lease cash security deposit

$ 1,201,000

$ 632,000

Operating lease right-of-use assets

23,427,000

0

Lease liability

23,490,000

Rent expense

6,700,000

1,800,000

$ 1,200,000

Research and Development Expense

82,896,000

54,965,000

35,691,000

Contract Research Organization [Member]

Commitments and Contingencies Disclosure [Line Items]

Maximum milestone payments

$ 34,800,000

$ 36,400,000

Research and Development Expense

1,000,000

1,000,000

100,000

Accounting Standards Update 2016-02 [Member]

Commitments and Contingencies Disclosure [Line Items]

Operating lease right-of-use assets

$ 14,224,000

Reduction to accumulated deficit

(153,000)

Emeryville California [Member]

Commitments and Contingencies Disclosure [Line Items]

Operating leasing term

120 months

84 months

Operating lease cash security deposit

$ 600,000

$ 50,000

Operating leasing renewal option to extend lease

5 years

3 years

Operating lease for additional temporary space

12 months

Temporary space lease termination notice period

30 days

Operating leases number of renewal terms | renewal_term

2

Tenant improvement allowance

$ 4,000,000

Operating lease right-of-use assets

9,800,000

Lease liability

13,900,000

Lease expiration date

Oct. 31, 2019

Maximum lease termination effective period after rent commencement date under new lease

60 days

Adjustment to right-of-use assets and lease liability

1,800,000

Pleasanton [Member]

Commitments and Contingencies Disclosure [Line Items]

Operating leasing term

64 months

Operating leasing renewal option to extend lease

5 years

Tenant improvement allowance

$ 1,200,000

$ 100,000

Operating lease right-of-use assets

300,000

Lease liability

500,000

Lease expiration date

Nov. 30, 2024

Net rentable area | ft²

42,620

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

Lessor financed improvement incentives due but not yet received

$ 1,200,000

Pleasanton [Member] | Accounting Standards Update 2016-02 [Member]

Commitments and Contingencies Disclosure [Line Items]

Operating lease right-of-use assets

4,000,000

Leased building derecognized

14,500,000

Financing obligation derecognized

10,500,000

Unamortized tenant improvement allowance

4,000,000

Reduction to accumulated deficit

200,000

Cambridge Massachusetts [Member]

Commitments and Contingencies Disclosure [Line Items]

Operating leasing term

24 months

24 months

24 months

67 months

Operating lease cash security deposit

$ 300,000

$ 300,000

Operating leasing renewal option to extend lease

12 months

3 years

Tenant improvement allowance

$ 2,100,000

Lease expiration date

Aug. 31, 2020

Base rent paid for the year

$ 1,300,000

$ 3,200,000

$ 1,300,000

Operating leases rent expense

$ 100,000

100,000

Approximate annual base rent

$ 3,400,000

Revised operating lease termination date

2021-08

Cambridge Massachusetts [Member] | Deposit [Member]

Commitments and Contingencies Disclosure [Line Items]

Operating lease cash security deposit

$ 300,000

Cambridge Massachusetts [Member] | Accounting Standards Update 2016-02 [Member]

Commitments and Contingencies Disclosure [Line Items]

Operating lease right-of-use assets

$ 900,000

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Components of Lease Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Lease cost
Operating lease cost	$ 6,382	$ 0	$ 0
Short-term lease cost	271
Total lease cost	$ 6,653

Commitments and Contingencies_3

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

Commitments and Contingencies_4

Commitments and Contingencies - Schedule of Minimum Annual Rental Payments Under Company's Operating Lease Agreements (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Operating Leases
2020	$ 5,366
2021	6,359
2022	3,742
2023	3,459
2024	3,449
Thereafter	12,530
Total minimum payments	34,905
Less: Amounts representing interest expense	(9,827)
Less: Amounts representing tenant improvement allowance	(1,588)
Present value of future minimum lease payments	23,490
Less: Current portion of lease liability	(2,505)	$ 0
Noncurrent portion of lease liability	$ 20,985	$ 0

Collaboration and License Agr_3

Collaboration and License Agreements - Additional Information (Detail) - USD ($)	1 Months Ended			12 Months Ended
	Oct. 31, 2018	Sep. 30, 2018	Aug. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Preferred stock shares issued				0	0
Purchase of common stock upon Company IPO					$ 20,935,000	$ 95,798,000
Collaboration and license revenue				$ (4,297,000)
Deferred revenue				$ 14,516,000	18,813,000
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
Purchase of common stock upon Company IPO	$ 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				$ 4,300,000
Deferred revenue				14,500,000
Receivables				0
Contract assets				0
Bluebird Bio Inc [Member] | Collaboration Agreement [Member] | Series C [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Preferred stock shares issued			768,115
Offering price per share			$ 13.04
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
Milestone payment included in research and development expense		$ 2,500,000		3,000,000
Reimbursements expense materials and personnel costs				$ 400,000	$ 400,000

Collaboration and License Agr_4

Collaboration and License Agreements - Schedule of Changes in Deferred Revenue Balance (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Revenue From Contract With Customer [Abstract]
Balance at December 31, 2018	$ 18,813
Additions	0
Deductions	(4,297)
Balance at December 31, 2019	$ 14,516

Balance Sheet Components - Sche

Balance Sheet Components - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Prepaid Expense And Other Assets Current [Abstract]
Prepaid insurance	$ 1,252	$ 966
Interest and other receivables	482	462
Prepaid research and development-related expenses	1,473	1,789
Prepaid rent		860
Other	290	449
Total prepaid expenses and other current assets	$ 3,497	$ 4,526

Balance Sheet Components - Sc_2

Balance Sheet Components - Schedule of Deposits and Other Long-Term Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Other Assets Unclassified [Abstract]
Lease security deposit	$ 1,201	$ 632
Prepaid research and development-related expenses	585	554
Restricted cash	992	992
Other		250
Total deposits and other long-term assets	$ 2,778	$ 2,428

Balance Sheet Components - Sc_3

Balance Sheet Components - Schedule of Accrued Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Deferred rent		$ 445
Property and equipment	$ 761
Other	280	295
Total accrued current liabilities	$ 1,041	$ 740

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($)	Oct. 20, 2017	Oct. 31, 2019	Apr. 30, 2019	Oct. 31, 2018	Aug. 31, 2018	Jul. 31, 2018	Jun. 30, 2018	Sep. 30, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Oct. 02, 2018
Subsidiary Sale Of Stock [Line Items]
Common stock, shares authorized									300,000,000	300,000,000
Common stock, par value									$ 0.0001	$ 0.0001
Preferred stock, shares authorized									10,000,000	10,000,000
Preferred stock, par value									$ 0.0001	$ 0.0001
Preferred stock, shares issued									0	0
Preferred stock, shares outstanding									0	0
Common stock, shares issued									36,332,956	28,823,130
Common stock, shares outstanding									36,332,956	28,823,130
Common stock voting rights									Holders of the Company’s common stock are entitled to one vote per share.
Net proceeds from issuance of stock			$ 69,700,000						$ 75,202,000	$ 92,586,000	$ 14,000
Common stock, preferred stock, debt securities, warrants and units covered in Registration Statement		$ 250,000,000
Percentage of gross proceeds as underwriter compensation		3.00%
Maximum offering expense agreed to be reimbursed		$ 50,000
Maximum [Member]
Subsidiary Sale Of Stock [Line Items]
Net proceeds from issuance of stock		$ 75,000,000
Series B Preferred Stock [Member]
Subsidiary Sale Of Stock [Line Items]
Temporary equity issued, price per share	$ 10.76							$ 10.76
Proceed from issuance of stock	$ 95,800,000							$ 95,800,000
Temporary equity shares issued	8,919,302							8,919,302
Payment for issuance cost	$ 200,000							$ 200,000			$ 210,000
Stock issued, price per share											$ 10.76
Series C Preferred Stock [Member]
Subsidiary Sale Of Stock [Line Items]
Temporary equity issued, price per share					$ 13.04	$ 13.04	$ 13.04
Proceed from issuance of stock					$ 10,000,000	$ 2,000,000	$ 8,900,000
Temporary equity shares issued					768,115	153,360	690,128
Payment for issuance cost							$ 100,000
Amended and Restated Certificate of Incorporation [Member]
Subsidiary Sale Of Stock [Line Items]
Common stock, shares authorized												300,000,000
Common stock, par value												$ 0.0001
Preferred stock, shares authorized												10,000,000
Preferred stock, par value												$ 0.0001
Underwritten Public Offering [Member]
Subsidiary Sale Of Stock [Line Items]
Stock issued, price per share			$ 11.50
ATM Equity Offering Program [Member]
Subsidiary Sale Of Stock [Line Items]
Stock issued, number of shares									490,880
Net proceeds from issuance of stock									$ 3,800,000
Payment of commissions on sale of shares									100,000
Payment of other offering costs on sale of shares									300,000
Common Stock [Member] | IPO [Member]
Subsidiary Sale Of Stock [Line Items]
Payment for issuance cost										$ 10,276,000
Stock issued, number of shares				6,854,202						6,854,202
Stock issued, price per share				$ 15						$ 15
Net proceeds from issuance of stock				$ 92,500,000
Conversion of preferred stock shares into common stock shares				19,409,132
Reclassified common stock and additional paid in capital				$ 177,900,000						$ 14,000
Common Stock [Member] | Over-Allotment Option [Member]
Subsidiary Sale Of Stock [Line Items]
Stock issued, number of shares				187,535
Common Stock [Member] | Underwritten Public Offering [Member]
Subsidiary Sale Of Stock [Line Items]
Stock issued, number of shares			6,500,000
Common Stock [Member] | ATM Equity Offering Program [Member]
Subsidiary Sale Of Stock [Line Items]
Payment for issuance cost									$ 301,000
Stock issued, number of shares									490,880

Stock Based Compensation - Addi

Stock Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for future issuance		2,851,883	2,695,110
Weighted-average grant-date fair value granted employee stock options granted		$ 8.08	$ 6.59	$ 2
Intrinsic value of stock option awards exercised		$ 2,970	$ 750	$ 80
Unrecognized compensation cost related to non-vested employee and consultant options		$ 11,500
Unrecognized compensation cost related to non-vested employee and consultant		2 years 9 months 3 days
Fair value of shares vested		$ 5,000
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]
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
Share-based Compensation Arrangement by Share-based Payment Award, maximum contribution of employee's eligible compensation		15.00%

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Fair Value of Stock Purchase Rights Granted Under Employee Stock Purchase Plan (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%	0.00%
Expected term	6 years 3 days	6 years 14 days	6 years 14 days
Risk-free interest rate	2.30%	2.80%	2.00%
Expected volatility	83.00%	88.00%	94.00%
2018 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%
Expected term	6 months
Risk-free interest rate	2.20%
Expected volatility	74.00%

Stock-Based Compensation - Sc_2

Stock-Based Compensation - Schedule of Weighted-Average Assumptions Used in Black-Scholes Option-Pricing Model to Estimate Fair Value of Employee Stock Options at Grant Date (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Expected dividend yield	0.00%	0.00%	0.00%
Expected term	6 years 3 days	6 years 14 days	6 years 14 days
Risk-free interest rate	2.30%	2.80%	2.00%
Expected volatility	83.00%	88.00%	94.00%

Stock-Based Compensation - Sc_3

Stock-Based Compensation - Schedule of Stock Options Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Options Available for Grant
Number of options beginning balance	2,695,110
Authorized	1,160,000
Granted	(1,312,610)
Exercised	0
Cancelled	295,484
Repurchased	13,899
Number of options ending balance	2,851,883	2,695,110
Outstanding Options
Outstanding at beginning of period	2,429,859
Granted	1,312,610
Exercised	(281,024)
Cancelled	(295,484)
Outstanding at end of period	3,165,961	2,429,859
Options Outstanding, Number of Shares, Vested and exercisable	1,124,222
Options Outstanding, Number of Shares, Vested and expected to vest	2,939,089
Weighted Average Exercise Price
Beginning Balance	$ 5.31
Granted	11.30
Exercised	1.61
Cancelled	8.46
Repurchased	0.35
Ending Balance	7.83	$ 5.31
Weighted-Average Exercise Price, Vested and exercisable	5.16
Weighted-Average Exercise Price, Vested and expected to vest	$ 7.54
Options Outstanding, Weighted-Average Remaining Contractual Term (in years)	8 years 5 months 12 days	8 years 10 months 9 days
Options Outstanding, Weighted-Average Remaining Contractual Term, Vested and exercisable (in years)	7 years 10 months 17 days
Options Outstanding, Weighted-Average Remaining Contractual Term, Vested and expected to vest (in years)	8 years 4 months 17 days
Options Outstanding, Aggregate Intrinsic Value	$ 8,875	$ 25,646
Options Outstanding, Aggregate Intrinsic Value, Vested and exercisable	5,433
Options Outstanding, Aggregate Intrinsic Value, Vested and expected to vest	$ 8,864

Stock-Based Compensation - Sc_4

Stock-Based Compensation - Schedule of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock Based Compensation Expense	$ 5,299	$ 2,995	$ 1,126
Research and Development Expense [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock Based Compensation Expense	3,437	2,081	888
General and Administrative Expense [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock Based Compensation Expense	$ 1,862	$ 914	$ 238

Income Taxes - Schedule of Reco

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Statutory federal income tax rate	21.00%	21.00%	34.00%
State tax, net of federal benefit	4.20%	4.30%	2.20%
Permanent differences	(0.80%)	(1.00%)	(2.30%)
Effective change in enacted tax rate			(17.30%)
Research and development tax credits	2.60%	4.40%	4.10%
Other	(0.80%)	(0.20%)
Change in valuation allowance	(26.30%)	(28.50%)	(20.70%)
Total provision for income taxes	0.00%	0.00%	0.00%

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Income Taxes [Line Items]
U.S. federal statutory corporate tax rate	21.00%	21.00%	34.00%
Deduction for net operating losses percentage			80.00%
Reduction to the deferred tax assets			$ 7,100,000
Increase in valuation allowance	$ 24,900,000	$ 18,400,000
Changes in ownership interest rate period	3 years
Unrecognized tax benefits that would impact effective tax rate	$ 0	0
Accrued interest and penalties related to unrecognized tax benefits	0	$ 0	$ 0
Research and Development | California
Income Taxes [Line Items]
Income tax credit carryforwards	3,400,000
Research and Development | Massachusetts
Income Taxes [Line Items]
Income tax credit carryforwards	$ 300,000
Income tax credit carryforwards, expiration year	2032
Federal
Income Taxes [Line Items]
Net operating loss carryforwards	$ 180,800,000			$ 130,200,000
Net operating loss carryforwards, expiration year	2035
Federal | Research and Development
Income Taxes [Line Items]
Income tax credit carryforwards	$ 5,600,000
Income tax credit carryforwards, expiration year	2035
State
Income Taxes [Line Items]
Net operating loss carryforwards	$ 113,300,000
Net operating loss carryforwards, expiration year	2035
Maximum [Member]
Income Taxes [Line Items]
U.S. federal statutory corporate tax rate			35.00%
Minimum [Member]
Income Taxes [Line Items]
Cumulative changes in ownership interest of shareholders	50.00%

Income Taxes - Components of th

Income Taxes - Components of the Net Deferred Tax Assets/Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Federal and state net operating loss carryforwards	$ 45,228	$ 27,311
Research and development tax credits	6,205	4,503
Lease financing obligation	0	2,658
Lease liabilities	5,920	0
Accruals and other	2,172	1,387
Amortization	2,700	1,726
Deferred Revenue	3,659	0
Deferred tax liabilities:
Other depreciation	(638)	(655)
Leased building depreciation	0	(2,446)
Operating lease right-of-use assets	(5,904)	0
Total net deferred tax assets	59,342	34,484
Less valuation allowance	(59,342)	(34,484)
Deferred tax assets, net of allowance	$ 0	$ 0

Income Taxes - Schedule of Acti

Income Taxes - Schedule of Activity Related Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Beginning of year—unrecognized tax benefits	$ 2,077	$ 1,089	$ 230
Decrease for tax positions taken during prior periods	(711)	(453)	(47)
Increases for tax positions taken during current period	1,244	1,441	906
End of year—unrecognized tax benefits	$ 2,610	$ 2,077	$ 1,089

Net Loss Per Common Share - Com

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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss	$ (27,704)	$ (27,548)	$ (21,172)	$ (18,009)	$ (17,338)	$ (18,588)	$ (15,473)	$ (13,376)	$ (94,433)	$ (64,775)	$ (41,377)
Denominator:
Weighted-average common shares outstanding, basic and diluted									33,554,823	8,919,281	1,999,044
Net loss per share, basic and diluted	$ (0.77)	$ (0.77)	$ (0.63)	$ (0.62)	$ (0.61)	$ (7.60)	$ (6.57)	$ (6.03)	$ (2.81)	$ (7.26)	$ (20.70)

Net Loss Per Common Share - C_2

Net Loss Per Common Share - Computation of Potentially Anti-dilutive Securities (Detail) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities that were not included in the diluted per share calculations	3,209,915	2,656,826	19,728,915
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
Options Issued and Outstanding and ESPP Shares Issuable and Outstanding [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities that were not included in the diluted per share calculations	3,179,041	2,429,859	1,351,840
Early Exercised Common Stock Subject To Future Vesting [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities that were not included in the diluted per share calculations	30,874	226,967	539,289
Warrant to Purchase Common 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			40,257

Related-Party Transactions - Ad

Related-Party Transactions - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Apr. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Common stock, shares issued		36,332,956	28,823,130
Proceeds from Issuance of Common Stock	$ 69,700	$ 75,202	$ 92,586	$ 14
Certain Stockholders Related Parties [Member]
Related Party Transaction [Line Items]
Common stock, shares issued			333,333
Proceeds from Issuance of Common Stock			$ 5,000

Defined Contribution Plan - Add

Defined Contribution Plan - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019
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 I

Subsequent Event - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 11, 2020	Apr. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Subsequent Event [Line Items]
Net proceeds from issuance of stock		$ 69,700	$ 75,202	$ 92,586	$ 14
ATM Equity Offering Program [Member]
Subsequent Event [Line Items]
Issuance of stock, net of issuance costs, shares			490,880
Net proceeds from issuance of stock			$ 3,800
Subsequent Event [Member] | ATM Equity Offering Program [Member]
Subsequent Event [Line Items]
Issuance of stock, net of issuance costs, shares	568,369
Net proceeds from issuance of stock	$ 5,600

Selected Quarterly Financial _3

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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue	$ 884	$ 984	$ 1,150	$ 1,347	$ 1,091	$ 96			$ 4,365	$ 1,187
Type of Revenue [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Loss from operations	$ (28,313)	$ (28,484)	$ (22,214)	$ (18,929)	$ (18,027)	$ (18,614)	$ (15,504)	$ (13,439)	$ (97,940)	$ (65,584)	$ (41,763)
Net loss	$ (27,704)	$ (27,548)	$ (21,172)	$ (18,009)	$ (17,338)	$ (18,588)	$ (15,473)	$ (13,376)	$ (94,433)	$ (64,775)	$ (41,377)
Net loss per share, basic and diluted	$ (0.77)	$ (0.77)	$ (0.63)	$ (0.62)	$ (0.61)	$ (7.60)	$ (6.57)	$ (6.03)	$ (2.81)	$ (7.26)	$ (20.70)