Principia Biopharma (PRNB) 10-K 10 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. 03, 2020	Jun. 30, 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
Trading Symbol	PRNB
Entity Registrant Name	Principia Biopharma Inc.
Entity Central Index Key	0001510487
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Public Float			$ 540
Entity Common Stock, Shares Outstanding		33,008,896
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	Yes
Title of 12(b) Security	Common Stock, Par Value $0.0001 Per Share
Security Exchange Name	NASDAQ
Entity File Number	001-38653
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-3487603
Entity Address, Address Line One	220 East Grand Avenue
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	416-7700
Document Annual Report	true
Document Transition Report	false
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE Portions of the Registrant’s definitive proxy statement relating to the 2020 Annual Meeting of Stockholders, to be filed with the U.S. Securities and Exchange Commission pursuant to Regulation 14A within 120 days after the end of the Registrant’s fiscal year ended December 31, 2019, are incorporated by reference into Part III of this Annual Report on Form 10-K where indicated.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 39,537	$ 34,489
Short-term marketable securities	276,043	142,436
Restricted cash	0	82
Prepaid expenses and other current assets	4,165	3,765
Total current assets	319,745	180,772
Property and equipment, net	9,687	1,666
Long-term restricted cash	567	567
Long-term marketable securities	52,257	3,712
Other long-term assets	480	8,804
Total assets	382,736	195,521
Current liabilities:
Accounts payable	1,850	4,439
Deferred rent, current portion	1,181	344
Deferred revenue	0	5,616
Accrued research and development liabilities	5,791	1,520
Accrued other liabilities	602	649
Accrued compensation	6,715	4,312
Total current liabilities	16,139	16,880
Long-term deferred rent	7,619	8,781
Commitments and contingencies (Note 7)
Stockholders' equity
Common stock, $0.0001 par value, 500,000,000 shares authorized at December 31, 2019 and 2018; 32,950,836 and 23,865,451 shares issued and outstanding at December 31, 2019 and 2018, respectively	3	2
Additional paid-in capital	544,709	302,393
Accumulated other comprehensive income (loss)	9	(128)
Accumulated deficit	(185,743)	(132,407)
Total stockholders’ equity	358,978	169,860
Total liabilities and stockholders’ equity	$ 382,736	$ 195,521

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	32,950,836	23,865,451
Common stock, shares outstanding	32,950,836	23,865,451

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Revenue	$ 35,160	$ 69,137	$ 5,247
Operating expenses:
Research and development	74,083	40,533	25,390
General and administrative	19,842	11,462	6,443
Total operating expenses	93,925	51,995	31,833
Income (loss) from operations	(58,765)	17,142	(26,586)
Other income (expense), net	0	(653)	5,096
Interest income	4,973	1,678	16
Interest expense	0	0	(7,223)
Net income (loss)	(53,792)	18,167	(28,697)
Net income (loss) attributable to common stockholders	$ (53,792)	$ 4,917	$ (28,697)
Net income (loss) per share attributable to common stockholders
Basic	$ (2.09)	$ 0.65	$ (50.37)
Diluted	$ (2.09)	$ 0.57	$ (50.37)
Weighted-average shares used to calculate net income (loss) per share attributable to common stockholders
Basic	25,778,122	7,622,602	569,719
Diluted	25,778,122	8,627,473	569,719

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ (53,792)	$ 18,167	$ (28,697)
Other comprehensive income (loss):
Net unrealized gain (loss) on available-for-sale securities	137	(38)	0
Foreign currency translation adjustment	0	0	52
Comprehensive income (loss)	$ (53,655)	$ 18,129	$ (28,645)

Consolidated Statements of Chan

Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at Dec. 31, 2016	$ (116,650)			$ 5,369	$ (142)	$ (121,877)
Balance, Shares at Dec. 31, 2016		10,316,873
Balance at Dec. 31, 2016		$ 104,926
Balance, Shares at Dec. 31, 2016			530,997
Stock-based compensation expense	1,027			1,027
Exercise of stock options and vesting of early exercise shares	318		$ 1	317
Exercise of stock options and vesting of early exercise shares, Shares			95,616
Issuance of preferred stock on conversion of notes payable		$ 23,605
Issuance of preferred stock on conversion of notes payable, Shares		1,968,561
Foreign currency translation adjustment	52				52
Beneficial conversion features of the convertible loan	488			488
Net income (loss)	(28,697)					(28,697)
Balance at Dec. 31, 2017	(143,462)		$ 1	7,201	(90)	(150,574)
Balance, Shares at Dec. 31, 2017		12,285,434
Balance at Dec. 31, 2017		$ 128,531
Balance, Shares at Dec. 31, 2017			626,613
Issuance of preferred stock from Series C offering		$ 49,800
Issuance of preferred stock from Series C offering, Shares		3,474,668
Conversion of preferred stock at initial public offering	178,332		$ 1	178,331
Conversion of preferred stock at initial public offering, Shares		(15,760,102)
Conversion of preferred stock at initial public offering		$ (178,331)
Conversion of preferred stock at initial public offering, Shares			15,760,102
Conversion of warrants at initial public offering	2,153			2,153
Issuance of common stock from initial public / equity offering	110,574			110,574
Issuance of stock from public / equity offering, Shares			7,187,500
Stock-based compensation expense	2,801			2,801
Exercise of stock options and vesting of early exercise shares	1,277			1,277
Exercise of stock options and vesting of early exercise shares, Shares			287,660
Issuance of common stock upon exercise of warrants	56			56
Issuance of common stock upon exercise of warrants, Shares			3,576
Unrealized gain (loss) on available-for-sale securities	(38)				(38)
Net income (loss)	18,167					18,167
Balance at Dec. 31, 2018	$ 169,860		$ 2	302,393	(128)	(132,407)
Balance, Shares at Dec. 31, 2018	23,865,451		23,865,451
Issuance of common stock from initial public / equity offering	$ 226,474		$ 1	226,473
Issuance of stock from public / equity offering, Shares			8,625,000
Stock-based compensation expense	12,182			12,182
Exercise of stock options and vesting of early exercise shares	2,125			2,125
Exercise of stock options and vesting of early exercise shares, Shares			295,729
Issuance of common stock upon exercise of warrants, Shares			66,215
Issuance of shares under the Employee Stock Purchase Plan	1,536			1,536
Issuance of shares under the Employee Stock Purchase Plan, Shares			98,441
Cumulative-effect adjustment from adoption of ASC 606 accounting standard on revenue recognition | ASC 606	456					456
Unrealized gain (loss) on available-for-sale securities	137				137
Net income (loss)	(53,792)					(53,792)
Balance at Dec. 31, 2019	$ 358,978		$ 3	$ 544,709	$ 9	$ (185,743)
Balance, Shares at Dec. 31, 2019	32,950,836		32,950,836

Consolidated Statements of Ch_2

Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018	Sep. 18, 2018	Dec. 31, 2017
Statement Of Stockholders Equity [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities:
Net income (loss)	$ (53,792)	$ 18,167	$ (28,697)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Change in fair value of convertible preferred stock warrant liability	0	577	526
Change in fair value of redemption features liability	0	0	(5,586)
Amortization of discount on marketable securities	(1,563)	(387)	0
Depreciation	1,832	252	178
Stock-based compensation	12,182	2,801	1,027
Deferred rent	(325)	745	0
Amortization of debt discount	0	0	5,495
Interest expense on convertible notes	0	0	1,669
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(400)	(2,187)	(843)
Deferred revenue	(5,160)	(44,137)	49,753
Accounts payable	(2,082)	1,034	1,280
Accrued liabilities	6,762	1,962	2,046
Net cash provided by (used in) operating activities	(42,546)	(21,173)	26,848
Investing activities:
Purchases of property and equipment	(2,171)	(1,067)	(90)
Maturities of marketable securities	199,916	13,500	0
Purchases of marketable securities	(380,368)	(159,299)	0
Net cash used in investing activities	(182,623)	(146,866)	(90)
Financing activities:
Net proceeds from equity offerings	226,474	110,574	0
Net proceeds from Series C preferred stock issuance	0	49,800	0
Proceeds from issuances of common stock upon exercise of options and participation in employee stock purchase plan	3,661	1,511	317
Proceeds from exercise of common stock warrants	0	56	0
Proceeds from issuance of convertible notes	0	0	8,060
Net cash provided by financing activities	230,135	161,941	8,377
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash	0	0	52
Net increase (decrease) in cash, cash equivalents and restricted cash	4,966	(6,098)	35,187
Cash, cash equivalents and restricted cash at beginning of period	35,138	41,236	6,049
Cash, cash equivalents and restricted cash, at end of period	40,104	35,138	41,236
Supplemental disclosures of cash flow information
Non cash tenant improvement allowance used for leasehold improvements	8,324	0	0
Non cash tenant improvement allowance receivables	0	8,324	0
Purchases of property and equipment accrued but not yet paid	0	642	0
Conversion of convertible note and interest into convertible preferred stock	$ 0	$ 0	$ 23,605

Organization

Organization	12 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	1. Organization Description of Business We, Principia Biopharma Inc. (“Principia”), are a late-stage biopharmaceutical company focused on developing potential treatments for immune-mediated diseases Initial Public Offering On September 13, 2018, our Registration Statement on Form S-1 was declared effective by the U.S. Securities and Exchange Commission (“SEC”) for our initial public offering (“IPO”). In connection with the IPO, we issued and sold an aggregate of 7,187,500 shares of common stock, including 937,500 shares issued and sold pursuant to the underwriters’ full exercise of their over-allotment option to purchase additional shares, at an offering price to the public of $17.00 per share. Proceeds from the IPO, net of underwriting discounts and commissions, were $113.6 million In addition, all of our convertible preferred stock warrants were converted into warrants to purchase shares of common stock. In connection with our IPO, on September 18, 2018, our certificate of incorporation was amended and restated to provide for 500,000,000 authorized shares of common stock with a par value of $0.0001 per share and 20,000,000 authorized shares of preferred stock with a par value of $0.0001 per share . Subsequent Equity Offering In October 2019, we completed an equity offering of 8,625,000 shares of our common stock (which included 1,125,000 shares of our common stock issued and sold pursuant to the underwriters’ option to purchase additional shares) at a price to the public of $28.00 per share. We received net proceeds of approximately $226.5 million after deducting underwriting discounts and commissions and estimated offering expenses payable by us.

Significant Accounting Policies

Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Significant Accounting Policies	2. Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) and include the accounts of Principia and our wholly-owned Australian subsidiary. All intercompany accounts, transactions and balances have been eliminated. Use of Estimates The preparation of our financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, as well as related disclosure of contingent assets and liabilities. Significant estimates include amounts to determine the fair value of common stock-based awards, warrants, and other issuances, embedded derivatives, accruals for research and development costs and uncertain tax positions, and the estimated periods of performance used in the determination of collaboration revenues. We base our estimates on historical experience and on various other market specific and relevant assumptions that our management believes to be reasonable under the circumstances. Actual results could differ materially from our estimates. Concentration of Credit Risk and Other Risks and Uncertainties Financial instruments that potentially subject us to a significant concentration of credit risk consist primarily of cash, cash equivalents and marketable securities. Our cash and cash equivalents are maintained with a limited number of financial institutions in the United States. Amounts on deposit have exceeded and will continue to exceed federally insured limits. We have not experienced any losses on our cash deposits. Additionally, we have established guidelines regarding the diversification of our investments in approved instruments, their credit quality ratings and maturities. The guidelines are designed to preserve principal balances and provide liquidity. We are subject to a number of risks similar to other late-stage or collect , or due to us, Cash and Cash Equivalents We consider all highly liquid financial instruments with original Marketable Securities We carry marketable securities consisting primarily of money market funds, U.S. Treasury securities and obligations of government-sponsored enterprises and corporate bonds and commercial paper. Marketable securities with maturities greater than 90 days at the time of purchase and that mature less than one year from the consolidated balance sheet date are classified as short-term. Marketable securities with a maturity date greater than one year at each balance sheet date are classified as long-term. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on marketable securities are included in other income (expense), net. The cost of securities sold is based on the specific-identification method. Interest earned on marketable securities is included in interest income. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts and such amortization and accretion are included as a component of interest income. Restricted Cash As of December 31, 2019 and 2018, we had $0.6 million in long-term restricted cash for a lease security deposit. This amount is separated from cash and cash equivalents on the consolidated balance sheets. Segments We have one operating segment. Our chief operating decision maker, our President and Property and Equipment Property and equipment consist of laboratory equipment, computer equipment, furniture and fixtures and leasehold improvements and are stated at cost, net of accumulated depreciation. Maintenance and repairs are charged to expense as incurred. Depreciation is recognized using the straight-line method over the estimated useful lives of the assets generally ranging from two years for computer equipment to five years for office furniture and fixtures. Leasehold improvements are amortized over the shorter of their estimated useful lives or the related lease term. Leases We enter into lease agreements for our laboratory and office facilities. These leases are classified as operating leases. Rent expense is recognized on a straight-line basis over the term of the lease. The difference between rent expense recognized and rental payments is recorded as deferred rent in the consolidated balance sheets Lease incentives and allowances provided by our landlord for the construction of leasehold improvements are recorded as lease incentive obligations as the related construction costs are incurred Revenue Recognition Effective January 1, 2019, we adopted Accounting Standards Codification (“ASC”) No. 2014-09, Revenue from Contracts with Customers, or ASC 606, using the modified retrospective approach. Under this approach, we recorded a cumulative adjustment to decrease accumulated deficit and deferred revenue by $0.4 million as of the adoption date. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. We only apply the five-step model to contracts when it is probable that we will collect the consideration we are entitled to in exchange for the goods and services we transfer to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, we assess the goods or services promised within each contract, determine those that are performance obligations and assess whether each promised good or service is distinct. We then recognize as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied . We have entered into licensing and collaboration agreements that are within the scope of ASC 606. Licenses of Intellectual Property: If the license to our intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, we recognize revenues from non-refundable, upfront fees allocated to the license when the license is transferred to the licensee and the licensee is able to use and benefit from the license. For licenses that are bundled with other promised goods or services, we utilize judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue from non-refundable, upfront fees. We evaluate the measure of progress each reporting period and, if necessary, adjust the measure of performance and related revenue recognition. Milestone Payments: At the inception of each arrangement that includes development, regulatory or commercial milestone payments, we evaluate whether the milestones are considered probable of being reached and estimate the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant reversal of cumulative revenue would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within our control or that of our licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received or the underlying activity has been completed. The transaction price is then allocated to each performance obligation on a relative stand-alone selling price basis, for which we recognize revenue as or when the performance obligations under the contract are satisfied. At the end of each subsequent reporting period, we re-evaluate the probability of achievement of such development milestones and any related constraint, and if necessary, adjust our estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect collaboration revenue in the period of adjustment. Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, we recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, we have not recognized any royalty revenue resulting from any of our licensing arrangements. The impact of the adoption of ASC 606 on the accompanying consolidated balance sheet as of January 1, 2019 was as follows (in thousands): December 31, 2018 Adjustments Due to ASC 606 January 1, 2019 Liabilities Deferred revenue $ 5,616 $ (456 ) $ 5,160 Stockholders' Equity Accumulated deficit (132,407 ) 456 (131,951 ) The impact of the adoption of ASC 606 on our consolidated statement of operations for the year ended December 31, 2019 was as follows (in thousands): Year Ended December 31, 2019 As Reported Adjustments Balance without ASC 606 Adoption Revenue $ 35,160 $ 456 $ 35,616 Loss from operations (58,765 ) 456 (58,309 ) Net loss (53,792 ) 456 (53,336 ) Net loss per share, basic and diluted (2.09 ) 0.02 (2.07 ) During the year ended December 31, 2019, we did not recognize any revenue from performance obligations satisfied in previous periods. Convertible Preferred Stock Warrants In connection with the issuance of certain convertible notes in 2016 and 2017, we issued warrants to purchase our capital stock. Freestanding warrants to purchase our convertible preferred stock were recorded as a liability on our consolidated balance sheets because the underlying shares of convertible preferred stock are contingently redeemable, which, therefore, may obligate us to transfer assets to settle those warrants. The warrants are subject to revaluation at each balance sheet date, with changes in fair value recognized as a component of other income (expense), net, on the consolidated statements of operations. Upon the completion of our IPO in September 2018, all of our convertible preferred stock warrants were converted into warrants to purchase shares of common stock. We re-valued the convertible preferred stock warrants upon completion of our IPO and reclassified the estimated fair value of the warrants to additional paid-in capital. See Note 8, Debt Obligation, for additional information. Convertible Notes Redemption Features We recorded a derivative liability related to redemption features embedded within our convertible notes. The convertible notes issued in 2016 and 2017 included features that were determined to be an embedded derivative requiring bifurcation and separate accounting. This bifurcated redemption feature was initially recorded at fair value and was subject to revaluation at each balance sheet date. Changes in fair value were recognized as a component of other income (expense), net, on the consolidated statements of operations. In December 2017, the convertible notes and interest payable were converted into Series B-3 preferred stock and the convertible notes redemption features liability was determined to have a fair value of zero as of December 31, 2017. See Note 8, Debt Obligation, for additional information Research and Development Expenses Research and development expenses consist primarily of personnel costs, including salaries, benefits and stock-based compensation, costs related to clinical and preclinical studies, contract manufacturing, consulting fees, laboratory supplies, and allocated overhead and facility occupancy costs. Research and development costs are expensed as incurred. Non-refundable advance payments for goods or services that will be used or rendered for future research and development activities are capitalized and expensed as the goods are delivered or the related services are performed. Costs associated with co-development activities are included in research and development expenses, with any reimbursement of costs by our collaboration partners reflected as a reduction of research and development expenses. Stock-based Compensation We use the Black-Scholes valuation model to estimate the grant date fair value of stock option awards with time-based vesting terms. The determination of fair value for stock-based awards on the date of grant using an option-pricing model (“OPM”) requires management to make certain assumptions regarding a number of variables. Stock-based compensation expense is recognized based on the grant date fair value on a straight-line basis over the requisite service period and is reduced for forfeitures as they occur. Equity-based payments granted to non-employees are recorded at grant date fair value and are re-measured at the current fair value at the end of each reporting period until the underlying equity instruments vest. Non-employee stock-based compensation expense was immaterial for all periods presented. Income Taxes We account for income taxes under the asset and liability method. Deferred tax assets and liabilities are recognized for future tax consequences attributable to the differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and net operating loss and tax credit carryforwards. Valuation allowances are established when necessary to reduce deferred tax assets to the amount expected to be realized. Deferred tax assets and liabilities are measured at the balance sheet date using the enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in the period such tax rate changes are enacted. We record uncertain tax positions on the basis of a two-step process in which (1) it determines whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, we recognize the largest amount of tax benefit that is more than 50 percent likely to be realized upon ultimate settlement with the related tax authority. Our policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the underpayment of income taxes. Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in stockholders’ equity (deficit) of a business enterprise during a period, resulting from transactions from non-owner sources, and consists primarily of unrealized gains or losses related to our available-for-sale marketable securities, which are carried at estimated fair values on the consolidated balance sheets. Net Income (Loss) per Share Basic net income (loss) per share attributable to common stockholders is calculated by dividing the net income (loss) attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period. Diluted net income (loss) per share includes the effect of potentially dilutive securities, which include outstanding warrants and stock options if the effect of their inclusion would be dilutive. In periods of net loss, diluted net loss per share is the same as basic net loss per share as the inclusion of potentially dilutive securities in the calculation would be anti-dilutive. We have issued securities other than common stock that participate in dividends to the extent declared (“Participating Securities”), and therefore utilize the two-class method to calculate net income (loss) per share. These Participating Securities include Series A, Series B-1, Series B-2, Series B-3 and Series C redeemable convertible preferred stock. The two-class method requires a portion of net income (loss) to be allocated to the Participating Securities to determine net income (loss) attributable to common stockholders. Net income (loss) attributable to common stockholders is equal to the net income (loss) less dividends paid on preferred stock with any remaining earnings allocated in accordance with the bylaws between the outstanding common and redeemable convertible preferred stock as of the end of each period. In September 2018, upon our IPO, all shares of convertible preferred stock were converted into an equal number of shares of common stock. Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board, or FASB Recently Adopted Accounting Standards Updates In November 2016, the FASB issued accounting standard update (“ASU”) 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash We early adopted ASU 2016-18 during the fourth quarter of 2018 and the adoption did not have a material impact on our consolidated financial statements and related disclosures. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) Entities have the option of applying either a full retrospective approach to all periods presented, or a modified approach that reflects differences prior to the date of adoption as an adjustment to equity. We adopted ASC No. 2014-09 on January 1, 2019 using the modified retrospective method of transition applied to contracts that were not completed at January 1, 2019. Therefore, comparative information will not be adjusted and the impact of the transition is reflected as an adjustment to the opening accumulated deficit. A completed contract is a contract for which all, or substantially all, of the revenue was recognized in accordance with revenue guidance in effect before the date of initial application. The new revenue recognition standard differs from the previous accounting standard in many respects, such as in the accounting for variable consideration and the measurement of progress toward completion of performance obligations. Recently Issued Accounting Standards or Updates Not Yet Effective In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases (Topic 842): Targeted Improvements In June 2018, the FASB issued ASU No. 2018-07, Compensation–Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework–Changes to the Disclosure Requirements for Fair Value Measurement . This ASU eliminates, modifies and adds disclosure requirements for fair value measurements. The amendments in this ASU are effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. We and related disclosures. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. We and related disclosures.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements Financial assets and liabilities are recorded at fair value. We determine fair value using the fair value hierarchy, which establishes three levels of inputs that may be used to measure fair value, as follows: Level 1 inputs include quoted prices in active markets for identical assets or liabilities. Level 2 inputs include observable inputs other than Level 1 inputs, such as quoted prices for similar assets or liabilities in active markets; quoted prices for identical or similar assets or liabilities in markets that are not active; and other inputs that are observable or can be corroborated by observable market data for substantially the full term of the asset or liability. Level 3 inputs include unobservable inputs that are supported by little or no market activity and are significant to the fair value of the underlying asset or liability. Such inputs reflect our best estimate of what market participants would use in pricing the asset or liability at the reporting date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model. Assets and liabilities measured at fair value are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. Our assessment of the significance of a particular input to the fair value measurement in its entirety requires our management to make judgments and consider factors specific to the asset or liability. Assets and liabilities measured at fair value on a recurring basis as of December 31, 2019 and 2018 were as follows (in thousands): December 31, 2019 Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 17,664 $ — $ — $ 17,664 Corporate commercial paper — 6,983 — 6,983 Corporate debt securities — 10,204 — 10,204 Short-term marketable securities Corporate commercial paper — 95,088 — 95,088 Corporate debt securities — 154,690 — 154,690 Government‑sponsored enterprise securities — 26,265 — 26,265 Long-term marketable securities Corporate debt securities — 14,202 — 14,202 Government‑sponsored enterprise securities — 38,055 — 38,055 Total $ 17,664 $ 345,487 $ — $ 363,151 December 31, 2018 Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 19,861 $ — $ — $ 19,861 Corporate commercial paper — 7,465 — 7,465 Corporate debt securities — 4,499 — 4,499 Short-term marketable securities Corporate commercial paper — 36,180 — 36,180 Corporate debt securities — 71,903 — 71,903 Government‑sponsored enterprise securities — 9,906 — 9,906 U.S. Treasury securities — 24,447 — 24,447 Long-term marketable securities Corporate debt securities — 3,712 — 3,712 Total $ 19,861 $ 158,112 $ — $ 177,973 The carrying amounts of accounts payable and accrued liabilities approximate their fair values due to their short-term maturities. Our Level 2 securities are valued using third-party pricing sources. 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. There were no transfers of assets or liabilities between the fair value measurement levels during the years ended December 31, 2019 and 2018.

Cash Equivalents and Marketable

Cash Equivalents and Marketable Securities	12 Months Ended
	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Cash Equivalents and Marketable Securities	4. Cash Equivalents and Marketable Securities Cash equivalents and marketable securities consisted of the following as of December 31, 2019 and 2018 (in thousands): December 31, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents Money market funds $ 17,664 $ — $ — $ 17,664 Corporate commercial paper 6,982 1 — 6,983 Corporate debt securities 10,205 — (1 ) 10,204 Total cash equivalents 34,851 1 (1 ) 34,851 Short-term marketable securities Corporate commercial paper 95,061 36 (9 ) 95,088 Corporate debt securities 154,678 54 (42 ) 154,690 Government‑sponsored enterprise securities 26,231 34 — 26,265 Total short-term marketable securities 275,970 124 (51 ) 276,043 Long-term marketable securities Corporate debt securities 14,194 8 — 14,202 Government‑sponsored enterprise securities 38,037 18 — 38,055 Total long-term marketable securities 52,231 26 — 52,257 Total $ 363,052 $ 151 $ (52 ) $ 363,151 December 31, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents Money market funds $ 19,861 $ — $ — $ 19,861 Corporate commercial paper 7,466 — (1 ) 7,465 Corporate debt securities 4,499 — — 4,499 Total cash equivalents 31,826 — (1 ) 31,825 Short-term marketable securities Corporate commercial paper 36,197 — (17 ) 36,180 Corporate debt securities 71,920 8 (25 ) 71,903 Government‑sponsored enterprise securities 9,911 — (5 ) 9,906 U.S. Treasury securities 24,451 — (4 ) 24,447 Total short-term marketable securities 142,479 8 (51 ) 142,436 Long-term marketable securities Corporate debt securities 3,706 6 — 3,712 Total long-term marketable securities 3,706 6 — 3,712 Total $ 178,011 $ 14 $ (52 ) $ 177,973 All our marketable securities are considered available-for-sale. There were no sales of available-for-sale marketable securities in any of the periods presented. The carrying value of marketable securities that were in unrealized loss positions totaled approximately $119.5 million as of December 31, 2019. We have determined that (i) we do not have the intent to sell any of these investments, and (ii) it is not more likely than not that we will be required to sell any of these investments before recovery of the entire amortized cost basis. We anticipate that we will recover the entire amortized cost basis of such securities and have determined there were no other-than-temporary impairments associated with any of these marketable securities before recovery of the entire amortized cost basis during the year ended December 31, 2019. At December 31, 2019, the remaining contractual maturities of long-term marketable securities were less than two years .

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2019
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	5. Balance Sheet Components The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheets that sum to the total of the amounts shown in the consolidated statements of cash flows (in thousands): December 31, 2019 December 31, 2018 December 31, 2017 December 31, 2016 Cash and cash equivalents $ 39,537 $ 34,489 $ 41,054 $ 5,867 Restricted cash, current — 82 100 100 Restricted cash, non-current 567 567 82 82 Total $ 40,104 $ 35,138 $ 41,236 $ 6,049 Property and equipment as of December 31, 2019 and 2018 consisted of the following (in thousands): December 31, 2019 December 31, 2018 Laboratory equipment $ 2,007 $ 2,500 Computer equipment 289 508 Furniture and fixtures 1,549 392 Leasehold improvements 8,324 420 12,169 3,820 Less accumulated depreciation and amortization (2,482 ) (2,154 ) Total $ 9,687 $ 1,666 Depreciation and leasehold improvement amortization expenses were $1.8 million and $0.3 million for the years ended December 31, 2019 and 2018, respectively. Prepaid expenses and other current assets as of December 31, 2019 and 2018 consisted of the following (in thousands): December 31, 2019 December 31, 2018 Other accounts receivable $ 1,272 $ 2,104 Prepaid expenses 2,893 1,661 Total $ 4,165 $ 3,765 Other long-term assets as of December 31, 2019 and 2018 consisted of the following (in thousands): December 31, 2019 December 31, 2018 Tenant improvement allowance receivables $ — $ 8,324 Long-term deposits 480 480 Total $ 480 $ 8,804

License and Collaboration Agree

License and Collaboration Agreements	12 Months Ended
	Dec. 31, 2019
Research And Development [Abstract]
License and Collaboration Agreements	6. License and Collaboration Agreements Sanofi In November 2017, we entered into a strategic collaboration agreement with Sanofi, or the Sanofi Agreement, for an exclusive license to PRN2246/SAR442168 and backup molecules for development in multiple sclerosis (“MS”) and other central nervous system (“CNS”) diseases. Under the Sanofi Agreement, we have completed the Phase 1 trials and Sanofi is taking on all further development activities. We and Sanofi each have been responsible for certain early development costs, and Sanofi is responsible for all further development and commercialization costs, subject to our Phase 3 option described below. Sanofi has an exclusive license for PRN2246/SAR442168 and its backups for the CNS field, which includes indications of the CNS, retina and ophthalmic nerve. We have agreed not to develop other BTK inhibitors within the CNS field, and Sanofi has agreed not to develop PRN2246/SAR442168 or its backups for any indications outside the CNS field. In the event we cease all development and commercialization of our other BTK inhibitors or unilaterally decide to offer Sanofi a field expansion, Sanofi could expand its field upon a field expansion payment to us as well as potential milestone payments and royalties within the expanded field. In December 2017, we received a $40.0 million upfront payment from Sanofi. In May 2018, we amended the Sanofi Agreement to include additional activities under the early development plan and to modify the definition of one of the milestone payments. Pursuant to the amendment, we received a $10.0 million payment in July 2018 for the completion of a major part of the Phase 1 trial. In August 2018, we received a $5.0 million payment for the successful completion of preclinical toxicology studies. In November 2018, we received $10.0 million in additional payments from Sanofi for successful development activities of PRN2246/SAR442168 related to the early development plan. In June 2019, we received a $30.0 million milestone payment from Sanofi for the initiation of the Phase 2b clinical trial of PRN2246/SAR442168. Under the amended Sanofi Agreement, we may receive development, regulatory and commercial milestone payments of up to an aggregate of $765.0 million, as well as royalties up to the mid-teens. We have an option to fund a portion of Phase 3 development costs in return for, at our option, either a profit and loss sharing arrangement within the United States, or an additional worldwide royalty that would result in royalties up to the high-teens. The additional royalty option would only be available if we develop rilzabrutinib We identified the following performance obligations under the Sanofi Agreement: (i) granting a license of rights to PRN2246/SAR442168, (ii) transferring of technology (know-how) related to PRN2246/SAR442168, and (iii) providing research and development services related to our responsibilities under the early development plan. We concluded that the delivered license is not distinct at inception of the arrangement due to our proprietary expertise with respect to the licensed compound and related developmental participation under the agreement, which is required for Sanofi to fully realize the value from the delivered license. Therefore, we combined these performance obligations as one unit of accounting and recognized the $40.0 million upfront payment and an aggregate of $25.0 million in milestone payments over the performance period under the Sanofi Agreement, which ended in December 2018. On January 1, 2019, we adopted ASC 606 using the modified retrospective approach. The transaction price was determined to be $65.0 million, which includes the $40.0 million upfront payment we received from Sanofi in December 2017 and an aggregate of $25.0 million in milestone payments received in 2018. All potential future milestones and other payments were considered constrained at the inception of the Sanofi Agreement since we could not conclude it is probable that a significant reversal in the amount recognized will not occur. Upon adoption of ASC 606, we also determined that variable considerations related to certain milestones not previously recognized were constrained because they were not probable, due to the inherent uncertainty related to the achievement of these milestones. In May 2019, we achieved a clinical development milestone upon Sanofi’s initiation of its Phase 2b clinical trial of PRN2246/SAR442168 We concluded upon analysis that there is no adjustment necessary in revenue recognized through December 31, 2018 under ASC 606 for the Sanofi Agreement. All deliverables under ASC 605 and all performance obligations under ASC 606 had been completed as of December 31, 2018. For the years ended December 31, 2019, 2018 and 2017, we recognized $30.0 million, $63.1 million and $1.9 million in revenue from Sanofi, respectively. There was no deferred revenue related to the Sanofi Agreement at December 31, 2019 and 2018 . AbbVie In June 2017, we entered into a collaboration agreement with AbbVie, or the AbbVie Agreement, to research and develop oral immunoproteasome inhibitors and received an upfront payment of $15.0 million. We identified the following performance obligations under the AbbVie Agreement: (i) granting a license of rights to certain licensed compounds to develop and commercialize oral immunoproteasome inhibitors, (ii) transferring of technology (know-how) related to the oral immunoproteasome inhibitors program, and We determined that each of these performance obligations individually is not distinct, rather each one represents a component of a single performance obligation of the contract. Prior to the adoption of ASC 606, we concluded under ASC 605 that the delivered license did not have standalone value at the inception of the arrangement due to our proprietary expertise with respect to the licensed compounds and related ongoing research participation under the AbbVie Agreement, which is required for AbbVie to fully realize the value from the licensed compound. Therefore, the $15.0 million received related to this combined unit of accounting was being recognized as revenue ratably over the estimated performance period, which was estimated to continue through the fourth quarter of 2019. On January 1, 2019, we adopted ASC 606 using the modified retrospective approach. Upon adoption, the fixed, non-refundable and non-creditable upfront payment of $15.0 million received in 2017 was determined to be the transaction price. Revenue is recognized based on a measurement of progress toward the completion of the performance obligation of providing research services for two years, which was subject to an extension for up to six months. The measurement is calculated using an input-method based on research costs incurred by us during each reporting period compared to the total research costs projected to provide research services by us pursuant to the AbbVie Agreement. Such costs include external direct costs and internal direct labor consisting of the efforts of certain of our employees that dedicate their time providing research services pursuant to the AbbVie Agreement. Based on this methodology, we concluded that under ASC 606, approximately $9.8 million in revenue should be recognized through December 31, 2018, as compared to $9.4 million under ASC 605. We recorded a cumulative adjustment to decrease accumulated deficit and deferred revenue by $0.4 million as of the adoption date. In March 2019, we and AbbVie agreed to conclude the collaboration and as of the date of termination, there were no further financial obligations between us. We recognized the remaining balance of the transaction price of $5.2 million in revenue in March 2019, upon termination of the AbbVie Agreement. For the years ended December 31, 2019, 2018 and 2017, we recognized $5.2 million, $6.0 million and $3.4 million in revenue from AbbVie, respectively. There was no deferred revenue related to the AbbVie Agreement at December 31, 2019. Deferred revenue related to the AbbVie Agreement was $5.6 million at December 31, 2018 . University of California License Agreements In November 2009 and September 2011, we entered into license agreements with the Regents of the University of California, or the Regents, which were subsequently amended and restated at various dates (the “UC Agreements”). Under the UC Agreements, the Regents have granted to us exclusive, worldwide licenses, with the right to grant sublicenses, under the Regents’ patent rights in certain patent applications to make, use, sell, offer for sale, and import products and services and practice methods covered by such patent applications in all fields of use. We have paid the Regents license fees of $40,000 in total under the UC Agreements and are required to pay annual license maintenance fees totaling $40,000 prior to launching any licensed product. We may be obligated to make one-time regulatory and development milestone payments under the UC Agreements for future drug candidates and are obligated to pay tiered royalty payments in the low single digits on net sales of the licensed products. Additionally, we are obligated to pay the Regents payments on non-royalty licensing revenue we receive from our sub-licensees for products covered by UC patents. Pursuant to the UC Agreements, we also made IPO milestone payments to the Regents totaling approximately $140,000 in October 2018. Under the UC Agreements, we are required to diligently proceed with the development, manufacture, regulatory approval, and sale of licensed products which include obligations to meet certain development-stage milestones within specified periods of time and to market the resulting licensed products in sufficient quantity to meet market demand. We have the right and option to extend the date by which we must meet any milestone in one year extensions by paying an extension fee for second and subsequent extension, provided we can demonstrate we made diligent efforts to meet the milestone.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies Leases Our corporate headquarters are located in South San Francisco, California. In April 2018, we signed a lease (the “Lease Agreement”) for approximately 47,500 square feet of office, research and development and laboratory space with occupancy commencing on and we have utilized, The additional tenant improvement allowance used in connection with this option is amortized and was added to monthly rent payments upon completion of the tenant improvements in May 2019. Reimbursable construction costs incurred were recorded as a leasehold improvement with a corresponding lease incentive obligation, which was classified as a component of deferred rent. Amounts that were reimbursed under the tenant improvement allowance were recorded as deferred rent and are being amortized as a reduction to rent expense over the lease term. We utilized the aggregate amount of allowances available to us and have recorded of tenant improvement costs as leasehold improvements as of December 31, 2019. We recognize rent expense on a straight-line basis over the non-cancellable lease term and record the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. Where leases contain escalation clauses, rent abatements, and/or concessions such as rent holidays and landlord or tenant incentives or allowances, we apply them in the determination of straight-line rent expense over the lease term. We recorded tenant improvement allowances as deferred rent and the associated expenditures as leasehold improvements. Leasehold improvements are being amortized over the shorter of their estimated useful life or the term of the lease. We do not assume renewals in our determination of the lease term unless renewals are deemed by management to be reasonably assured at lease inception. We have determined that our lease related to the Lease Agreement commenced on August 1, 2018, when we had right to use or control physical access to the new leased premises. Future minimum lease payments for operating leases at December 31, 2019 are as follows (in thousands): Year ended December 31, 2020 $ 3,093 2021 3,193 2022 3,296 2023 3,403 2024 3,514 2025 and beyond 3,933 Total $ 20,432 Our previous lease, which expired on January 31, 2019, was for 30,000 square feet of office, research and development, and laboratory space in South San Francisco. It was subject to several amendments to secure additional space, sublease certain office and laboratory space and/or extend the lease term. In 2015, we entered into agreements to sublease certain office and laboratory space to two different subtenants. Pursuant to the sublease agreements, we received sublease payments of $0.1 million for year ended December 31, 2017, which were recognized as an offset to rent expense. Both subleases were terminated during 2017. We recorded rent expense of $3.1 million and $2.7 million for the years ended December 31, 2019 and 2018, respectively, and rent expense, net of sublease payments, of $1.6 million for the year ended December 31, 2017. Indemnifications We are required to recognize a liability for the fair value of any obligations we assume upon the issuance of a guarantee. We have certain agreements with licensors, licensees, collaborators and service providers that contain indemnification provisions. In such provisions, we typically agree to indemnify the licensor, licensee collaborator or service provider against certain types of third party claims. The maximum amount of the indemnifications is usually not limited. We accrue for known indemnification issues when a loss is probable and can be reasonably estimated. There were no accruals for expenses related to indemnification issues for any periods presented. We indemnify each of our officers and directors for certain events or occurrences, subject to certain limitations, while the officer or director is or was serving at our request in such capacity, as permitted under Delaware law and in accordance with our certificate of incorporation, our bylaws and certain indemnification agreements between us and each of our directors and officers. The term of the indemnification period lasts as long as an officer or a 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, we currently hold director and officer liability insurance. This insurance allows the transfer of risk associated with our exposure and may enable us to recover a portion of any future amounts paid. We believe that the fair value of these indemnification obligations is minimal. Accordingly, we have not recognized any liabilities relating to these obligations for any period presented.

Debt Obligation

Debt Obligation	12 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Debt Obligation	8. Debt Obligation Convertible Note Financing In August 2016, we entered into a Note and Warrant Purchase Agreement (the “Convertible Note Agreement”), as amended in September 2016 and March 2017, with certain of our existing investors, pursuant to which we issued convertible notes (the “Notes”) in the aggregate amount of $21.5 million. The Notes had an annual interest rate of 8.0% and a maturity date of December 31, 2017. At the time of issuance of the Notes in 2016 and 2017, we determined that a beneficial conversion feature existed based on the amount by which the fair value of the underlying shares of Series B-3 preferred stock exceeded the effective conversion rate of the Notes. An incremental debt discount of $0.5 million and $1.9 million was recorded related to this beneficial conversion feature in 2017 and 2016, respectively. Notes Redemption Features Liability The Notes also had redemption features that were determined to be an embedded derivative requiring bifurcation and separate accounting. The fair value of the embedded derivative upon issuance was determined to be $3.4 million and $1.9 million for the Notes issued in 2016 and 2017, respectively, and was recognized as a debt discount and as a derivative liability on the consolidated balance sheets. The embedded derivative associated with the Notes required re-measurement to fair value using Level 3 inputs while the Notes were outstanding at the end of each reporting period. In December 2017, the holders of the Notes exercised their option to convert the Notes and accrued interest into Series B-3 convertible preferred stock. In connection with the conversion, we issued 1,968,561 shares of Series B-3 convertible preferred stock at $11.9907 per share in a non-cash transaction and derecognized our convertible notes payable and related notes interest payable. Upon conversion, the fair value of the convertible notes redemption features liability was re-measured to zero. Prior to the conversion, we determined the fair value of the convertible notes redemption features liability based on a lattice model that utilizes projected outcomes and the probability of those outcomes. The inputs used to determine the estimated fair value of the derivative instrument included the probability of an underlying event triggering the exercise of the redemption features and its timing. The fair value measurement was based upon significant inputs not observable in the market at each valuation date. The inputs included our evaluation of the likelihood of various settlement scenarios for the convertible notes, including the probabilities of (a) subsequent preferred stock sales at various price ranges, (b) the completion of an IPO, (c) a change in control of the company, (d) conversion of the Notes upon maturity, and (e) repayment of the notes upon maturity. The settlement scenarios that would have triggered payments under the redemption features included the conversion to a future round of financing at a discounted price and a change in control prior to the conversion or repayment of the Notes. The other scenarios would not have triggered payment under the redemption features. Changes in fair value related to the notes redemption features liability were recorded as a component of other income (expense), net. The following table summarizes the changes in fair value during the year ended December 31, 2017 (in thousands): Estimated Fair Value of Convertible Notes Redemption Features Liability Balance at December 31, 2016 $ 3,648 Fair value of the redemption features related to second tranche of convertible notes 1,938 Change in estimated fair value of redemption features liability included in other income (expense), net (5,586 ) Balance at December 31, 2017 $ — In September 2018, upon our IPO, all shares of convertible preferred stock were converted into an equal number shares of common stock. Preferred Stock Warrant Liability In connection with the issuance of the Notes, we issued warrants to purchase our capital stock. Upon conversion of the Notes into Series B-3 preferred stock on December 29, 2017, we determined the total number of shares underlying such warrants to be 168,046 shares of our capital stock at an exercise price of $8.99 per share. The warrants are exercisable upon the issuance date and have a five-year term. The estimated fair value of the warrants upon issuance, of $1.1 million, was determined using an option pricing model. We recorded the fair value of the warrants at issuance as a debt discount and as a convertible preferred stock warrant liability. The debt discount was accreted using the effective interest method as additional interest expense over the term of the Notes. The estimated fair value of the warrants was subject to re-measurement at each reporting period. As of December 31, 2017, the fair value of the outstanding warrants was $1.5 million. In September 2011, in connection with entering into a debt facility agreement, we issued warrants to purchase 4,954 shares of Series A convertible preferred stock at a price of $9.0839 per share. The warrants have a 10-year term and can be exercised by paying cash or converted into the number of shares of convertible preferred stock equal in value to the intrinsic value of the warrants (“Net Exercises”). The estimated fair value of the warrants was subject to re-measurement at each balance sheet date. As of December 31, 2017, the fair value of outstanding Series A warrants was $27,000. The following table summarizes the changes in the estimated fair value of our convertible preferred stock warrant liability related to Series A and Series B-3 warrants for the years ended December 31, 2018 and 2017 (in thousands): Estimated Fair Value of Convertible Preferred Stock Warrant Liability Balance at December 31, 2016 $ 1,050 Change in estimated fair value of convertible preferred stock warrant liability included in other income (expense), net 526 Balance at December 31, 2017 1,576 Change in estimated fair value of convertible preferred stock warrant liability included in other income (expense), net 634 Reclassification of warrant liability to additional paid-in capital upon exercise of warrants (57 ) Reclassification of warrant liability to additional paid-in capital upon initial public offering (2,153 ) Balance at December 31, 2018 $ — The fair value of the preferred stock warrants was determined using a hybrid approach that takes into account the probability of IPO and non-IPO liquidity events. The non-IPO liquidity event scenario uses an option pricing model to allocate the total enterprise value to the various securities within our capital structure based on future expectations. The IPO scenario also uses an option pricing model to estimate the value of the warrant in the case of an IPO. The two OPM models’ inputs reflect assumptions that market participants would use in pricing the instrument in a current period transaction. The fair values of our convertible preferred stock warrant liability related to Series A and Series B-3 warrants totaled approximately $1.6 million as of December 31, 2017. As of December 31, 2018, no preferred stock warrants were outstanding. Interest expense We had no interest expense during the years ended December 31, 2019 and 2018. Interest expense of approximately $7.2 million was recognized during the year ended December 31, 2017, which included $5.5 million of accretion of debt discounts related to the beneficial conversion features, the convertible notes redemption features liability and the convertible preferred stock warranty liability, and $1.7 million of accrued interest expense on the Notes which was converted into preferred stock in December 2017.

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Convertible Preferred Stock	9. Convertible Preferred Stock We have issued two classes of stock, convertible preferred stock and common stock. Convertible preferred stock is carried at issuance price, net of issuance costs. In August 2018, we sold 3,474,668 shares of Series C convertible preferred stock at a price of $14.3898 per share for net proceeds of $49.8 million. Upon the completion of the IPO in September 2018, all the outstanding shares of the convertible preferred stock were converted into 15,760,102 shares of common stock, including shares of Series C convertible preferred stock that was issued in August 2018. In addition, all of our convertible preferred stock warrants were converted into warrants to purchase shares of common stock. As of December 31, 2018, we had no remaining shares of convertible preferred stock issued or outstanding. As of December 31, 2017, convertible preferred stock consisted of the following (in thousands, except share numbers): Shares Authorized Shares Issued and Outstanding Liquidation Amount Carrying Value Series A 40,094,998 4,408,893 $ 40,050 $ 39,630 Series B-1 22,957,067 2,527,221 25,253 25,028 Series B-2 19,130,887 2,106,014 25,252 25,027 Series B-3 31,497,330 3,243,306 38,890 38,846 Total 113,680,282 12,285,434 $ 129,445 $ 128,531 Series A Preferred Stock In 2011 and 2012, we sold a total of 4,408,893 shares of Series A convertible preferred stock, at a price per share of $9.0839, for gross proceeds of approximately $40.1 million. Series B Preferred Stock In April 2014, we completed the first tranche (“Series B-1”) of our Series B round of financing. Pursuant to the financing terms, we issued 2,527,221 shares of Series B-1 convertible preferred stock at $9.9923 per share for gross cash proceeds of $25.3 million. The Series B purchase agreement contemplated the sale in an additional closing, the second tranche (“Series B-2”), of up to 2,106,014 additional shares of our Series B preferred stock for aggregate gross proceeds of approximately $25.3 million. Series B-2 closed in November 2014 after we met the pre-defined milestones. Under the terms of the stock purchase agreement, we issued 2,106,014 shares of Series B-2 convertible preferred stock at $11.9907 per share for gross cash proceeds of approximately $25.3 million. In July 2015, we completed another financing with existing investors for Series B-3 convertible preferred stock. Pursuant to the financing terms, we issued 1,274,745 shares of Series B-3 convertible preferred stock at $11.9907 per share for gross cash proceeds of approximately $15.3 million. In December 2017, the holders of the outstanding convertible notes exercised their option to convert the Notes and accrued interest into Series B-3 convertible preferred stock. In connection with the conversion, we issued 1,968,561 shares of Series B-3 convertible preferred stock at $11.9907 per share in a non-cash transaction. Prior to their conversion upon closing of the IPO, the significant rights and obligations of our convertible preferred stock were as follows: Voting Rights —Each holder of convertible preferred stock is entitled to the number of votes equal to the number of shares of common stock into which such shares of convertible preferred stock are convertible. So long as 20% of the shares of convertible preferred stock remain outstanding, the holders of such shares of preferred stock are entitled to elect five members of our Board of Directors. Dividends— Holders of Series A, B-1, B-2, and B-3 convertible preferred stock are entitled to receive non-cumulative dividends, at the rate of 8.0%, 8.8%, 10.56%, and 10.56% per annum, respectively, for each share of Series A, B-1, B-2, and B-3 convertible preferred stock outstanding, when, as, and if declared by our Board of Directors. These dividends are payable in preference to common stock dividends. To date, we have not declared or paid any dividends. Liquidation— In the event of any liquidation, dissolution, or winding up of the company, the holders of Series A, B-1, B-2, and B-3 convertible preferred stock shall be entitled to receive payment from all assets legally available at a liquidation preference of $9.0839, $9.9923, $11.9907, and $11.9907 per share, respectively. In the event that the available funds and assets are insufficient for full payment to the holders of convertible preferred stock on a per-share basis as outlined above, our entire assets and funds legally available for distribution shall be distributed ratably among the holders of convertible preferred stock in proportion to the liquidation preference amount each such holder is otherwise entitled to receive. Upon completion of the distribution of assets as set forth above, all of the remaining assets, if any, shall be distributed ratably among the holders of preferred stock and common stock pro rata based on the number of shares of common stock held by each (assuming full conversion of all preferred stock to common stock at the then effective conversion rate). Conversion— Each share of convertible preferred stock is convertible at a 1:1 ratio at the option of the holder into fully paid and non-assessable shares of common stock. Conversion of each series of convertible preferred stock into common stock is automatic upon the earlier of (i) the closing of an IPO of our common stock, registered under the Securities Act of 1933, as amended, which results in aggregate proceeds equal to or exceeding $50.0 million to us, and the value of our shares immediately prior to the IPO is not less than $22.71 per share (as adjusted for any stock splits, stock dividends, combinations, subdivisions, recapitalizations or the like); or (ii) at any time upon the agreement of the holders of at least 60% of the then outstanding shares of preferred stock

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 Equity Incentive Plans In August 2018, our Board of Directors adopted the 2018 Equity Incentive Plan (the “2018 Plan”). The 2018 Plan provides for the grant of incentive stock options to employees and for the grant of non-statutory stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance stock awards, performance cash awards and other forms of stock awards to employees, directors and consultants. The 2018 Plan initially reserved 2,270,000 of new shares for issuance with automatic annual increases on January 1 of each year equal to 4% of the total number of shares of our capital stock outstanding on the last day of the preceding year, or a lesser number of shares determined by our Board of Directors. The exercise price of stock options is determined to be the market price of our common stock on the date immediately preceding the date of grant. Options granted under the 2018 Plan typically will vest over a four-year period but may be granted with different vesting terms. Awards that expire or terminate without being exercised are added back to the number of shares available for issuance under the 2018 Plan. Following our IPO and in connection with the effectiveness of the 2018 Plan, the 2008 Equity Incentive Plan (the “2008 Plan”) terminated and no further awards will be granted under that plan. All outstanding awards under the 2008 Plan will continue to be governed by their existing terms. The shares subject to stock options or other stock awards granted under the 2008 Plan and the shares that remained outstanding for issuance under the 2008 Plan were transferred into the 2018 Plan. As of December 31, 2019, there was an aggregate 5,863,621 shares of common stock authorized for issuance under the 2018 Plan. Prior to its termination, the 2008 Plan provided for the grant of incentive and non-statutory stock options to eligible employees, officers, directors, advisors and consultants. Options granted under the 2008 Plan generally vest over a four-year period and vested options are exercisable from the vesting date and expire 10 years from the date of grant. The 2008 Plan allowed for early exercise of certain options prior to vesting. Upon termination of employment, the unvested shares are subject to repurchase at the original exercise price. Stock Option Activity The following table summarizes our stock option activity under our stock plans and related information: Options Available for Grant Options Outstanding Weighted Average Exercise Price Weighted Average Remaining Contractual Life Aggregate Intrinsic Value (years) (in thousands) Balance at December 31, 2018 2,195,788 2,998,590 $ 9.39 8.05 $ 53,994 Options granted (1,418,000 ) 1,418,000 $ 35.32 Options forfeited 240,205 (240,205 ) $ 19.45 Options exercised — (287,468 ) $ 7.20 Options expired 1,137 (1,137 ) $ 30.30 Options repurchased 738 — Shares added to plans 955,973 — Balance at December 31, 2019 1,975,841 3,887,780 $ 18.38 7.69 $ 141,505 Vested and expected to vest 3,887,780 $ 18.38 7.69 $ 141,505 Exercisable as of December 31, 2019 2,270,563 $ 8.17 7.70 $ 105,835 The weighted-average fair values of stock options granted during t he years 2019, 2018 and 2017 was $24.8, $10.5 and $4.3 per share, respectively he years 2019, 2018 and 2017 was $8.6 million, $1.7 million and $0.1 million, respectively. At December 31, 2019 and 2018, 25,081 shares and 33,882 shares, respectively, related to the early exercise of stock options, were subject to repurchase by us at the original exercise price in the event the optionee’s employment is terminated either voluntarily or involuntarily. We classify cash received from the exercise of unvested options as a short term liability. These liabilities were approximately $0.2 million as of December 31, 2019 and 2018, respectively, and were included in accrued liabilities on the consolidated balance sheets. Amounts from such liabilities are reclassified into common stock and additional paid-in capital as the shares vest, which is generally over 48 months. Common shares are presented as outstanding on our consolidated balance sheets and consolidated statements of changes in convertible preferred stock and stockholders’ equity (deficit) as the shares vest and the underlying right of repurchase lapses. Valuation Assumptions We use the Black-Scholes valuation model to estimate fair value of stock options at the grant date. The Black-Scholes valuation model requires us to make certain estimates and assumptions, including assumptions related to the expected term, expected volatility, risk-free interest rate, and the expected dividend yield for our stock. The fair values for the stock options granted to employees and non-employee directors were estimated at the date of grant using the Black-Scholes valuation model with the following weighted-average assumptions: Year Ended December 31, 2019 2018 2017 Risk-free interest rate 1.3% - 2.6% 2.5% - 3.1% 1.8% - 2.2% Expected term (years) 5.0 - 6.0 5.0 - 6.1 5.8 - 6.1 Expected volatility 73.0% - 80.0% 80.0% - 84.5% 81.0% - 85.3% Dividend yield —% —% —% The valuation assumptions were determined as follows: • Risk-free interest rate—We base the risk-free interest rate on the interest rate payable on U.S. Treasury securities in effect at the time of grant for a period corresponding with the expected option term. • Expected term—The expected term of options represents the period of time options are expected to be outstanding. The expected term of the options granted is derived using the “simplified” method (that is, estimating the expected term as the mid-point between the vesting date and the end of the contractual term for each option). We use the simplified method because we do not have sufficient historical option exercise data to provide a reasonable basis upon which to estimate the expected term. • Expected volatility—The expected volatility used is based on historical volatilities of similar entities within our industry for periods corresponding with our expected term assumption. • Expected dividend yield—Historically, we have never paid dividends on our common stock and do not expect to pay a dividend in the foreseeable future. Therefore, we used an expected dividend yield of zero. Stock Options Granted to Employees Employee stock-based compensation is measured at the grant date based on the fair value of the award and is recognized as an expense, net of forfeitures, over the requisite service period, which is generally the vesting period of the respective award. Employee stock-based compensation expense includes expenses related to options granted to employees and non-employee directors. The following table summarizes total stock-based compensation expense related to our 2018 Equity Incentive Plan, 2008 Equity Incentive Plan and 2018 Employee Stock Purchase Plan Year Ended December 31, 2019 2018 2017 Research and development $ 6,636 $ 1,427 $ 462 General and administrative 5,546 1,352 559 Total $ 12,182 $ 2,779 $ 1,021 As of December 31, 2019, there was approximately $36.2 million of unamortized compensation cost related to stock option awards that is expected to be recognized as expense over a weighted-average period of approximately 2.9 years. The weighted-average remaining contractual term of options outstanding at December 31, 2019 was 7.7 years. Our stock-based compensation expense for the year ended December 31, 2019 includes approximately $0.7 million in expense related to modifications of stock options for members of our Board of Directors. Stock Options Granted to Outside Consultants We did not grant any stock options to consultants for the years ended December 31, 2019 and 2017. For the year ended December 31, 2018, we granted to consultants a total of 1,101 stock options. Stock-based compensation expense related to outside consultants was insignificant for each of the years ended December 31, 2019, 2018 and 2017. Employee Stock Purchase Plan In September 2018, we adopted the 2018 Employee Stock Purchase Plan (the “ESPP”), which initially reserved 250,000 shares of our common stock for employee purchases under terms and provisions established by the Board of Directors, with automatic annual increases on January 1 of each year equal to the lesser of (a) 1% of the total number of shares of our common stock outstanding on the last day of the preceding year, or (b) 500,000 shares of our common stock. Under the ESPP, employees may purchase common stock through payroll deductions at a price equal to 85% of the lower of the fair market value of common stock on the first trading day of each offering period or on the exercise date. The ESPP provides for consecutive 12-month offering periods. The offering periods may start on the first trading day on or after May 16 or November 16 of each year, except for the first offering period which commenced on September 13, 2018, the pricing date and the effective date of our registration statement. Contributions under the ESPP are limited to a maximum of 15% of an employee's eligible compensation. We recorded approximately $0.6 million and $0.2 million of stock-based compensation expense related to our ESPP for the years ended December 31, 2019 and 2018, respectively. No income tax benefit has been recognized related to stock-based compensation expense, and no tax benefits have been realized from exercised stock options.

Net Income (Loss) per Share

Net Income (Loss) per Share	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Net Income (Loss) per Share	11. Net Income (Loss) per Share Net income is allocated between our common stock and other participating securities based on their participation rights. Our preferred stock (see Note 9) represents participating securities. Additionally, during the periods in which we have net income, the diluted net income per share has been computed using the weighted average number of shares of common stock outstanding and other dilutive securities. For the periods presented with a net loss herein, the computation of basic earnings per share using the two-class method is not applicable as the participating securities do not have contractual rights and obligations to share in our losses. The following table presents a reconciliation of the numerators and denominators of the basic and diluted net income (loss) per share computations and the calculation of basic and diluted net income (loss) per share (in thousands, except share and per share data): Year Ended December 31, 2019 2018 2017 Numerator Net income (loss) $ (53,792 ) $ 18,167 $ (28,697 ) Allocation of undistributed earnings to participating securities — (13,250 ) — Net income (loss) attributable to common stockholders (53,792 ) 4,917 (28,697 ) Denominator Weighted-average common shares outstanding, basic 25,778,122 7,622,602 569,719 Effect of dilutive shares — 1,004,871 — Weighted-average shares outstanding used in per share calculation, diluted 25,778,122 8,627,473 569,719 Net income (loss) per share, basic $ (2.09 ) $ 0.65 $ (50.37 ) Net income (loss) per share, diluted $ (2.09 ) $ 0.57 $ (50.37 ) The following outstanding shares of common stock equivalents were excluded in the computation of diluted net income (loss) per share attributable to common stockholders, because their effect would have been antidilutive for the periods presented. Year Ended December 31, 2019 2018 2017 Series A-1 convertible preferred stock — — 4,408,893 Series B-1 convertible preferred stock — — 2,527,221 Series B-2 convertible preferred stock — — 2,106,014 Series B-3 convertible preferred stock — — 3,243,306 Employee stock purchase plan 8,054 27,751 — Warrants to purchase capital stock 81,966 168,046 173,000 Common stock options issued and outstanding 3,887,780 1,993,720 1,921,453 Early exercised common stock subject to future vesting 25,081 33,882 4,146 Total 4,002,881 2,223,399 14,384,033

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	12. Income Taxes Income (loss) before provision for income taxes consisted of the following (in thousands): Year Ended December 31, 2019 2018 2017 United States $ (53,789 ) $ 18,226 $ (28,589 ) Foreign (3 ) (59 ) (108 ) Total $ (53,792 ) $ 18,167 $ (28,697 ) The income tax provision is summarized as follows: Year Ended December 31, 2019 2018 2017 Current Federal $ — $ — $ — State 800 800 800 International — — — Total current tax expense 800 800 800 Deferred Federal — — — State — — — International — — — Total deferred tax expense — — — Total tax expense $ 800 $ 800 $ 800 Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes, and operating losses and tax credit carryforwards. The tax effects of significant items comprising our net deferred tax assets are as follows (in thousands): December 31, 2019 2018 Deferred tax assets: Net operating loss carryforwards $ 40,501 $ 29,983 Stock-based compensation 2,046 453 Deferred revenue — 1,179 Other accrued expenses 3,330 1,079 Total deferred tax assets 45,877 32,694 Deferred tax liabilities: Fixed assets and intangibles (1,577 ) (141 ) Total deferred tax liabilities (1,577 ) (141 ) Valuation allowance (44,300 ) (32,553 ) Net deferred tax assets $ — $ — ASC 740 requires that the tax benefit of net operating losses, temporary differences and credit carryforwards be recorded as an asset to the extent that management assesses that realization is “more likely than not.” Realization of the future tax benefits is dependent on our ability to generate sufficient taxable income within the carryforward period. Because of our recent history of operating losses, we believe that recognition of the deferred tax assets arising from the above-mentioned future tax benefits is currently not likely to be realized and, accordingly, have provided a valuation allowance. The valuation allowance increased by $11.7 million and decreased by $4.5 million and $8.0 million during the years ended December 31, 2019, 2018 and 2017, respectively. Net operating losses and tax credit carryforwards as of December 31, 2019 are as follows (amounts in thousands): Amount Expiration Years Net operating losses, federal (post December 31, 2017) $ 50,403 No expiration Net operating losses, federal (pre January 1, 2018) 103,950 2031 - 2037 Net operating losses, state 111,770 2031 - 2036 Tax credits, federal 10,100 2031 - 2039 Tax credits, state 4,934 No expiration The Tax Reform Act of 1986 limits the use of net operating loss carryforwards in certain situations where changes occur in the stock ownership of a company. The annual limitation may result in the expiration of net operating losses and credits before utilization. The Company does not expect any previous ownership changes, as defined under Section 382 and 383 of the Internal Revenue Code, to result in a limitation that will materially reduce the total amount of net operating loss carryforwards and credits that can be utilized. The effective tax rate of our provision for (benefit from) income taxes differs from the federal statutory income tax provision as follows: Year Ended December 31, 2019 2018 2017 Statutory rate 21.0 % 21.0 % 35.0 % State tax 0.0 % 0.0 % 0.0 % Stock option expense 1.1 % 1.6 % (0.6 %) General business credit 10.8 % (14.7 %) 4.2 % Uncertain tax positions (10.8 %) 14.7 % (4.2 %) Change in valuation allowance (21.8 %) (24.8 %) 30.1 % Other permanent differences (0.2 %) 2.2 % (3.3 %) Tax reform—tax rate change 0.0 % 0.0 % (61.2 %) Total 0.0 % 0.0 % 0.0 % A reconciliation of our unrecognized tax benefits for the years ended December 31, 2019 and 2018 is as follows (in thousands): 2019 2018 Balance at beginning of year $ 8,958 $ 6,119 Addition for tax positions in prior years — 736 Addition for tax positions in current year 6,076 2,103 Balance at end of year $ 15,034 $ 8,958 There is approximately $15.0 million of unrecognized tax benefits included in our consolidated balance sheets. Because of our valuation allowance in the United States, none of the $15.0 million of uncertain tax positions would, if recognized, affect the effective tax rate. We do not believe that our unrecognized tax benefits will significantly change within the next 12 months. It is our practice to recognize interest and penalties related to income tax matters in income tax expense. As of December 31, 2019, we had no accrued interest and penalties related to uncertain tax positions. We file federal, state and foreign income tax returns with varying statutes of limitations. A number of our tax returns remain subject to examination by the tax authorities. These include the U.S. federal and California returns for June 30, 2011 and later years.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2019
Selected Quarterly Financial Information [Abstract]
Selected Quarterly Financial Data (Unaudited)	13. Selected Quarterly Financial Data (Unaudited) The following tables present certain unaudited quarterly financial information for the years ended December 31, 2019 and 2018 (in thousands, except per share amounts): Quarter Ended December 31, 2019 September 30, 2019 June 30, 2019 March 31, 2019 Revenue $ — $ — $ 30,000 $ 5,160 Income (loss) from operations $ (26,611 ) $ (23,333 ) $ 6,049 $ (14,871 ) Net income (loss) $ (24,924 ) $ (22,296 ) $ 7,115 $ (13,687 ) Net income (loss) per share - basic $ (0.80 ) $ (0.93 ) $ 0.30 $ (0.57 ) Net income (loss) per share - diluted $ (0.80 ) $ (0.93 ) $ 0.28 $ (0.57 ) Quarter Ended December 31, 2018 September 30, 2018 June 30, 2018 March 31, 2018 Revenue $ 26,137 $ 18,564 $ 12,987 $ 11,449 Income from operations $ 8,262 $ 6,477 $ 1,871 $ 532 Net income $ 9,377 $ 6,683 $ 1,797 $ 310 Net income per share - basic $ 0.39 $ — $ — $ — Net income per share - diluted $ 0.37 $ — $ — $ —

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) and include the accounts of Principia and our wholly-owned Australian subsidiary. All intercompany accounts, transactions and balances have been eliminated.
Use of Estimates	Use of Estimates The preparation of our financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, as well as related disclosure of contingent assets and liabilities. Significant estimates include amounts to determine the fair value of common stock-based awards, warrants, and other issuances, embedded derivatives, accruals for research and development costs and uncertain tax positions, and the estimated periods of performance used in the determination of collaboration revenues. We base our estimates on historical experience and on various other market specific and relevant assumptions that our management believes to be reasonable under the circumstances. Actual results could differ materially from our estimates.
Concentration of Credit Risk and Other Risks and Uncertainties	Concentration of Credit Risk and Other Risks and Uncertainties Financial instruments that potentially subject us to a significant concentration of credit risk consist primarily of cash, cash equivalents and marketable securities. Our cash and cash equivalents are maintained with a limited number of financial institutions in the United States. Amounts on deposit have exceeded and will continue to exceed federally insured limits. We have not experienced any losses on our cash deposits. Additionally, we have established guidelines regarding the diversification of our investments in approved instruments, their credit quality ratings and maturities. The guidelines are designed to preserve principal balances and provide liquidity. We are subject to a number of risks similar to other late-stage or collect , or due to us,
Cash and Cash Equivalents	Cash and Cash Equivalents We consider all highly liquid financial instruments with original
Marketable Securities	Marketable Securities We carry marketable securities consisting primarily of money market funds, U.S. Treasury securities and obligations of government-sponsored enterprises and corporate bonds and commercial paper. Marketable securities with maturities greater than 90 days at the time of purchase and that mature less than one year from the consolidated balance sheet date are classified as short-term. Marketable securities with a maturity date greater than one year at each balance sheet date are classified as long-term. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on marketable securities are included in other income (expense), net. The cost of securities sold is based on the specific-identification method. Interest earned on marketable securities is included in interest income. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts and such amortization and accretion are included as a component of interest income.
Restricted Cash	Restricted Cash As of December 31, 2019 and 2018, we had $0.6 million in long-term restricted cash for a lease security deposit. This amount is separated from cash and cash equivalents on the consolidated balance sheets.
Segments	Segments We have one operating segment. Our chief operating decision maker, our President and
Property and Equipment	Property and Equipment Property and equipment consist of laboratory equipment, computer equipment, furniture and fixtures and leasehold improvements and are stated at cost, net of accumulated depreciation. Maintenance and repairs are charged to expense as incurred. Depreciation is recognized using the straight-line method over the estimated useful lives of the assets generally ranging from two years for computer equipment to five years for office furniture and fixtures. Leasehold improvements are amortized over the shorter of their estimated useful lives or the related lease term.
Leases	Leases We enter into lease agreements for our laboratory and office facilities. These leases are classified as operating leases. Rent expense is recognized on a straight-line basis over the term of the lease. The difference between rent expense recognized and rental payments is recorded as deferred rent in the consolidated balance sheets Lease incentives and allowances provided by our landlord for the construction of leasehold improvements are recorded as lease incentive obligations as the related construction costs are incurred
Revenue Recognition	Revenue Recognition Effective January 1, 2019, we adopted Accounting Standards Codification (“ASC”) No. 2014-09, Revenue from Contracts with Customers, or ASC 606, using the modified retrospective approach. Under this approach, we recorded a cumulative adjustment to decrease accumulated deficit and deferred revenue by $0.4 million as of the adoption date. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. We only apply the five-step model to contracts when it is probable that we will collect the consideration we are entitled to in exchange for the goods and services we transfer to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, we assess the goods or services promised within each contract, determine those that are performance obligations and assess whether each promised good or service is distinct. We then recognize as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied . We have entered into licensing and collaboration agreements that are within the scope of ASC 606. Licenses of Intellectual Property: If the license to our intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, we recognize revenues from non-refundable, upfront fees allocated to the license when the license is transferred to the licensee and the licensee is able to use and benefit from the license. For licenses that are bundled with other promised goods or services, we utilize judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue from non-refundable, upfront fees. We evaluate the measure of progress each reporting period and, if necessary, adjust the measure of performance and related revenue recognition. Milestone Payments: At the inception of each arrangement that includes development, regulatory or commercial milestone payments, we evaluate whether the milestones are considered probable of being reached and estimate the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant reversal of cumulative revenue would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within our control or that of our licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received or the underlying activity has been completed. The transaction price is then allocated to each performance obligation on a relative stand-alone selling price basis, for which we recognize revenue as or when the performance obligations under the contract are satisfied. At the end of each subsequent reporting period, we re-evaluate the probability of achievement of such development milestones and any related constraint, and if necessary, adjust our estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect collaboration revenue in the period of adjustment. Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, we recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, we have not recognized any royalty revenue resulting from any of our licensing arrangements. The impact of the adoption of ASC 606 on the accompanying consolidated balance sheet as of January 1, 2019 was as follows (in thousands): December 31, 2018 Adjustments Due to ASC 606 January 1, 2019 Liabilities Deferred revenue $ 5,616 $ (456 ) $ 5,160 Stockholders' Equity Accumulated deficit (132,407 ) 456 (131,951 ) The impact of the adoption of ASC 606 on our consolidated statement of operations for the year ended December 31, 2019 was as follows (in thousands): Year Ended December 31, 2019 As Reported Adjustments Balance without ASC 606 Adoption Revenue $ 35,160 $ 456 $ 35,616 Loss from operations (58,765 ) 456 (58,309 ) Net loss (53,792 ) 456 (53,336 ) Net loss per share, basic and diluted (2.09 ) 0.02 (2.07 ) During the year ended December 31, 2019, we did not recognize any revenue from performance obligations satisfied in previous periods.
Convertible Preferred Stock Warrants	Convertible Preferred Stock Warrants In connection with the issuance of certain convertible notes in 2016 and 2017, we issued warrants to purchase our capital stock. Freestanding warrants to purchase our convertible preferred stock were recorded as a liability on our consolidated balance sheets because the underlying shares of convertible preferred stock are contingently redeemable, which, therefore, may obligate us to transfer assets to settle those warrants. The warrants are subject to revaluation at each balance sheet date, with changes in fair value recognized as a component of other income (expense), net, on the consolidated statements of operations. Upon the completion of our IPO in September 2018, all of our convertible preferred stock warrants were converted into warrants to purchase shares of common stock. We re-valued the convertible preferred stock warrants upon completion of our IPO and reclassified the estimated fair value of the warrants to additional paid-in capital. See Note 8, Debt Obligation, for additional information.
Convertible Notes Redemption Features	Convertible Notes Redemption Features We recorded a derivative liability related to redemption features embedded within our convertible notes. The convertible notes issued in 2016 and 2017 included features that were determined to be an embedded derivative requiring bifurcation and separate accounting. This bifurcated redemption feature was initially recorded at fair value and was subject to revaluation at each balance sheet date. Changes in fair value were recognized as a component of other income (expense), net, on the consolidated statements of operations. In December 2017, the convertible notes and interest payable were converted into Series B-3 preferred stock and the convertible notes redemption features liability was determined to have a fair value of zero as of December 31, 2017. See Note 8, Debt Obligation, for additional information
Research and Development Expenses	Research and Development Expenses Research and development expenses consist primarily of personnel costs, including salaries, benefits and stock-based compensation, costs related to clinical and preclinical studies, contract manufacturing, consulting fees, laboratory supplies, and allocated overhead and facility occupancy costs. Research and development costs are expensed as incurred. Non-refundable advance payments for goods or services that will be used or rendered for future research and development activities are capitalized and expensed as the goods are delivered or the related services are performed. Costs associated with co-development activities are included in research and development expenses, with any reimbursement of costs by our collaboration partners reflected as a reduction of research and development expenses.
Stock-based Compensation	Stock-based Compensation We use the Black-Scholes valuation model to estimate the grant date fair value of stock option awards with time-based vesting terms. The determination of fair value for stock-based awards on the date of grant using an option-pricing model (“OPM”) requires management to make certain assumptions regarding a number of variables. Stock-based compensation expense is recognized based on the grant date fair value on a straight-line basis over the requisite service period and is reduced for forfeitures as they occur. Equity-based payments granted to non-employees are recorded at grant date fair value and are re-measured at the current fair value at the end of each reporting period until the underlying equity instruments vest. Non-employee stock-based compensation expense was immaterial for all periods presented.
Income Taxes	Income Taxes We account for income taxes under the asset and liability method. Deferred tax assets and liabilities are recognized for future tax consequences attributable to the differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and net operating loss and tax credit carryforwards. Valuation allowances are established when necessary to reduce deferred tax assets to the amount expected to be realized. Deferred tax assets and liabilities are measured at the balance sheet date using the enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in the period such tax rate changes are enacted. We record uncertain tax positions on the basis of a two-step process in which (1) it determines whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, we recognize the largest amount of tax benefit that is more than 50 percent likely to be realized upon ultimate settlement with the related tax authority. Our policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the underpayment of income taxes. Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes, and operating losses and tax credit carryforwards. ASC 740 requires that the tax benefit of net operating losses, temporary differences and credit carryforwards be recorded as an asset to the extent that management assesses that realization is “more likely than not.” Realization of the future tax benefits is dependent on our ability to generate sufficient taxable income within the carryforward period. Because of our recent history of operating losses, we believe that recognition of the deferred tax assets arising from the above-mentioned future tax benefits is currently not likely to be realized and, accordingly, have provided a valuation allowance.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in stockholders’ equity (deficit) of a business enterprise during a period, resulting from transactions from non-owner sources, and consists primarily of unrealized gains or losses related to our available-for-sale marketable securities, which are carried at estimated fair values on the consolidated balance sheets.
Net Income (Loss) per Share	Net Income (Loss) per Share Basic net income (loss) per share attributable to common stockholders is calculated by dividing the net income (loss) attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period. Diluted net income (loss) per share includes the effect of potentially dilutive securities, which include outstanding warrants and stock options if the effect of their inclusion would be dilutive. In periods of net loss, diluted net loss per share is the same as basic net loss per share as the inclusion of potentially dilutive securities in the calculation would be anti-dilutive. We have issued securities other than common stock that participate in dividends to the extent declared (“Participating Securities”), and therefore utilize the two-class method to calculate net income (loss) per share. These Participating Securities include Series A, Series B-1, Series B-2, Series B-3 and Series C redeemable convertible preferred stock. The two-class method requires a portion of net income (loss) to be allocated to the Participating Securities to determine net income (loss) attributable to common stockholders. Net income (loss) attributable to common stockholders is equal to the net income (loss) less dividends paid on preferred stock with any remaining earnings allocated in accordance with the bylaws between the outstanding common and redeemable convertible preferred stock as of the end of each period. In September 2018, upon our IPO, all shares of convertible preferred stock were converted into an equal number of shares of common stock.
Recent Accounting Pronouncements	Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board, or FASB Recently Adopted Accounting Standards Updates In November 2016, the FASB issued accounting standard update (“ASU”) 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash We early adopted ASU 2016-18 during the fourth quarter of 2018 and the adoption did not have a material impact on our consolidated financial statements and related disclosures. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) Entities have the option of applying either a full retrospective approach to all periods presented, or a modified approach that reflects differences prior to the date of adoption as an adjustment to equity. We adopted ASC No. 2014-09 on January 1, 2019 using the modified retrospective method of transition applied to contracts that were not completed at January 1, 2019. Therefore, comparative information will not be adjusted and the impact of the transition is reflected as an adjustment to the opening accumulated deficit. A completed contract is a contract for which all, or substantially all, of the revenue was recognized in accordance with revenue guidance in effect before the date of initial application. The new revenue recognition standard differs from the previous accounting standard in many respects, such as in the accounting for variable consideration and the measurement of progress toward completion of performance obligations. Recently Issued Accounting Standards or Updates Not Yet Effective In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases (Topic 842): Targeted Improvements In June 2018, the FASB issued ASU No. 2018-07, Compensation–Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework–Changes to the Disclosure Requirements for Fair Value Measurement . This ASU eliminates, modifies and adds disclosure requirements for fair value measurements. The amendments in this ASU are effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. We and related disclosures. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. We and related disclosures.
Fair Value Measurements	Financial assets and liabilities are recorded at fair value. We determine fair value using the fair value hierarchy, which establishes three levels of inputs that may be used to measure fair value, as follows: Level 1 inputs include quoted prices in active markets for identical assets or liabilities. Level 2 inputs include observable inputs other than Level 1 inputs, such as quoted prices for similar assets or liabilities in active markets; quoted prices for identical or similar assets or liabilities in markets that are not active; and other inputs that are observable or can be corroborated by observable market data for substantially the full term of the asset or liability. Level 3 inputs include unobservable inputs that are supported by little or no market activity and are significant to the fair value of the underlying asset or liability. Such inputs reflect our best estimate of what market participants would use in pricing the asset or liability at the reporting date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model. Assets and liabilities measured at fair value are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. Our assessment of the significance of a particular input to the fair value measurement in its entirety requires our management to make judgments and consider factors specific to the asset or liability.

Significant Accounting Polici_3

Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Impact of the Adoption of ASC 606 on Consolidated Financial Statements	The impact of the adoption of ASC 606 on the accompanying consolidated balance sheet as of January 1, 2019 was as follows (in thousands): December 31, 2018 Adjustments Due to ASC 606 January 1, 2019 Liabilities Deferred revenue $ 5,616 $ (456 ) $ 5,160 Stockholders' Equity Accumulated deficit (132,407 ) 456 (131,951 ) The impact of the adoption of ASC 606 on our consolidated statement of operations for the year ended December 31, 2019 was as follows (in thousands): Year Ended December 31, 2019 As Reported Adjustments Balance without ASC 606 Adoption Revenue $ 35,160 $ 456 $ 35,616 Loss from operations (58,765 ) 456 (58,309 ) Net loss (53,792 ) 456 (53,336 ) Net loss per share, basic and diluted (2.09 ) 0.02 (2.07 )

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis	Assets and liabilities measured at fair value on a recurring basis as of December 31, 2019 and 2018 were as follows (in thousands): December 31, 2019 Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 17,664 $ — $ — $ 17,664 Corporate commercial paper — 6,983 — 6,983 Corporate debt securities — 10,204 — 10,204 Short-term marketable securities Corporate commercial paper — 95,088 — 95,088 Corporate debt securities — 154,690 — 154,690 Government‑sponsored enterprise securities — 26,265 — 26,265 Long-term marketable securities Corporate debt securities — 14,202 — 14,202 Government‑sponsored enterprise securities — 38,055 — 38,055 Total $ 17,664 $ 345,487 $ — $ 363,151 December 31, 2018 Level 1 Level 2 Level 3 Total Assets Cash equivalents Money market funds $ 19,861 $ — $ — $ 19,861 Corporate commercial paper — 7,465 — 7,465 Corporate debt securities — 4,499 — 4,499 Short-term marketable securities Corporate commercial paper — 36,180 — 36,180 Corporate debt securities — 71,903 — 71,903 Government‑sponsored enterprise securities — 9,906 — 9,906 U.S. Treasury securities — 24,447 — 24,447 Long-term marketable securities Corporate debt securities — 3,712 — 3,712 Total $ 19,861 $ 158,112 $ — $ 177,973

Cash Equivalents and Marketab_2

Cash Equivalents and Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Summary of Cash Equivalents and Marketable Securities	Cash equivalents and marketable securities consisted of the following as of December 31, 2019 and 2018 (in thousands): December 31, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents Money market funds $ 17,664 $ — $ — $ 17,664 Corporate commercial paper 6,982 1 — 6,983 Corporate debt securities 10,205 — (1 ) 10,204 Total cash equivalents 34,851 1 (1 ) 34,851 Short-term marketable securities Corporate commercial paper 95,061 36 (9 ) 95,088 Corporate debt securities 154,678 54 (42 ) 154,690 Government‑sponsored enterprise securities 26,231 34 — 26,265 Total short-term marketable securities 275,970 124 (51 ) 276,043 Long-term marketable securities Corporate debt securities 14,194 8 — 14,202 Government‑sponsored enterprise securities 38,037 18 — 38,055 Total long-term marketable securities 52,231 26 — 52,257 Total $ 363,052 $ 151 $ (52 ) $ 363,151 December 31, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents Money market funds $ 19,861 $ — $ — $ 19,861 Corporate commercial paper 7,466 — (1 ) 7,465 Corporate debt securities 4,499 — — 4,499 Total cash equivalents 31,826 — (1 ) 31,825 Short-term marketable securities Corporate commercial paper 36,197 — (17 ) 36,180 Corporate debt securities 71,920 8 (25 ) 71,903 Government‑sponsored enterprise securities 9,911 — (5 ) 9,906 U.S. Treasury securities 24,451 — (4 ) 24,447 Total short-term marketable securities 142,479 8 (51 ) 142,436 Long-term marketable securities Corporate debt securities 3,706 6 — 3,712 Total long-term marketable securities 3,706 6 — 3,712 Total $ 178,011 $ 14 $ (52 ) $ 177,973

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2019
Balance Sheet Related Disclosures [Abstract]
Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheets that sum to the total of the amounts shown in the consolidated statements of cash flows (in thousands): December 31, 2019 December 31, 2018 December 31, 2017 December 31, 2016 Cash and cash equivalents $ 39,537 $ 34,489 $ 41,054 $ 5,867 Restricted cash, current — 82 100 100 Restricted cash, non-current 567 567 82 82 Total $ 40,104 $ 35,138 $ 41,236 $ 6,049
Summary of Property and Equipment	Property and equipment as of December 31, 2019 and 2018 consisted of the following (in thousands): December 31, 2019 December 31, 2018 Laboratory equipment $ 2,007 $ 2,500 Computer equipment 289 508 Furniture and fixtures 1,549 392 Leasehold improvements 8,324 420 12,169 3,820 Less accumulated depreciation and amortization (2,482 ) (2,154 ) Total $ 9,687 $ 1,666
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets as of December 31, 2019 and 2018 consisted of the following (in thousands): December 31, 2019 December 31, 2018 Other accounts receivable $ 1,272 $ 2,104 Prepaid expenses 2,893 1,661 Total $ 4,165 $ 3,765
Summary of Other Long-Term Assets	Other long-term assets as of December 31, 2019 and 2018 consisted of the following (in thousands): December 31, 2019 December 31, 2018 Tenant improvement allowance receivables $ — $ 8,324 Long-term deposits 480 480 Total $ 480 $ 8,804

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Lease Payments For Operating Leases	Future minimum lease payments for operating leases at December 31, 2019 are as follows (in thousands): Year ended December 31, 2020 $ 3,093 2021 3,193 2022 3,296 2023 3,403 2024 3,514 2025 and beyond 3,933 Total $ 20,432

Debt Obligation (Tables)

Debt Obligation (Tables)	12 Months Ended
	Dec. 31, 2019
Convertible Notes Redemption Features Liability
Debt Instrument [Line Items]
Summary of Changes in Fair Value Related to Notes Redemption Features Liability and Warrant Liability Related to Series A and B-3 Warrants	Changes in fair value related to the notes redemption features liability were recorded as a component of other income (expense), net. The following table summarizes the changes in fair value during the year ended December 31, 2017 (in thousands): Estimated Fair Value of Convertible Notes Redemption Features Liability Balance at December 31, 2016 $ 3,648 Fair value of the redemption features related to second tranche of convertible notes 1,938 Change in estimated fair value of redemption features liability included in other income (expense), net (5,586 ) Balance at December 31, 2017 $ —
Convertible Preferred Stock Warrant Liability
Debt Instrument [Line Items]
Summary of Changes in Fair Value Related to Notes Redemption Features Liability and Warrant Liability Related to Series A and B-3 Warrants	The following table summarizes the changes in the estimated fair value of our convertible preferred stock warrant liability related to Series A and Series B-3 warrants for the years ended December 31, 2018 and 2017 (in thousands): Estimated Fair Value of Convertible Preferred Stock Warrant Liability Balance at December 31, 2016 $ 1,050 Change in estimated fair value of convertible preferred stock warrant liability included in other income (expense), net 526 Balance at December 31, 2017 1,576 Change in estimated fair value of convertible preferred stock warrant liability included in other income (expense), net 634 Reclassification of warrant liability to additional paid-in capital upon exercise of warrants (57 ) Reclassification of warrant liability to additional paid-in capital upon initial public offering (2,153 ) Balance at December 31, 2018 $ —

Convertible Preferred Stock (Ta

Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Summary of Convertible Preferred Stock	As of December 31, 2017, convertible preferred stock consisted of the following (in thousands, except share numbers): Shares Authorized Shares Issued and Outstanding Liquidation Amount Carrying Value Series A 40,094,998 4,408,893 $ 40,050 $ 39,630 Series B-1 22,957,067 2,527,221 25,253 25,028 Series B-2 19,130,887 2,106,014 25,252 25,027 Series B-3 31,497,330 3,243,306 38,890 38,846 Total 113,680,282 12,285,434 $ 129,445 $ 128,531

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Activity	The following table summarizes our stock option activity under our stock plans and related information: Options Available for Grant Options Outstanding Weighted Average Exercise Price Weighted Average Remaining Contractual Life Aggregate Intrinsic Value (years) (in thousands) Balance at December 31, 2018 2,195,788 2,998,590 $ 9.39 8.05 $ 53,994 Options granted (1,418,000 ) 1,418,000 $ 35.32 Options forfeited 240,205 (240,205 ) $ 19.45 Options exercised — (287,468 ) $ 7.20 Options expired 1,137 (1,137 ) $ 30.30 Options repurchased 738 — Shares added to plans 955,973 — Balance at December 31, 2019 1,975,841 3,887,780 $ 18.38 7.69 $ 141,505 Vested and expected to vest 3,887,780 $ 18.38 7.69 $ 141,505 Exercisable as of December 31, 2019 2,270,563 $ 8.17 7.70 $ 105,835
Summary of Fair Values for Stock Options Granted at Estimated Date of Grant	The fair values for the stock options granted to employees and non-employee directors were estimated at the date of grant using the Black-Scholes valuation model with the following weighted-average assumptions: Year Ended December 31, 2019 2018 2017 Risk-free interest rate 1.3% - 2.6% 2.5% - 3.1% 1.8% - 2.2% Expected term (years) 5.0 - 6.0 5.0 - 6.1 5.8 - 6.1 Expected volatility 73.0% - 80.0% 80.0% - 84.5% 81.0% - 85.3% Dividend yield —% —% —%
Summary of Stock-based Compensation Expense	The following table summarizes total stock-based compensation expense related to our 2018 Equity Incentive Plan, 2008 Equity Incentive Plan and 2018 Employee Stock Purchase Plan Year Ended December 31, 2019 2018 2017 Research and development $ 6,636 $ 1,427 $ 462 General and administrative 5,546 1,352 559 Total $ 12,182 $ 2,779 $ 1,021

Net Income (Loss) per Share (Ta

Net Income (Loss) per Share (Tables)	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Schedule of Reconciliation of Numerators and Denominators Used In Computations and Calculation of Basic and Diluted Net Income (Loss) Per Share	The following table presents a reconciliation of the numerators and denominators of the basic and diluted net income (loss) per share computations and the calculation of basic and diluted net income (loss) per share (in thousands, except share and per share data): Year Ended December 31, 2019 2018 2017 Numerator Net income (loss) $ (53,792 ) $ 18,167 $ (28,697 ) Allocation of undistributed earnings to participating securities — (13,250 ) — Net income (loss) attributable to common stockholders (53,792 ) 4,917 (28,697 ) Denominator Weighted-average common shares outstanding, basic 25,778,122 7,622,602 569,719 Effect of dilutive shares — 1,004,871 — Weighted-average shares outstanding used in per share calculation, diluted 25,778,122 8,627,473 569,719 Net income (loss) per share, basic $ (2.09 ) $ 0.65 $ (50.37 ) Net income (loss) per share, diluted $ (2.09 ) $ 0.57 $ (50.37 )
Summary of Common Stock Equivalents Excluded Used in Computation of Diluted Net Income (loss) Per Share	The following outstanding shares of common stock equivalents were excluded in the computation of diluted net income (loss) per share attributable to common stockholders, because their effect would have been antidilutive for the periods presented. Year Ended December 31, 2019 2018 2017 Series A-1 convertible preferred stock — — 4,408,893 Series B-1 convertible preferred stock — — 2,527,221 Series B-2 convertible preferred stock — — 2,106,014 Series B-3 convertible preferred stock — — 3,243,306 Employee stock purchase plan 8,054 27,751 — Warrants to purchase capital stock 81,966 168,046 173,000 Common stock options issued and outstanding 3,887,780 1,993,720 1,921,453 Early exercised common stock subject to future vesting 25,081 33,882 4,146 Total 4,002,881 2,223,399 14,384,033

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Summary of Income (Loss) Before Provision for Income Taxes	Income (loss) before provision for income taxes consisted of the following (in thousands): Year Ended December 31, 2019 2018 2017 United States $ (53,789 ) $ 18,226 $ (28,589 ) Foreign (3 ) (59 ) (108 ) Total $ (53,792 ) $ 18,167 $ (28,697 )
Components of Income Tax Provision	The income tax provision is summarized as follows: Year Ended December 31, 2019 2018 2017 Current Federal $ — $ — $ — State 800 800 800 International — — — Total current tax expense 800 800 800 Deferred Federal — — — State — — — International — — — Total deferred tax expense — — — Total tax expense $ 800 $ 800 $ 800
Schedule of Components of Net Deferred Tax Assets	The tax effects of significant items comprising our net deferred tax assets are as follows (in thousands): December 31, 2019 2018 Deferred tax assets: Net operating loss carryforwards $ 40,501 $ 29,983 Stock-based compensation 2,046 453 Deferred revenue — 1,179 Other accrued expenses 3,330 1,079 Total deferred tax assets 45,877 32,694 Deferred tax liabilities: Fixed assets and intangibles (1,577 ) (141 ) Total deferred tax liabilities (1,577 ) (141 ) Valuation allowance (44,300 ) (32,553 ) Net deferred tax assets $ — $ —
Summary of Net Operating Losses and Tax Credit Carryforwards	Net operating losses and tax credit carryforwards as of December 31, 2019 are as follows (amounts in thousands): Amount Expiration Years Net operating losses, federal (post December 31, 2017) $ 50,403 No expiration Net operating losses, federal (pre January 1, 2018) 103,950 2031 - 2037 Net operating losses, state 111,770 2031 - 2036 Tax credits, federal 10,100 2031 - 2039 Tax credits, state 4,934 No expiration
Summary of Effective Tax Rate Provision For (Benefit From) Income Taxes Differs From The Federal Statutory Income Tax Provision	The effective tax rate of our provision for (benefit from) income taxes differs from the federal statutory income tax provision as follows: Year Ended December 31, 2019 2018 2017 Statutory rate 21.0 % 21.0 % 35.0 % State tax 0.0 % 0.0 % 0.0 % Stock option expense 1.1 % 1.6 % (0.6 %) General business credit 10.8 % (14.7 %) 4.2 % Uncertain tax positions (10.8 %) 14.7 % (4.2 %) Change in valuation allowance (21.8 %) (24.8 %) 30.1 % Other permanent differences (0.2 %) 2.2 % (3.3 %) Tax reform—tax rate change 0.0 % 0.0 % (61.2 %) Total 0.0 % 0.0 % 0.0 %
Schedule of Reconciliation of Unrecognized Tax Benefits	A reconciliation of our unrecognized tax benefits for the years ended December 31, 2019 and 2018 is as follows (in thousands): 2019 2018 Balance at beginning of year $ 8,958 $ 6,119 Addition for tax positions in prior years — 736 Addition for tax positions in current year 6,076 2,103 Balance at end of year $ 15,034 $ 8,958

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2019
Selected Quarterly Financial Information [Abstract]
Schedule of Unaudited Quarterly Financial Information	The following tables present certain unaudited quarterly financial information for the years ended December 31, 2019 and 2018 (in thousands, except per share amounts): Quarter Ended December 31, 2019 September 30, 2019 June 30, 2019 March 31, 2019 Revenue $ — $ — $ 30,000 $ 5,160 Income (loss) from operations $ (26,611 ) $ (23,333 ) $ 6,049 $ (14,871 ) Net income (loss) $ (24,924 ) $ (22,296 ) $ 7,115 $ (13,687 ) Net income (loss) per share - basic $ (0.80 ) $ (0.93 ) $ 0.30 $ (0.57 ) Net income (loss) per share - diluted $ (0.80 ) $ (0.93 ) $ 0.28 $ (0.57 ) Quarter Ended December 31, 2018 September 30, 2018 June 30, 2018 March 31, 2018 Revenue $ 26,137 $ 18,564 $ 12,987 $ 11,449 Income from operations $ 8,262 $ 6,477 $ 1,871 $ 532 Net income $ 9,377 $ 6,683 $ 1,797 $ 310 Net income per share - basic $ 0.39 $ — $ — $ — Net income per share - diluted $ 0.37 $ — $ — $ —

Organization - Additional Infor

Organization - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	Sep. 13, 2018	Oct. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Sep. 18, 2018	Dec. 31, 2017
Organization And Presentation Of Financial Statements [Line Items]
Date of incorporation			Oct. 6, 2008
Common stock, shares authorized			500,000,000	500,000,000	500,000,000
Common stock, par value			$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001
Preferred stock shares authorized					20,000,000
Preferred stock par value					$ 0.0001
Common Stock | IPO
Organization And Presentation Of Financial Statements [Line Items]
Issuance of stock, shares	7,187,500
Offering price per share	$ 17
Proceeds from IPO, net of underwriting discounts and commissions	$ 113.6
Number of common stock issued on conversion of convertible preferred stock	15,760,102
Common Stock | IPO | Underwriters
Organization And Presentation Of Financial Statements [Line Items]
Issuance of stock, shares	937,500
Common Stock | Underwritten Public Offering
Organization And Presentation Of Financial Statements [Line Items]
Issuance of stock, shares		8,625,000
Offering price per share		$ 28
Proceeds from IPO, net of underwriting discounts and commissions		$ 226.5
Common Stock | Underwritten Public Offering | Underwriters Option
Organization And Presentation Of Financial Statements [Line Items]
Issuance of stock, shares		1,125,000

Significant Accounting Polici_4

Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2019USD ($)Segment	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Summary Of Significant Accounting Policies [Line Items]
Cash cash equivalents and marketable securities	$ 367,800,000	$ 180,600,000
Restricted cash	$ 600,000	$ 600,000
Number of operating segment | Segment	1
Revenue recognized from performance obligations satisfied in previous periods	$ 0
Unrecognized tax benefits, penalties and interest expense	0
Series B-3 Convertible Preferred Stock
Summary Of Significant Accounting Policies [Line Items]
Fair value of convertible notes redemption features liability			$ 0
ASC 606 | Adjustments
Summary Of Significant Accounting Policies [Line Items]
Cumulative adjustment to decrease accumulated deficit and deferred revenue	$ 400,000
Computer Equipment
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of assets	2 years
Office Furniture and Fixtures
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of assets	5 years

Significant Accounting Polici_5

Significant Accounting Policies - Summary of Impact of the Adoption of Topic 606 on Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Liabilities
Deferred revenue	$ 0		$ 5,616
Stockholders' equity
Accumulated deficit	$ (185,743)		$ (132,407)
ASC 606
Liabilities
Deferred revenue		$ 5,160
Stockholders' equity
Accumulated deficit		(131,951)
Adjustments | ASC 606
Liabilities
Deferred revenue		(456)
Stockholders' equity
Accumulated deficit		$ 456

Significant Accounting Polici_6

Significant Accounting Policies - Summary of Impact of the Adoption of ASC 606 on Unaudited Condensed Consolidated Statement of Operations (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
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 0	$ 0	$ 30,000	$ 5,160	$ 26,137	$ 18,564	$ 12,987	$ 11,449	$ 35,160	$ 69,137	$ 5,247
Loss from operations	(26,611)	(23,333)	6,049	(14,871)	8,262	6,477	1,871	532	(58,765)	17,142	(26,586)
Net income (loss)	$ (24,924)	$ (22,296)	$ 7,115	$ (13,687)	$ 9,377	$ 6,683	$ 1,797	$ 310	$ (53,792)	$ 18,167	$ (28,697)
Net loss per share, basic and diluted									$ (2.09)
Adjustments | ASC 606
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenue									$ 456
Loss from operations									456
Net income (loss)									$ 456
Net loss per share, basic and diluted									$ 0.02
Balance without ASC 606 Adoption | ASC 606
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenue									$ 35,616
Loss from operations									(58,309)
Net income (loss)									$ (53,336)
Net loss per share, basic and diluted									$ (2.07)

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets
Assets	$ 363,151	$ 177,973
Level 1
Assets
Assets	17,664	19,861
Level 2
Assets
Assets	345,487	158,112
Level 3
Assets
Assets	0	0
Cash Equivalents | Money Market Funds
Assets
Assets	17,664	19,861
Cash Equivalents | Corporate Commercial Paper
Assets
Assets	6,983	7,465
Cash Equivalents | Corporate Debt Securities
Assets
Assets	10,204	4,499
Cash Equivalents | Level 1 | Money Market Funds
Assets
Assets	17,664	19,861
Cash Equivalents | Level 1 | Corporate Commercial Paper
Assets
Assets	0	0
Cash Equivalents | Level 1 | Corporate Debt Securities
Assets
Assets	0	0
Cash Equivalents | Level 2 | Money Market Funds
Assets
Assets	0	0
Cash Equivalents | Level 2 | Corporate Commercial Paper
Assets
Assets	6,983	7,465
Cash Equivalents | Level 2 | Corporate Debt Securities
Assets
Assets	10,204	4,499
Cash Equivalents | Level 3 | Money Market Funds
Assets
Assets	0	0
Cash Equivalents | Level 3 | Corporate Commercial Paper
Assets
Assets	0	0
Cash Equivalents | Level 3 | Corporate Debt Securities
Assets
Assets	0	0
Short-term marketable securities | Corporate Commercial Paper
Assets
Assets	95,088	36,180
Short-term marketable securities | Corporate Debt Securities
Assets
Assets	154,690	71,903
Short-term marketable securities | Government- sponsored Enterprise Securities
Assets
Assets	26,265	9,906
Short-term marketable securities | Level 1 | Corporate Commercial Paper
Assets
Assets	0	0
Short-term marketable securities | Level 1 | Corporate Debt Securities
Assets
Assets	0	0
Short-term marketable securities | Level 1 | Government- sponsored Enterprise Securities
Assets
Assets	0	0
Short-term marketable securities | Level 2 | Corporate Commercial Paper
Assets
Assets	95,088	36,180
Short-term marketable securities | Level 2 | Corporate Debt Securities
Assets
Assets	154,690	71,903
Short-term marketable securities | Level 2 | Government- sponsored Enterprise Securities
Assets
Assets	26,265	9,906
Short-term marketable securities | Level 3 | Corporate Commercial Paper
Assets
Assets	0	0
Short-term marketable securities | Level 3 | Corporate Debt Securities
Assets
Assets	0	0
Short-term marketable securities | Level 3 | Government- sponsored Enterprise Securities
Assets
Assets	0	0
Long-term marketable securities | Corporate Debt Securities
Assets
Assets	14,202	3,712
Long-term marketable securities | Government- sponsored Enterprise Securities
Assets
Assets	38,055
Long-term marketable securities | Level 1 | Corporate Debt Securities
Assets
Assets	0	0
Long-term marketable securities | Level 1 | Government- sponsored Enterprise Securities
Assets
Assets	0
Long-term marketable securities | Level 2 | Corporate Debt Securities
Assets
Assets	14,202	3,712
Long-term marketable securities | Level 2 | Government- sponsored Enterprise Securities
Assets
Assets	38,055
Long-term marketable securities | Level 3 | Corporate Debt Securities
Assets
Assets	0	0
Long-term marketable securities | Level 3 | Government- sponsored Enterprise Securities
Assets
Assets	$ 0
Short Term Marketable Securities | U.S. treasury securities
Assets
Assets		24,447
Short Term Marketable Securities | Level 1 | U.S. treasury securities
Assets
Assets		0
Short Term Marketable Securities | Level 2 | U.S. treasury securities
Assets
Assets		24,447
Short Term Marketable Securities | Level 3 | U.S. treasury securities
Assets
Assets		$ 0

Cash Equivalents and Marketab_3

Cash Equivalents and Marketable Securities - Summary of Short-Term Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	$ 363,052	$ 178,011
Unrealized Gains, Total	151	14
Unrealized Losses, Total	(52)	(52)
Fair Value, Total	363,151	177,973
Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	34,851	31,826
Unrealized Gains, Total	1	0
Unrealized Losses, Total	(1)	(1)
Fair Value, Total	34,851	31,825
Cash Equivalents | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	10,205	4,499
Unrealized Gains, Total	0	0
Unrealized Losses, Total	(1)	0
Fair Value, Total	10,204	4,499
Cash Equivalents | Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	17,664	19,861
Unrealized Gains, Total	0	0
Unrealized Losses, Total	0	0
Fair Value, Total	17,664	19,861
Cash Equivalents | Corporate Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	6,982	7,466
Unrealized Gains, Total	1	0
Unrealized Losses, Total	0	(1)
Fair Value, Total	6,983	7,465
Short-term marketable securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	275,970	142,479
Unrealized Gains, Total	124	8
Unrealized Losses, Total	(51)	(51)
Fair Value, Total	276,043	142,436
Short-term marketable securities | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	154,678	71,920
Unrealized Gains, Total	54	8
Unrealized Losses, Total	(42)	(25)
Fair Value, Total	154,690	71,903
Short-term marketable securities | Corporate Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	95,061	36,197
Unrealized Gains, Total	36	0
Unrealized Losses, Total	(9)	(17)
Fair Value, Total	95,088	36,180
Short-term marketable securities | Government- sponsored Enterprise Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	26,231	9,911
Unrealized Gains, Total	34	0
Unrealized Losses, Total	0	(5)
Fair Value, Total	26,265	9,906
Short-term marketable securities | U.S. treasury securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total		24,451
Unrealized Gains, Total		0
Unrealized Losses, Total		(4)
Fair Value, Total		24,447
Long-term marketable securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	52,231	3,706
Unrealized Gains, Total	26	6
Unrealized Losses, Total	0	0
Fair Value, Total	52,257	3,712
Long-term marketable securities | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	14,194	3,706
Unrealized Gains, Total	8	6
Unrealized Losses, Total	0	0
Fair Value, Total	14,202	$ 3,712
Long-term marketable securities | Government- sponsored Enterprise Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost, Total	38,037
Unrealized Gains, Total	18
Unrealized Losses, Total	0
Fair Value, Total	$ 38,055

Cash Equivalents and Marketab_4

Cash Equivalents and Marketable Securities - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Sales of available-for-sale marketable securities	$ 0	$ 0
Marketable securities in unrealized loss positions	119,500,000
Other than temporary impairments	$ 0
Maximum
Schedule Of Available For Sale Securities [Line Items]
Remaining contractual maturities of long-term marketable securities	2 years

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 39,537	$ 34,489	$ 41,054	$ 5,867
Restricted cash, current	0	82	100	100
Restricted cash, non-current	567	567	82	82
Total	$ 40,104	$ 35,138	$ 41,236	$ 6,049

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 12,169	$ 3,820
Less accumulated depreciation and amortization	(2,482)	(2,154)
Total	9,687	1,666
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,007	2,500
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	289	508
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,549	392
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 8,324	$ 420

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Depreciation and leasehold improvement amortization expense	$ 1.8	$ 0.3

Balance Sheet Components - Su_3

Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Prepaid Expense And Other Assets Current [Abstract]
Other accounts receivable	$ 1,272	$ 2,104
Prepaid expenses	2,893	1,661
Total	$ 4,165	$ 3,765

Balance Sheet Components - Su_4

Balance Sheet Components - Summary of Other Long-Term Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Other Assets Noncurrent Disclosure [Abstract]
Tenant improvement allowance receivables	$ 0	$ 8,324
Long-term deposits	480	480
Total	$ 480	$ 8,804

License and Collaboration Agr_2

License and Collaboration Agreements - Additional Information (Details) - USD ($)

Jan. 02, 2019

Jun. 30, 2019

Mar. 31, 2019

Nov. 30, 2018

Aug. 31, 2018

Jul. 31, 2018

Dec. 31, 2017

Nov. 30, 2017

Jun. 30, 2017

Jun. 30, 2019

Dec. 31, 2019

Dec. 31, 2018

Dec. 31, 2017

May 01, 2019

Jan. 01, 2019

Oct. 01, 2018

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Deferred revenue

$ 0

$ 5,616,000

ASC 606

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Deferred revenue

$ 5,160,000

ASC 606 | Adjustments

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Deferred revenue

(456,000)

University Of California San Francisco

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Licensing fees

40,000

Annual licenses maintenance fee

40,000

University Of California San Francisco | IPO

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Milestone payments

$ 140,000

Sanofi Agreement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Payment received

$ 30,000,000

$ 10,000,000

$ 5,000,000

$ 10,000,000

$ 40,000,000

Royalty expiration period

10 years

Upfront payment recognized

40,000,000

Aggregate milestone payments

25,000,000

Transaction price determined

65,000,000

Revenue recognized

30,000,000

63,100,000

$ 1,900,000

Deferred revenue

0

0

Sanofi Agreement | Clinical Development Milestones

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Milestone payments

$ 30,000,000

Revenue recognized

$ 30,000,000

Increase in transaction price

$ 30,000,000

Sanofi Agreement | Maximum

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

License agreement milestone payment receive regulatory milestones and commercial milestones

$ 765,000,000

AbbVie Agreement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Payment received

$ 15,000,000

15,000,000

Revenue recognized

$ 5,200,000

5,200,000

6,000,000

$ 3,400,000

Research and development period

2 years

Deferred revenue

$ 0

5,600,000

AbbVie Agreement | ASC 606

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenue recognized

9,800,000

AbbVie Agreement | ASC 606 | ASC 605

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Revenue recognized

$ 9,400,000

AbbVie Agreement | ASC 606 | Adjustments

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Decrease in accumulated deficit

$ 400,000

Decrease in deferred revenue

$ 400,000

AbbVie Agreement | Maximum

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Extension of research and development period

6 months

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	1 Months Ended	12 Months Ended
	Apr. 30, 2018USD ($)ft²	Dec. 31, 2019USD ($)ft²	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2015Tenant
Commitments And Contingencies [Line Items]
Area under lease | ft²		30,000
Lease expiration date		Jan. 31, 2019
Rent expense net of sublease payment		$ 3,100,000	$ 2,700,000	$ 1,600,000
Number of subtenants | Tenant					2
Sublease payments received				$ 100,000
Sublease agreement termination year		2017
Description of subleases to terminate		Both subleases were terminated during 2017.
Indemnification Guarantee
Commitments And Contingencies [Line Items]
Accruals for expenses related to indemnification issues		$ 0
April 2018 Lease Agreement
Commitments And Contingencies [Line Items]
Area under lease | ft²	47,500
Commencement date		Feb. 1, 2019
Term of lease agreement		7 years
Option to extend period		7 years
Letter of credit		$ 600,000	$ 600,000
Tenant improvement		$ 8,300,000
Lease agreement commencement date		Aug. 1, 2018
April 2018 Lease Agreement | Leasehold Improvements
Commitments And Contingencies [Line Items]
Tenant improvement allowance claim date		Jul. 1, 2019
April 2018 Lease Agreement | Maximum
Commitments And Contingencies [Line Items]
Tenant improvement		$ 7,100,000
Additional tenant improvement allowance	$ 1,200,000

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Future Minimum Lease Payments For Operating Leases (Details) $ in Thousands	Dec. 31, 2019USD ($)
Commitments And Contingencies Disclosure [Abstract]
2020	$ 3,093
2021	3,193
2022	3,296
2023	3,403
2024	3,514
2025 and beyond	3,933
Total	$ 20,432

Debt Obligation - Additional in

Debt Obligation - Additional information (Details)	Dec. 29, 2017USD ($)$ / sharesshares	Sep. 30, 2018shares	Dec. 31, 2017USD ($)$ / sharesshares	Aug. 31, 2016USD ($)	Sep. 30, 2011$ / sharesshares	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)Model	Dec. 31, 2017USD ($)Model$ / shares	Dec. 31, 2016USD ($)	Apr. 30, 2017USD ($)	Sep. 30, 2016USD ($)
Debt Instrument [Line Items]
Incremental debt discount								$ 500,000	$ 1,900,000
Debt instrument annual interest rate				8.00%
Debt instrument, maturity date				Dec. 31, 2017
Fair value of embedded derivative upon issuance										$ 1,900,000	$ 3,400,000
Convertible Notes
Debt Instrument [Line Items]
Interest expense						$ 0	$ 0	7,200,000
Series A Warrants
Debt Instrument [Line Items]
Warrants term					10 years
Convertible preferred stock warrant liability outstanding			$ 27,000					$ 27,000
Series A Warrants | Common Stock
Debt Instrument [Line Items]
Shares issued upon conversion of convertible securities | shares		3,576
Series B-3 Convertible Preferred Stock
Debt Instrument [Line Items]
Shares issued upon conversion of convertible securities | shares			1,968,561
Offering price per share | $ / shares			$ 11.9907					$ 11.9907
Remeasurement of fair value of convertible notes redemption features liability			$ 0					$ 0
Warrants issued during period to purchase capital stock | shares	168,046
Exercise price per share | $ / shares	$ 8.99
Warrants term	5 years
Convertible preferred stock warrant liability outstanding	$ 1,100,000		1,500,000					1,500,000
Series A Convertible Preferred Stock | Series A Warrants
Debt Instrument [Line Items]
Exercise price per share | $ / shares					$ 9.0839
Warrants to purchase convertible shares | shares					4,954
Convertible Preferred Stock Warrant Liability
Debt Instrument [Line Items]
Convertible preferred stock warrant liability outstanding			$ 1,576,000				$ 0	$ 1,576,000	$ 1,050,000
Number of option pricing models | Model							2	2
Convertible Notes Redemption Features Liability | Convertible Notes
Debt Instrument [Line Items]
Interest expense								$ 5,500,000
Convertible Preferred Stock | Convertible Notes
Debt Instrument [Line Items]
Interest expense								$ 1,700,000
Note and Warrant Purchase Agreement
Debt Instrument [Line Items]
Aggregate amount of convertible notes issued				$ 21,500,000

Debt Obligation - Summary of Ch

Debt Obligation - Summary of Changes in Fair Value Related to Notes Redemption Features Liability (Details) - Convertible Notes Redemption Features Liability $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 3,648
Fair value of the redemption features related to second tranche of convertible notes	1,938
Change in estimated fair value of warrant liability included in other income (expense), net	(5,586)
Ending Balance	$ 0

Debt Obligation - Summary of _2

Debt Obligation - Summary of Changes in Estimated Fair Value of Warrant Liability Related to Series A and B-3 Warrants (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Class Of Warrant Or Right [Line Items]
Change in estimated fair value of convertible preferred stock warrant liability included in other income (expense), net	$ 0	$ (577,000)	$ (526,000)
Convertible Preferred Stock Warrant Liability
Class Of Warrant Or Right [Line Items]
Beginning Balance	$ 0	1,576,000	1,050,000
Change in estimated fair value of convertible preferred stock warrant liability included in other income (expense), net		634,000	526,000
Reclassification of Warrant Liability to Additional Paid-in-Capital Upon Exercise of Warrants		(57,000)
Reclassification of Warrant Liability to Additional Paid-in-Capital Upon Initial Public Offering		(2,153,000)
Ending Balance		$ 0	$ 1,576,000

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Details) $ / shares in Units, $ in Thousands	Sep. 13, 2018shares	Sep. 30, 2018BoardofDirector$ / shares	Aug. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017$ / sharesshares	Jul. 31, 2015USD ($)$ / sharesshares	Nov. 30, 2014USD ($)$ / sharesshares	Apr. 30, 2014USD ($)$ / sharesshares	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2012USD ($)$ / sharesshares	Dec. 31, 2016shares
Temporary Equity [Line Items]
Proceeds from sale of convertible preferred stock | $								$ 0	$ 49,800	$ 0
Convertible preferred stock, conversion term		Each share of convertible preferred stock is convertible at a 1:1 ratio at the option of the holder into fully paid and non-assessable shares of common stock. Conversion of each series of convertible preferred stock into common stock is automatic upon the earlier of (i) the closing of an IPO of our common stock, registered under the Securities Act of 1933, as amended, which results in aggregate proceeds equal to or exceeding $50.0 million to us, and the value of our shares immediately prior to the IPO is not less than $22.71 per share (as adjusted for any stock splits, stock dividends, combinations, subdivisions, recapitalizations or the like); or (ii) at any time upon the agreement of the holders of at least 60% of the then outstanding shares of preferred stock
Common Stock
Temporary Equity [Line Items]
Conversion ratio, convertible preferred stock to common stock		100.00%
Minimum percentage of convertible preferred shareholders required to request conversion		60.00%
Minimum | Common Stock
Temporary Equity [Line Items]
Proceeds from IPO | $								$ 50,000
IPO | Common Stock
Temporary Equity [Line Items]
Issuance of stock, shares	7,187,500
Number of convertible preferred shares issued upon conversion of convertible notes	15,760,102
IPO | Minimum | Common Stock
Temporary Equity [Line Items]
Share price | $ / shares		$ 22.71
Series C Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, Shares Outstanding									0
Convertible preferred stock, Shares Issued									0
Series C Convertible Preferred Stock | IPO
Temporary Equity [Line Items]
Issuance of stock, shares			3,474,668
Shares price per share | $ / shares			$ 14.3898
Proceeds from sale of convertible preferred stock | $			$ 49,800
Number of convertible preferred shares issued upon conversion of convertible notes	15,760,102
Series A
Temporary Equity [Line Items]
Issuance of stock, shares											4,408,893
Shares price per share | $ / shares											$ 9.0839
Proceeds from sale of convertible preferred stock | $											$ 40,100
Convertible preferred stock, Shares Outstanding				4,408,893						4,408,893
Convertible preferred stock, Shares Issued				4,408,893						4,408,893
Percentage of non-cumulative dividends		8.00%
Convertible preferred stock, liquidation preference per share | $ / shares		$ 9.0839
Series B-1
Temporary Equity [Line Items]
Issuance of stock, shares							2,527,221
Shares price per share | $ / shares							$ 9.9923
Proceeds from sale of convertible preferred stock | $							$ 25,300
Convertible preferred stock, Shares Outstanding				2,527,221						2,527,221
Convertible preferred stock, Shares Issued				2,527,221						2,527,221
Percentage of non-cumulative dividends		8.80%
Convertible preferred stock, liquidation preference per share | $ / shares		$ 9.9923
Series B-2
Temporary Equity [Line Items]
Issuance of stock, shares						2,106,014
Shares price per share | $ / shares						$ 11.9907
Proceeds from sale of convertible preferred stock | $						$ 25,300
Convertible preferred stock, Shares Outstanding				2,106,014						2,106,014
Convertible preferred stock, Shares Issued				2,106,014						2,106,014
Percentage of non-cumulative dividends		10.56%
Convertible preferred stock, liquidation preference per share | $ / shares		$ 11.9907
Series B-2 | Maximum
Temporary Equity [Line Items]
Issuance of stock, shares						2,106,014
Series B-3 Convertible Preferred Stock
Temporary Equity [Line Items]
Issuance of stock, shares				1,968,561	1,274,745
Shares price per share | $ / shares				$ 11.9907	$ 11.9907					$ 11.9907
Proceeds from sale of convertible preferred stock | $					$ 15,300
Number of convertible preferred shares issued upon conversion of convertible notes				1,968,561
Convertible preferred stock, Shares Outstanding				3,243,306						3,243,306
Convertible preferred stock, Shares Issued				3,243,306						3,243,306
Percentage of non-cumulative dividends		10.56%
Convertible preferred stock, liquidation preference per share | $ / shares		$ 11.9907
Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, Shares Outstanding				12,285,434						12,285,434		10,316,873
Convertible preferred stock, Shares Issued				12,285,434						12,285,434
Convertible preferred stock voting rights, description		Each holder of convertible preferred stock is entitled to the number of votes equal to the number of shares of common stock into which such shares of convertible preferred stock are convertible. So long as 20% of the shares of convertible preferred stock remain outstanding, the holders of such shares of preferred stock are entitled to elect five members of our Board of Directors
Percentage of converitble preferred stock shares remain outstanding		20.00%
Number of directors are entitled to elect by preferred stock holders | BoardofDirector		5

Convertible Preferred Stock - S

Convertible Preferred Stock - Summary of Convertible Preferred Stock (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Series A
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized	40,094,998
Convertible preferred stock, Shares Issued	4,408,893
Convertible preferred stock, Shares Outstanding	4,408,893
Convertible preferred stock, Liquidation Amount	$ 40,050
Convertible preferred stock, Carrying Value	$ 39,630
Series B-1
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized	22,957,067
Convertible preferred stock, Shares Issued	2,527,221
Convertible preferred stock, Shares Outstanding	2,527,221
Convertible preferred stock, Liquidation Amount	$ 25,253
Convertible preferred stock, Carrying Value	$ 25,028
Series B-2
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized	19,130,887
Convertible preferred stock, Shares Issued	2,106,014
Convertible preferred stock, Shares Outstanding	2,106,014
Convertible preferred stock, Liquidation Amount	$ 25,252
Convertible preferred stock, Carrying Value	$ 25,027
Series B-3
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized	31,497,330
Convertible preferred stock, Shares Issued	3,243,306
Convertible preferred stock, Shares Outstanding	3,243,306
Convertible preferred stock, Liquidation Amount	$ 38,890
Convertible preferred stock, Carrying Value	$ 38,846
Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, Shares Authorized	113,680,282
Convertible preferred stock, Shares Issued	12,285,434
Convertible preferred stock, Shares Outstanding	12,285,434	10,316,873
Convertible preferred stock, Liquidation Amount	$ 129,445
Convertible preferred stock, Carrying Value	$ 128,531	$ 104,926

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Sep. 30, 2018	Aug. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period			48 months
Weighted average fair value stock options granted			$ 24.8	$ 10.5	$ 4.3
Total intrinsic value of options exercised			$ 8,600	$ 1,700	$ 100
Fair value of stock options vested			$ 6,800	$ 2,600	$ 1,100
Weighted-average remaining contractual term options outstanding			7 years 8 months 8 days	8 years 18 days
Stock options granted			1,418,000
Income tax benefit recognized from stock-based compensation expense			$ 0
Tax benefits realized from exercised stock options			0
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance	250,000
Automatic annual increase of capital stock outstanding	1.00%
Stock-based compensation expense			$ 600	$ 200
Additional shares of common stock	500,000
Percentage of purchase price of common stock to fair value	85.00%
Offering period	12 months
Consultant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted			0	1,101	0
Early Exercised Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares repurchase in the event of employment termination			25,081	33,882
Unvested stock options liabilities			$ 200	$ 200
Employees and Nonemployee Directors Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unamortized compensation cost related to stock awards			$ 36,200
Expected weighted average period			2 years 10 months 24 days
Weighted-average remaining contractual term options outstanding			7 years 8 months 12 days
Modifications of Stock Options | Board of Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			$ 700
Maximum | Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of employee's eligible compensation contribution	15.00%
2018 Equity Incentive Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance		2,270,000
Automatic annual increase of capital stock outstanding		4.00%
Options vesting period		4 years
Number of shares of common stock authorized for issuance			5,863,621
2008 Equity Incentive Plan | Employees and Nonemployees Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period			4 years
Options expiration period			10 years

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Options Available for Grant, Beginning Balance	2,195,788
Options Available for Grant, Options granted	(1,418,000)
Options Available for Grant, Options forfeited	240,205
Options Available for Grant, Options exercised	0
Options Available for Grant, Options expired	1,137
Options Available for Grant, Options repurchased	738
Options Available for Grant, Shares added to plans	955,973
Options Available for Grant, Ending Balance	1,975,841	2,195,788
Options Outstanding, Beginning Balance	2,998,590
Options Outstanding, Options granted	1,418,000
Options Outstanding, Options forfeited	(240,205)
Options Outstanding, Options exercised	(287,468)
Options Outstanding, Options expired	(1,137)
Options Outstanding, Options repurchased	0
Options Outstanding, Shares added to plans	0
Options Outstanding, Ending Balance	3,887,780	2,998,590
Options Outstanding, Vested and expected to vest	3,887,780
Options Outstanding, Exercisable as of December 31, 2019	2,270,563
Weighted Average Exercise Price, Beginning Balance	$ 9.39
Weighted Average Exercise Price, Options granted	35.32
Weighted Average Exercise Price, Options forfeited	19.45
Weighted Average Exercise Price, Options exercised	7.20
Weighted Average Exercise Price, Options expired	30.30
Weighted Average Exercise Price, Ending Balance	18.38	$ 9.39
Weighted Average Exercise Price, Vested and expected to vest	18.38
Weighted Average Exercise Price, Exercisable as of December 31, 2019	$ 8.17
Weighted Average Remaining Contractual Life	7 years 8 months 8 days	8 years 18 days
Weighted Average Remaining Contractual Life, Vested and expected to vest	7 years 8 months 8 days
Weighted Average Remaining Contractual Life, Exercisable as of December 31, 2019	7 years 8 months 12 days
Aggregate Intrinsic Value	$ 141,505	$ 53,994
Aggregate Intrinsic Value, Vested and expected to vest	141,505
Aggregate Intrinsic Value, Exercisable as of December 31, 2019	$ 105,835

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Fair Values for Stock Options Granted at Estimated Date of Grant (Details) - Employees and Nonemployee Directors Stock Options	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	1.30%	2.50%	1.80%
Risk-free interest rate, maximum	2.60%	3.10%	2.20%
Expected volatility, minimum	73.00%	80.00%	81.00%
Expected volatility, maximum	80.00%	84.50%	85.30%
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	5 years	5 years	5 years 9 months 18 days
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	6 years	6 years 1 month 6 days	6 years 1 month 6 days

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Stock-based Compensation Expense (Details) - Employee Stock Purchase Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 600	$ 200
2018 Equity Incentive Plan/2008 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	12,182	2,779	$ 1,021
Research and Development | 2018 Equity Incentive Plan/2008 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	6,636	1,427	462
General and Administrative | 2018 Equity Incentive Plan/2008 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 5,546	$ 1,352	$ 559

Net Income (Loss) per Share - S

Net Income (Loss) per Share - Schedule of Reconciliation of Numerators and Denominators Used In Computations and Calculation of Basic and Diluted Net Income (Loss) Per Share (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
Numerator
Net income (loss)	$ (24,924)	$ (22,296)	$ 7,115	$ (13,687)	$ 9,377	$ 6,683	$ 1,797	$ 310	$ (53,792)	$ 18,167	$ (28,697)
Allocation of undistributed earnings to participating securities									0	(13,250)	0
Net income (loss) attributable to common stockholders									$ (53,792)	$ 4,917	$ (28,697)
Denominator
Weighted-average common shares outstanding, basic									25,778,122	7,622,602	569,719
Effect of dilutive shares									0	1,004,871	0
Weighted-average shares outstanding used in per share calculation, diluted									25,778,122	8,627,473	569,719
Net income (loss) per share, basic	$ (0.80)	$ (0.93)	$ 0.30	$ (0.57)	$ 0.39	$ 0	$ 0	$ 0	$ (2.09)	$ 0.65	$ (50.37)
Net income (loss) per share, diluted	$ (0.80)	$ (0.93)	$ 0.28	$ (0.57)	$ 0.37	$ 0	$ 0	$ 0	$ (2.09)	$ 0.57	$ (50.37)

Net Income (Loss) per Share -_2

Net Income (Loss) per Share - Summary of Common Stock Equivalents Excluded Used in Computation of Diluted Net Income (loss) Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	4,002,881	2,223,399	14,384,033
Series A-1 Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	0	0	4,408,893
Series B-1 Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	0	0	2,527,221
Series B-2 Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	0	0	2,106,014
Series B-3 Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	0	0	3,243,306
Employee Stock Purchase Plan
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	8,054	27,751	0
Warrants to Purchase Capital Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	81,966	168,046	173,000
Common Stock Options Issued and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	3,887,780	1,993,720	1,921,453
Early Exercised Common Stock Subject To Future Vesting
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	25,081	33,882	4,146

Income Taxes - Summary of Incom

Income Taxes - Summary of Income (Loss) Before Provision for Income Taxes (Details) - 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
Income Tax Disclosure [Abstract]
United States									$ (53,789)	$ 18,226	$ (28,589)
Foreign									(3)	(59)	(108)
Net income (loss)	$ (24,924)	$ (22,296)	$ 7,115	$ (13,687)	$ 9,377	$ 6,683	$ 1,797	$ 310	$ (53,792)	$ 18,167	$ (28,697)

Income Taxes - Components of In

Income Taxes - Components of Income Tax Provision (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current
Federal	$ 0	$ 0	$ 0
State	800	800	800
International	0	0	0
Total current tax expense	800	800	800
Deferred
Federal	0	0	0
State	0	0	0
International	0	0	0
Total deferred tax expense	0	0	0
Total tax expense	$ 800	$ 800	$ 800

Income Taxes - Summary of Net D

Income Taxes - Summary of Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 40,501	$ 29,983
Stock-based compensation	2,046	453
Deferred revenue	0	1,179
Other accrued expenses	3,330	1,079
Total deferred tax assets	45,877	32,694
Deferred tax liabilities:
Fixed assets and intangibles	(1,577)	(141)
Total deferred tax liabilities	(1,577)	(141)
Valuation allowance	(44,300)	(32,553)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Increase (decrease) in valuation allowance	$ 11,700,000	$ (4,500,000)	$ (8,000,000)
Unrecognized tax benefits	15,034,000	$ 8,958,000	$ 6,119,000
Unrecognized tax benefits that would impact effective tax rate	0
Accrued interest and penalties	$ 0

Income Taxes - Summary of Net O

Income Taxes - Summary of Net Operating Losses and Tax Credit Carryforwards (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Tax Credit Carryforward [Line Items]
Net operating losses, state, amount	$ 111,770
Federal
Tax Credit Carryforward [Line Items]
Tax credits	$ 10,100
Tax credits, Expiration years	2031
Tax credits, Expiration years	2039
State
Tax Credit Carryforward [Line Items]
Tax credits	$ 4,934
Net operating losses, Expiration years	2031
Net operating losses, Expiration years	2036
Tax credits, Expiration years	No expiration
Post December 31, 2017
Tax Credit Carryforward [Line Items]
Net operating losses, federal, amount	$ 50,403
Post December 31, 2017 | Federal
Tax Credit Carryforward [Line Items]
Net operating losses, Expiration years	No expiration
Pre January 1, 2018
Tax Credit Carryforward [Line Items]
Net operating losses, federal, amount	$ 103,950
Pre January 1, 2018 | Federal
Tax Credit Carryforward [Line Items]
Net operating losses, Expiration years	2031
Net operating losses, Expiration years	2037

Income Taxes - Summary of Effec

Income Taxes - Summary of Effective Tax Rate Provision For (Benefit From) Income Taxes Differs From The Federal Statutory Income Tax Provision (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]
Statutory rate	21.00%	21.00%	35.00%
State tax	0.00%	0.00%	0.00%
Stock option expense	1.10%	1.60%	(0.60%)
General business credit	10.80%	(14.70%)	4.20%
Uncertain tax positions	(10.80%)	14.70%	(4.20%)
Change in valuation allowance	(21.80%)	(24.80%)	30.10%
Other permanent differences	(0.20%)	2.20%	(3.30%)
Tax reform—tax rate change	0.00%	0.00%	(61.20%)
Total	0.00%	0.00%	0.00%

Income Taxes - Summary of Recon

Income Taxes - Summary of Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 8,958	$ 6,119
Addition for tax positions in prior years	0	736
Addition for tax positions in current year	6,076	2,103
Balance at end of year	$ 15,034	$ 8,958

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Unaudited) - Schedule of Unaudited 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
Selected Quarterly Financial Information [Abstract]
Revenue	$ 0	$ 0	$ 30,000	$ 5,160	$ 26,137	$ 18,564	$ 12,987	$ 11,449	$ 35,160	$ 69,137	$ 5,247
Income (loss) from operations	(26,611)	(23,333)	6,049	(14,871)	8,262	6,477	1,871	532	(58,765)	17,142	(26,586)
Net income (loss)	$ (24,924)	$ (22,296)	$ 7,115	$ (13,687)	$ 9,377	$ 6,683	$ 1,797	$ 310	$ (53,792)	$ 18,167	$ (28,697)
Net income (loss) per share, basic	$ (0.80)	$ (0.93)	$ 0.30	$ (0.57)	$ 0.39	$ 0	$ 0	$ 0	$ (2.09)	$ 0.65	$ (50.37)
Net income (loss) per share, diluted	$ (0.80)	$ (0.93)	$ 0.28	$ (0.57)	$ 0.37	$ 0	$ 0	$ 0	$ (2.09)	$ 0.57	$ (50.37)