Harpoon Therapeutics (HARP) 10-K 14 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Feb. 28, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	HARP
Entity Registrant Name	Harpoon Therapeutics, Inc.
Entity Central Index Key	0001708493
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	No
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Public Float			$ 0
Entity Common Stock, Shares Outstanding		24,339,830

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 89,493	$ 29,423
Prepaid expenses and other current assets	730	224
Total current assets	90,223	29,647
Property and equipment, net	2,998	2,087
Tenant improvement allowance receivable	5,784
Other assets	3,575	138
Total assets	102,580	31,872
Current liabilities
Accounts payable	4,357	1,174
Accrued liabilities	3,341	1,492
Deferred revenue, current	4,250	4,250
Total current liabilities	11,948	6,916
Deferred revenue, noncurrent	7,792	12,042
Other long-term liabilities	6,742	16
Total liabilities	26,482	18,974
Commitments and contingencies (Note 6)
Convertible preferred stock, $0.0001 par value; 82,000,000 shares and 50,000,000 shares authorized at December 31, 2018 and 2017, respectively; 16,618,448 and 6,989,973 shares issued and outstanding as of December 31, 2018 and 2017, respectively; aggregate liquidation preference of $130,178 as of December 31, 2018	129,577	39,841
Stockholders' deficit
Common stock, $0.0001 par value; 114,000,000 and 75,000,000 shares authorized as of December 31, 2018 and 2017, respectively; 1,383,221 and 1,193,629 shares issued and outstanding at December 31, 2018 and 2017, respectively	1	1
Additional paid-in capital	9,111	8,309
Note receivable from stockholder		(28)
Accumulated deficit	(62,591)	(35,225)
Total stockholders' deficit	(53,479)	(26,943)
Total liabilities, convertible preferred stock and stockholders' deficit	$ 102,580	$ 31,872

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Convertible preferred stock, authorized	82,000,000	50,000,000
Convertible preferred stock, aggregate liquidation preference, value	$ 130,178	$ 40,182
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, Authorized	114,000,000	75,000,000
Common stock, issued	1,383,221	1,193,629
Common stock, outstanding	1,383,221	1,193,629
Convertible Preferred Stock
Convertible preferred stock, par value	$ 0.0001	$ 0.0001
Convertible preferred stock, authorized	82,000,000	50,000,000
Convertible preferred stock, issued	16,618,448	6,989,973
Convertible preferred stock, outstanding	16,618,448	6,989,973
Convertible preferred stock, aggregate liquidation preference, value	$ 130,178

Statement of Operation and Comp

Statement of Operation and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue
Collaboration and license revenue	$ 1,063	$ 1,063	$ 1,063	$ 1,561	$ 708				$ 4,750	$ 708
Total revenue	1,063	1,063	1,063	1,561	708				4,750	708
Operating expenses
Research and development	8,717	5,967	6,151	5,533	4,791	$ 3,876	$ 2,801	$ 2,154	26,368	13,622	$ 7,778
General and administrative	2,215	1,942	967	982	912	1,012	847	843	6,106	3,614	3,369
Total operating expenses	10,932	7,909	7,118	6,515	5,703	4,888	3,648	2,997	32,474	17,236	11,147
Loss from operations	(9,869)	(6,846)	(6,055)	(4,954)	(4,995)	(4,888)	(3,648)	(2,997)	(27,724)	(16,528)	(11,147)
Interest income	148	108	66	73	57	13	2	6	395	78	6
Interest expense							(112)	(173)		(285)	(261)
Other expense	(8)	(22)	(5)	(2)	(4)	(5)		(86)	(37)	(95)	(4)
Net loss and comprehensive loss	$ (9,729)	$ (6,760)	$ (5,994)	$ (4,883)	$ (4,942)	$ (4,880)	$ (3,758)	$ (3,250)	$ (27,366)	$ (16,830)	$ (11,406)
Net loss per share, basic and diluted	$ (8.15)	$ (6.23)	$ (5.89)	$ (5.04)	$ (5.27)	$ (5.35)	$ (4.24)	$ (3.86)	$ (25.65)	$ (18.81)	$ (21.61)
Weighted-average common shares used in computing net loss per share, basic and diluted	1,193,797	1,084,477	1,017,336	969,235	938,372	912,728	886,332	840,905	1,066,877	894,901	527,931

Statements of Convertible Prefe

Statements of Convertible Preferred Stock and Stockholder's Deficit - USD ($) $ in Thousands	Total	2015 Notes	2016 Notes	2016 Notes and 2017 Notes	2017 Notes	Convertible Preferred Stock	Convertible Preferred StockSeries A	Convertible Preferred StockSeries A2015 Notes	Convertible Preferred StockSeries B	Convertible Preferred StockSeries B2016 Notes and 2017 Notes	Convertible Preferred StockSeries C	Common Stock	Additional Paid-In Capital	Additional Paid-In Capital2015 Notes	Additional Paid-In Capital2016 Notes	Additional Paid-In Capital2016 Notes and 2017 Notes	Additional Paid-In Capital2017 Notes	Note Receivable from Stockholder	Accumulated Deficit
Beginning balance, shares at Dec. 31, 2015												186,832
Beginning balance at Dec. 31, 2015	$ (6,897)												$ 92						$ (6,989)
Temporary equity, beginning balance, shares at Dec. 31, 2015						0
Temporary equity, beginning balance at Dec. 31, 2015						$ 0
Issuance of convertible preferred stock, net of issuance costs							$ 7,426
Issuance of convertible preferred stock, net of issuance costs, shares							1,525,164
Issuance of convertible preferred stock upon extinguishment of notes								$ 7,500
Issuance of convertible preferred stock upon extinguishment of notes, shares								1,525,165
Capital transaction with a related party upon extinguishment of notes		$ 337												$ 337
Issuance of warrants related to sale of notes			$ 199												$ 199
Reclassification of warrants from liability	23												23
Issuance of common stock for exercise of stock options	10												10
Issuance of common stock for exercise of stock options, shares												17,556
Issuance of common stock for services and a license agreement	153											$ 1	152
Issuance of common stock for services and a license agreement, shares												201,118
Vesting of early exercised stock options	35												35
Vesting of early exercised stock options, shares												58,714
Stock-based compensation	99												99
Vesting of Founder's shares, shares												357,624
Proceeds from Maverick spin-off transaction	7,031												7,031
Note receivable from stockholder	(28)																	$ (28)
Net loss and comprehensive loss	(11,406)																		(11,406)
Ending balance, shares at Dec. 31, 2016												821,844
Ending balance at Dec. 31, 2016	$ (10,444)											$ 1	7,978					(28)	(18,395)
Temporary equity, ending balance, shares at Dec. 31, 2016						3,050,329
Temporary equity, ending balance at Dec. 31, 2016						$ 14,926
Issuance of convertible preferred stock, net of issuance costs									$ 19,729
Issuance of convertible preferred stock, net of issuance costs, shares	6,989,973								3,128,541
Issuance of convertible preferred stock upon extinguishment of notes										$ 5,186
Issuance of convertible preferred stock upon extinguishment of notes, shares										811,103
Capital transaction with a related party upon extinguishment of notes				$ (204)												$ (204)
Issuance of warrants related to sale of notes					$ 144												$ 144
Issuance of common stock for exercise of stock options	$ 4												4
Issuance of common stock for exercise of stock options, shares												7,897
Issuance of common stock	2												2
Issuance of common stock, shares												3,050
Vesting of early exercised stock options	18												18
Vesting of early exercised stock options, shares												30,502
Stock-based compensation	367												367
Vesting of Founder's shares, shares												85,001
Net loss and comprehensive loss	(16,830)																		(16,830)
Ending balance, shares at Dec. 31, 2017												948,294
Ending balance at Dec. 31, 2017	(26,943)											$ 1	8,309					(28)	(35,225)
Temporary equity, ending balance, shares at Dec. 31, 2017						6,989,973
Temporary equity, ending balance at Dec. 31, 2017	$ 39,841					$ 39,841
Issuance of convertible preferred stock, net of issuance costs											$ 69,744
Issuance of convertible preferred stock, net of issuance costs, shares	16,618,448										6,499,935
Issuance of convertible preferred stock upon extinguishment of notes										$ 19,992
Issuance of convertible preferred stock upon extinguishment of notes, shares										3,128,540
Issuance of common stock for exercise of stock options	$ 122												122
Issuance of common stock for exercise of stock options, shares												125,533
Vesting of early exercised stock options	36												36
Vesting of early exercised stock options, shares												56,025
Stock-based compensation	672												644					$ 28
Net loss and comprehensive loss	(27,366)																		(27,366)
Ending balance, shares at Dec. 31, 2018												1,211,419
Ending balance at Dec. 31, 2018	(53,479)											$ 1	$ 9,111						$ (62,591)
Temporary equity, ending balance, shares at Dec. 31, 2018						16,618,448
Temporary equity, ending balance at Dec. 31, 2018	$ 129,577					$ 129,577

Statements of Convertible Pre_2

Statements of Convertible Preferred Stock and Stockholder's Deficit (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Series A Preferred Stock
Shares issued, price per share			$ 4.92
Payments of stock issuance costs			$ 74
Series A Preferred Stock | 2015 Notes
Shares issued, price per share			$ 4.92
Series B Preferred Stock
Shares issued, price per share		$ 6.39
Payments of stock issuance costs		$ 271
Series B Preferred Stock | 2016 Notes and 2017 Notes
Payments of stock issuance costs	$ 8	$ 271
Shares issued, price per share	$ 6.39	$ 6.39
Series B Preferred Stock | 2017 Notes
Payments of stock issuance costs	$ 8	$ 271
Shares issued, price per share	$ 6.39	$ 6.39
Series C Preferred Stock
Shares issued, price per share	$ 10.77
Payments of stock issuance costs	$ 256

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net loss	$ (27,366)	$ (16,830)	$ (11,406)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities
Stock-based compensation expense	672	367	99
Depreciation and amortization	644	366	198
Accrued interest on convertible notes payable		153	185
Amortization of debt discount		132	76
Change in fair value of warrant liability			4
Changes in operating assets and liabilities
Prepaid expenses and other assets	(507)	335	(327)
Other assets		(138)	76
Accounts payable	2,555	804	570
Accrued liabilities	1,048	257	(317)
Deferred revenue	(4,250)	16,292
Other long-term liabilities	78	(24)	48
Net cash (used in) provided by operating activities	(27,126)	1,714	(10,794)
Cash flows from investing activities
Purchases of property and equipment	(663)	(2,275)	(553)
Proceeds from repayment of note receivable		6,750
Net cash (used in) provided by investing activities	(663)	4,475	(553)
Cash flows from financing activities
Proceeds from issuance of convertible preferred stock, net of issuance costs	89,828	19,729	7,426
Proceeds from issuance of convertible notes, net of issuance costs		2,496	2,478
Proceeds from issuance of common stock upon exercise of stock options, net	158	22	81
Proceeds from issuance of common stock		2
Payments of deferred initial public offering costs	(1,660)
Net cash provided by financing activities	88,326	22,249	9,985
Net increase (decrease) in cash, cash equivalents, and restricted cash	60,537	28,438	(1,362)
Cash, cash equivalents, and restricted cash at beginning of period	29,423	985	2,347
Cash, cash equivalents, and restricted cash at end of period	89,960	29,423	985
Supplemental disclosures of non-cash investing and financing information
Issuance of Maverick promissory note to Harpoon			6,750
Derecognition of net liabilities related to Maverick spin-off transaction			281
Capital transaction with a related party upon extinguishment of Notes		(204)	337
Issuance of common stock in exchange for license and services			552
Reacquisition of unvested common stock on extinguishment of note receivable from stockholder			62
Purchases of property and equipment included in accounts payable	28		23
Deferred initial public offering costs included in accounts payable and accrued liabilities	1,309	78	10
Tenant improvements provided by landlord	6,648
Series A Preferred Stock | 2015 Notes
Supplemental disclosures of non-cash investing and financing information
Issuance of stock on extinguishment			$ 7,500
Series B Convertible Preferred Stock [Member] | 2016 Notes and 2017 Notes
Supplemental disclosures of non-cash investing and financing information
Issuance of stock on extinguishment		$ 5,186
Series C Preferred Stock
Supplemental disclosures of non-cash investing and financing information
Series C preferred stock issuance costs included in accounts payable and accrued liabilities	$ 92

Organization

Organization	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	HARPOON THERAPEUTICS, Inc. 1. Description of Business Harpoon Therapeutics, Inc. (the “Company”) is a clinical-stage immunotherapy company developing a novel class of T cell engagers that harness the power of the body’s immune system to treat patients suffering from cancer and other diseases. T cell engagers are engineered proteins that direct a patient’s own T cells to kill target cells that express specific proteins, or antigens, carried by the target cells. Using a proprietary Tri-specific T cell Activating Construct, or TriTAC, platform, the Company is developing a pipeline of novel T cell engagers, or TriTACs, initially focused on the treatment of solid tumors and hematologic malignancies. The Company was incorporated in Delaware in March 2015 and is headquartered in South San Francisco, California. Initial Public Offering On February 7, 2019, the Company’s registration statements on Form S-1 relating to its initial public offering (“IPO”) was declared effective by the Securities and Exchange Commission (“SEC”) and the shares of its common stock began trading on the NASDAQ Global Select Market on February 8, 2019. The public offering price of the shares sold in the IPO was $14.00 per share. The IPO closed in February 2019, pursuant to which the Company sold 5,769,201 shares of common stock, for gross proceeds of approximately $80.8 million, including the exercise in part of the underwriters’ option to purchase additional shares. The Company received net proceeds from the IPO of approximately $70.7 million, after underwriting discounts, commissions and Immediately prior to the completion of the IPO on February 12, 2019, all outstanding shares of redeemable convertible preferred stock, including preferred stock warrants, were converted into 17,181,491 shares of common stock and $129.6 million was reclassified from temporary equity to additional paid in capital on the balance sheet. Subsequent to the closing of the IPO, there were no shares of redeemable convertible preferred stock outstanding. Liquidity The Company has incurred significant losses and has negative cash flows from operations. As of December 31, 2018, the Company had an accumulated deficit of $62.6 million. Management expects to continue to incur additional substantial losses in the foreseeable future as a result of the Company’s research and development activities. As of December 31, 2018, the Company had unrestricted cash and cash equivalents of $89.5 million, which is available to fund future operations. The Company believes that the proceeds from the IPO, along with the Company’s cash and cash equivalents balance as of December 31, 2018, provide sufficient capital resources to continue its operations for at least 12 months from the issuance date of these financial statements. The Company will need to raise additional capital to support the completion of its research and development activities. The Company’s activities are subject to significant risks and uncertainties, including failing to secure additional funding to continue to operationalize the Company’s current technology and to advance the development of its product candidates.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Basis of Presentation The financial statements have been prepared in accordance with U.S. generally accepted accounting principles. Reverse Stock Split On January 28, 2019, the Company filed an amendment to the Company’s amended and restated certificate of incorporation to effect a reverse split of shares of the Company’s common stock and convertible preferred stock on a 4.9175-for-one basis (the “Reverse Stock Split”). The par value and the authorized number of shares of the convertible preferred stock and common stock were not adjusted in connection with the Reverse Stock Split. All references to common stock, convertible preferred stock, warrants to purchase common stock, options to purchase common stock, early exercised options, restricted stock, share data, per share data and related information contained in the financial statements have been retrospectively adjusted to reflect the effect of the Reverse Stock Split for all periods presented. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of expenses during the reporting period. Significant estimates and assumptions made in the accompanying financial statements include, but are not limited to, the fair value of common stock, the fair value of stock options, the research period of the collaboration agreement with AbbVie, income tax uncertainties and certain accruals. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates. Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the Company’s Chief Operating Decision Maker in making decisions regarding resource allocation and assessing performance. To date, the Company has viewed its operations and manages its business as one segment operating primarily in the United States. Cash, Cash Equivalents and Restricted Cash The Company considers all highly liquid investments purchased with original maturities of three months or less from the purchase date to be cash equivalents. Cash equivalents consist primarily of amounts invested in money market accounts and are stated at fair value. The Company maintained restricted cash of $0.5 million and zero as of December 31, 2018 and 2017, respectively. This amount as of December 31, 2018 is included within “Other assets” in the accompanying balance sheets and is comprised solely of a letter of credit required pursuant to the lease for the Company’s corporate headquarters entered into in August 2018 as discussed in Note 6. The following table provides a reconciliation of cash and cash equivalents and restricted cash reported within the balance sheets that sum to the total of the same amounts shown in the statement of cash flows. As of December 31, 2018 2017 2016 (in thousands) Balance Sheets Cash and cash equivalents $ 89,493 $ 29,423 $ 985 Restricted cash (included in other assets) 467 — — Cash, cash equivalents and restricted cash in Statements of Cash Flows $ 89,960 $ 29,423 $ 985 Concentration of Credit Risk Cash and cash equivalents consist of financial instruments that potentially subject the Company to a concentration of credit risk, to the extent of the amounts recorded on the balance sheets. The Company minimizes the amount of credit exposure by investing cash that is not required for immediate operating needs in money market funds. Fair Value Measurement Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability, or an exit price, in the principal or most advantageous market for that asset or liability in an orderly transaction between market participants on the measurement date, and established a fair value hierarchy that requires an entity to maximize the use of observable inputs, where available, and minimize the use of unobservable inputs when measuring fair value. The Company measures fair value based on a three-level hierarchy of inputs, of which the first two are considered observable and the last unobservable. Unobservable inputs reflect the Company’s own assumptions about current market conditions. The three-level hierarchy of inputs is as follows: Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2—Inputs (other than quoted prices included in Level 1) are either directly or indirectly observable for the asset or liability. These include quoted prices for similar assets or liabilities in active markets and quoted prices for identical or similar assets or liabilities in markets that are not active; and Level 3— Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The carrying amounts reflected in the accompanying balance sheets for cash and cash equivalents, prepaid expenses, other current assets, accounts payable, accrued expenses and other current liabilities approximate their fair values, due to their short-term nature. Property and Equipment, Net Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation is computed on a straight-line basis over the estimated useful lives of the related assets, generally three to five years. Leasehold improvements are amortized using the straight-line method over the shorter of the assets’ estimated useful lives or the remaining term of the lease. Depreciation and amortization begin at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss is reflected in operations. Impairment of Long-Lived Assets The Company evaluates its long-lived assets, including property and equipment, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets or group of assets may not be fully recoverable. If indicators of impairment exist and the undiscounted future cash flows that the assets are expected to generate are less than the carrying amount of the assets, the Company reduces the carrying amount of the assets through an impairment charge to their estimated fair values based on a discounted cash flow approach or, when available and appropriate, to comparable market values. There were no impairments of long-lived assets for any of the periods presented. Revenue Recognition Effective January 1, 2017, the Company early adopted Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers The Company enters into corporate collaborations under which it may obtain upfront license fees, research and development funding, and development, regulatory and commercial milestone payments and royalty payments. The Company’s performance obligations under these arrangements may include licenses of intellectual property, distribution rights, research and development services, delivery of manufactured product and/or participation on joint steering committees. Licenses of intellectual property: If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenue from upfront license fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. For licenses that are bundled with other promises, the Company utilizes judgement 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 proportional performance for purposes of recognizing revenue from non-refundable, up-front fees. The Company evaluates the measure of proportional performance each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Milestone payments: At the inception of each arrangement that includes development, regulatory or commercial milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price. Topic 606 suggests two alternatives to use when estimating the amount of variable consideration: the expected value method and the most likely amount method. Under the expected value method, an entity considers the sum of probability-weighted amounts in a range of possible consideration amounts. Under the most likely amount method, an entity considers the single most likely amount in a range of possible consideration amounts. Whichever method is used, it should be consistently applied throughout the life of the contract; however, it is not necessary for the Company to use the same approach for all contracts. The Company expects to use the most likely amount method for development and regulatory milestone payments. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s or the licensee’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The transaction price is then allocated to each performance obligation on a relative stand-alone selling price basis. The Company recognizes revenue as or when the performance obligations under the contract are satisfied. At the end of each subsequent reporting period, the Company re-evaluates the probability or achievement of each such milestone and any related constraint, and if necessary, adjusts its estimates of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. Commercial milestones and royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and in which the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue when the related sales occur. To date, the Company has not recognized any royalty revenue resulting from its collaboration arrangements. Upfront payments and fees are recorded as deferred revenue upon receipt or when due, and may require deferral of revenue recognition to a future period until the Company performs its obligations under these arrangements. Amounts payable to the Company are recorded as accounts receivable when the Company’s right to consideration is unconditional. Research and Development Expenses and Accrued Research and Development Costs The Company expenses research and development costs as incurred. Research and development expenses consist of personnel costs for the Company’s research and product development employees. Also included are non-personnel costs such as professional fees payable to third parties for preclinical studies, clinical trials, research services, laboratory supplies and equipment maintenance and depreciation, intellectual property licenses and other consulting costs. The Company estimates preclinical and clinical study and research expenses based on the services performed, pursuant to contracts with research institutions that conduct and manage preclinical studies, clinical trials and research services on its behalf. The Company estimates these expenses based on discussions with internal management personnel and external service providers as to the progress or stage of completion of services and the contracted fees to be paid for such services. The Company records the estimated costs of research and development activities based upon the estimated amount services provided but not yet invoiced, and includes these costs in development expenses. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service provides under the service agreements. The Company makes significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred. However, the status and timing of actual services performed may vary from the Company’s estimates, resulting in adjustments to expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations. Payments associated with licensing agreements to acquire exclusive license to develop, use, manufacture and commercialize products that have not reached technological feasibility and do not have alternate future use are expensed as incurred. Payments made to third parties under these arrangements in advance of the performance of the related services by the third parties are recorded as prepaid expenses until the services are rendered. Such payments are evaluated for current or long-term classification based on when such services are expected to be received. Stock-Based Compensation The Company maintains a stock-based compensation plan as a long-term incentive for employees, consultants and members of the Company’s board of directors (the “Board”). The plan allows for the issuance of non-statutory options (“NSOs”) and incentive stock options to employees and NSOs to nonemployees. Share-based payments are measured using fair-value-based measurements and recognized as compensation expense over the service period in which the awards are expected to vest. The Company’s fair-value-based measurements of awards to employees and directors as of the grant date utilize the single-option award-valuation approach, and the Company uses the straight-line method for expense attribution. The fair-value-based measurements of options granted to nonemployees are remeasured at each period end until the options vest and are amortized to expense as earned. The valuation model used for calculating the estimated fair value of stock awards is the Black-Scholes option-pricing model. The Black-Scholes model requires the Company to make assumptions and judgments about the variables used in the calculations, including the expected term (weighted-average period of time that the options granted are expected to be outstanding), the expected volatility of the Company’s common stock, the related risk-free interest rate and the expected dividend. The Company has elected to recognize forfeitures of share-based payment awards as they occur. Effective January 1, 2018, we early adopted ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting Convertible Preferred Stock The Company records all shares of convertible preferred stock at their respective fair values less issuance costs on the dates of issuance. The convertible preferred stock is recorded outside of stockholders’ deficit because, in the event of certain deemed liquidation events considered not solely within the Company’s control, such as a merger, acquisition and sale of all or substantially all of the Company’s assets, the convertible preferred stock will become redeemable at the option of the holders. In the event of a change of control of the Company, proceeds received from the sale of such shares will be distributed in accordance with the liquidation preferences set forth in the Company’s Amended and Restated Certificate of Incorporation unless the holders of convertible preferred stock have converted their shares of convertible preferred stock into shares of common stock. The Company has determined not to adjust the carrying values of the convertible preferred stock to the liquidation preferences of such shares because of the uncertainty of whether or when such an event would occur. Income Taxes Income taxes are accounted for under the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates applied to taxable income in the years in which those temporary differences are expected to be realized. The effect on deferred tax assets and liabilities of a change in tax rates is recognized as income in the period that includes the enactment date. A valuation allowance is established when necessary to reduce deferred tax assets to the amount expected to be realized. Financial statement effects of uncertain tax positions are recognized when it is more-likely-than-not, based on the technical merits of the position, that it will be sustained upon examination. Interest and penalties related to unrecognized tax benefits are included as a component of other expense. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits. The Company accounts for uncertain tax positions in accordance with ASC 740-10, Accounting for Uncertainty in Income Taxes The Company includes any penalties and interest expense related to income taxes as a component of provision for income tax as necessary. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits. Net Loss Per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration for potentially dilutive securities. As discussed in Note 11, the unvested portion of early exercised stock options are excluded from the computation of weighted average shares as the continuing vesting of such shares is contingent on the holders’ continued service to the Company. Diluted net loss per share is the same as basic net loss per share for each period presented, since the effects of potentially dilutive securities are antidilutive given the net loss of the Company. Comprehensive Income (Loss) The Company has no components of comprehensive income (loss) other than net loss. Thus, comprehensive loss is the same as net loss for all periods presented. Deferred Offering Costs The Company has deferred offering costs consisting of legal, accounting and other fees directly attributable to the Company’s IPO. The deferred offering costs were offset against the proceeds received upon completion of the IPO. As of December 31, 2018 and December 31, 2017, $3.0 million and zero of deferred offering costs were included in other assets on the condensed balance sheet, respectively. Emerging Growth Company Status The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (the “JOBS Act”). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that we (i) are no longer an emerging growth company or (ii) affirmatively and irrevocably opt out of the extended transition period provided in the JOBS Act. As a result, these financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates. As described in “Recently Adopted Accounting Pronouncements” below, the Company early adopted Revenue from Contracts with Customers Stock Compensation—Improvements to Employee Share-Based Payment Accounting Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, as the JOBS Act does not preclude an emerging growth company from adopting a new or revised accounting standard earlier than the time that such standard applies to private companies. (Topic 842), Leases Recently Adopted Accounting Pronouncements On January 1, 2018, the Company early adopted ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting On January 1, 2018, the Company adopted ASU 2017-09, Stock Compensation – Scope of Modification Accounting The Company elected to adopt this guidance prospectively to awards modified on or after the adoption date. The adoption Recently Issued Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2016-02 (Topic 842), Leases In February 2018, the FASB issued ASU No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income 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

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. The following table presents information about the Company’s financial assets that are measured at fair value and indicates the fair value hierarchy of the valuation: Fair Value Measurements at December 31, 2018 Total Level 1 Level 2 Level 3 (in thousands) Assets Money market funds $ 60,396 $ 60,396 $ — $ — Total $ 60,396 $ 60,396 $ — $ — Fair Value Measurements at December 31, 2017 Total Level 1 Level 2 Level 3 (in thousands) Assets Money market funds $ 28,575 $ 28,575 $ — $ — Total $ 28,575 $ 28,575 $ — $ — The Company classifies money market funds as Level 1. The Company has no Level 2 or Level 3 assets or liabilities as of December 31, 2018 or 2017. There were no transfers between Level 1 and Level 2 during the years ended December 31, 2018 and 2017. There were no unrealized or realized gains or losses during the years ended December 31, 2018 and 2017. The Company did not have any financial liabilities subject to fair value measurements on a recurring basis as of December 31, 2018 and 2017.

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	4. Property and Equipment, Net Property and equipment, net consists of the following: December 31, 2018 2017 (in thousands) Laboratory equipment $ 2,402 $ 1,752 Furniture and fixtures 312 268 Computer equipment and software 32 32 Leasehold improvements 360 327 Construction in progress 906 78 4,012 2,457 Less: Accumulated depreciation and amortization (1,014 ) (370 ) Total property and equipment, net $ 2,998 $ 2,087 Depreciation and amortization expense for property and equipment amounted to $0.7 million and $0.4 million for the years ended December 31, 2018 and 2017, respectively. Accrued Liabilities Accrued liabilities consist of the following: December 31, 2018 2017 Accrued research and development $ 504 $ 522 Accrued personnel costs 1,593 871 Accrued professional and consulting fees 333 — Accrued offering costs 709 — Other 202 99 Total accrued liabilities $ 3,341 $ 1,492

Convertible Notes

Convertible Notes	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Convertible Notes	5 . In November 2016, the Company issued an aggregate of $2.5 million in convertible promissory notes (the “2016 Notes”) to existing shareholders. In January 2017, the Company issued an additional $2.5 million in convertible promissory notes (the “2017 Notes”) to the same investors. Interest accrued on the aggregate principal balance of $5.0 million at an annual rate of 8%. Principal and interest were due and payable upon the earliest of (i) the one-year anniversary of the issuance date, (ii) an event of default or (iii) on demand. Principal and accrued interest thereon automatically converted into shares of convertible preferred stock in a Qualified Financing (defined as the first issuance of convertible preferred stock with gross proceeds to the Company of at least $25.0 million). In May 2017, the outstanding principal balance of the 2016 Notes and the 2017 Notes, and the then-outstanding balance of accrued interest, converted into 811,103 shares of the Company’s Series B convertible preferred stock in conjunction with the Series B Financing (see Note 8). The conversion of the 2016 Notes and the 2017 Notes into shares of Series B convertible preferred stock was accounted for as an extinguishment. The difference of $0.2 million between the net carrying value of the 2016 Notes and the 2017 Notes and accrued interest thereon, and the reacquisition price, was recorded through additional paid-in capital for the year ending December 31, 2017 as the extinguishment was deemed to be a capital transaction with a related party. Warrants Issued with 2016 and 2017 Notes In connection with the issuance of the 2016 Notes and the 2017 Notes, the Company issued warrants for the purchase of 152,514 shares of the Company’s common stock and for the purchase of 101,675 shares of the Company’s common stock, respectively, at an exercise price of $0.05 per share. The warrants are exercisable after the first issuance of convertible preferred stock with gross proceeds to the Company of at least $5.0 million until the ten-year anniversary of the issuance date, and were classified in equity as they meet the equity classification requirements. Upon issuance, the Company estimated the fair value of these warrants using the Black-Scholes option-pricing model to be $0.2 million and $0.1 million for the 2016 Notes and the 2017 Notes, respectively. Such amounts were recorded as a reduction to the carrying value of the 2016 Notes and the 2017 Notes, and such debt discount was accreted using the effective interest method. For the years ended December 31, 2018 and 2017, the Company recognized total amortization expense relating to the discount created by the warrants and debt issuance costs of zero and $0.1 million, respectively, through interest expense in the statements of operations and comprehensive loss. As of December 31, 2018 and 2017, the unamortized debt discount was zero as the 2016 and 2017 Notes were converted into shares of convertible preferred stock as noted above and no convertible promissory notes existed as of December 31, 2018 and 2017. As of December 31, 2018 and 2017, warrants for the purchase of an aggregate of 565,270, of the Company’s common stock were outstanding, of which 565,270 shares were exercisable. The weighted-average exercise price of the outstanding warrants was $0.05 per share as of both December 31, 2018 and 2017. In connection with the Company’s IPO, the Company’s outstanding warrants of 565,270 shares automatically net exercised at the IPO price of $14.00 per share.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	6 . Leases In February 2017, the Company entered into an operating lease agreement with Tizona Therapeutics, Inc. (“Tizona”) for its headquarters in South San Francisco, California. One member of the Board is also the Executive Chairman of Tizona and, as such, Tizona is deemed to be a related party. The lease term is for 36 months. In August 2018, the Company entered into a lease agreement for the office and laboratory space in South San Francisco, California. The lease has an initial term of eight years, beginning on the lease commencement date, with an option to extend the lease for an additional period of eight years. The lease commencement date is the later to occur of (i) July 1, 2019 and (ii) the date the premises are ready for occupancy. Pursuant to the terms of the lease, the Company is entitled to a tenant improvement allowance of approximately $5.2 million with the option for an additional tenant improvement allowance of approximately $1.4 million. The additional tenant improvement allowance of $1.4 million would be treated as a loan from the landlord and is expected to be paid back (including interest) by the Company through additional rental payments. In December 2018, the Company exercised this option for the use of the additional tenant improvement allowance of $1.4 million. As such, the total tenant improvement allowance of $6.6 million granted to the Company was recognized as a tenant improvement allowance receivable and a related deferred rent liability. As of December 31, 2018, the portion of the tenant improvement allowance utilized under this lease was $0.9 million, which was recorded as construction in progress and a reduction to the tenant improvement allowance receivable on the balance sheet. The Company will amortize the deferred rent liability as a reduction of rent expense over the expected lease term. In addition, the lease requires the Company to maintain a letter of credit for the benefit of the landlord in the amount of $0.5 million, commencing on the effective date of the agreement until the expiration of the lease. This amount is included within the other assets on the balance sheet. The initial annual base rent is approximately $2.2 million, and such amount will increase by 3.5% annually on each anniversary of the commencement date. The Company recognizes rent expense on a straight-line basis over the non-cancelable lease term and records 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 incentive and allowances, the Company applies them in the determination of straight-line rent expense over the lease term. The Company records tenant improvement allowances as deferred rent and associated expenditures as leasehold improvements that are being amortized over the shorter of their estimated useful life or the term of the lease. The Company does not assume renewals in its determination of the lease term unless the renewals are deemed by management to be reasonably assured at lease inception. As of December 31, 2018, future minimum payments under the Company’s non-cancelable operating lease are as follows (in thousands): Year Ended December 31, 2018 2019 $ 1,308 2020 2,762 2021 2,566 2022 2,647 2023 2,731 Thereafter 10,251 $ 22,265 Rent expense for each of the years ended December 31, 2018, 2017, and 2016 was $1.2 million, $0.8 million, and $0.8 million, respectively. Indemnification In the ordinary course of business, the Company enters into agreements that may include indemnification provisions. Pursuant to such agreements, the Company may indemnify, hold harmless and defend an indemnified party for losses suffered or incurred by the indemnified party. Some of the provisions will limit losses to those arising from third-party actions. In some cases, the indemnification will continue after the termination of the agreement. The maximum potential amount of future payments the Company could be required to make under these provisions is not determinable. The Company has never incurred material costs to defend lawsuits or settle claims related to these indemnification provisions. The Company has also entered into indemnification agreements with its directors and officers that may require the Company to indemnify its directors and officers against liabilities that may arise by reason of their status or service as directors or officers to the fullest extent permitted by Delaware corporate law. The Company currently has directors’ and officers’ insurance.

Collaboration & License Agreeme

Collaboration & License Agreements	12 Months Ended
	Dec. 31, 2018
Collaboration And License Agreements [Abstract]
Collaboration & License Agreements	7 . Collaboration Agreement with AbbVie On October 10, 2017, the Company entered into a Discovery Collaboration and License Agreement (the “Collaboration Agreement”) with AbbVie. Pursuant to the Collaboration Agreement, the Company granted to AbbVie worldwide exclusive rights to develop and commercialize products that incorporate its proprietary TriTAC technology together with soluble T cell receptors (“TCRs”) provided by AbbVie that bind to targets accepted by the parties. Under the terms of the Collaboration Agreement, AbbVie is allowed to designate up to two targets, subject to confirmation of target availability. During a period of up to four years following the date of AbbVie’s designation of each target for the products, and confirmation of target availability, the Company and AbbVie will conduct certain research and discovery activities under a mutually agreed discovery and research plan in connection with the creation and evaluation of constructs comprising the Company’s proprietary TriTAC technology in conjunction with the soluble TCR sequences directed against the agreed upon targets of interest. The Company may not, by itself or through any third party, develop or commercialize any competing product that binds to any of the included targets. Following the discovery phase, AbbVie will be solely responsible, at its cost, for the development, manufacture and commercialization of products that arise from the activities under the discovery plan. AbbVie is required to use commercially reasonable efforts to develop and commercialize one such product directed to each target for which the discovery activities were completed, in the United States and specified European markets. In addition to an upfront payment of $17 million, AbbVie will be required to make further payments to the Company of up to $600 million in the aggregate, for the achievement of specified development, regulatory, and commercial sale milestones for licensed products indicated for human therapeutic or prophylactic use, if such licensed products are successfully progressed against all included targets and indications. The Company will also receive tiered royalties on net sales by AbbVie, its affiliates and sublicensees of licensed products at percentages in the mid-single digits, subject to specified offsets and reductions. If licensed products are developed and commercialized for diagnostic or veterinary use, or certain screening or monitoring uses, the parties have agreed to negotiate an appropriate reduction in the economic terms applicable to such non-therapeutic and prophylactic applications. The Collaboration Agreement will terminate upon the date of the expiration of all AbbVie’s royalty payment obligations in all countries. The Collaboration Agreement may be terminated by either party immediately for the insolvency of the other party or on 90 days’ written notice for an uncured material breach of the Collaboration Agreement by the other party. In the case of a material breach with respect to commercialization diligence with respect to any major market, or with respect to only one of the included targets, the Company may terminate the Collaboration Agreement solely with respect to the affected major markets, or target, as applicable. AbbVie may also terminate the Collaboration Agreement in its entirety or on a target-by-target or country-by-country basis for any reason on 30 days’ written notice to the Company. In addition, AbbVie may terminate the Collaboration Agreement immediately in its entirety or on a target-by-target basis if AbbVie considers in good faith that there has been a failure of the discovery or development efforts with respect to such target, or that further development or commercialization of products directed to such target is not advisable as a result of a serious safety issue. The Company assessed the Collaboration Agreement in accordance with Topic 606 and concluded that AbbVie is a customer. The Company concluded that there are multiple promises under the Collaboration Agreement which include the research and development activities, regulatory documentation and know-how, exclusivity, and the license to the related technology. The Company combined these promises into a single performance obligation, as the Company is obliged to render specialized services for the research program, and other promises have either no significant value or are not distinct. The Company estimates that the $17.0 million upfront payment will be recognized over the estimated four-year period of ongoing research and development activities based on the projected activities to be performed over each reporting period relative to the total estimated performance period. Such estimates are reviewed by the Company on a periodic basis At the inception of the Collaboration Agreement, the Company determined that the transaction price was $17.0 million, which was all allocated to the single unit of accounting. The Company has evaluated the transaction price and has determined $17.0 million is still appropriate as of December 31, 2018. For the year ended December 31, 2018 and 2017, $4.3 million and $0.7 million of revenue has been recognized in the accompanying statement of operations and comprehensive loss, respectively. As of December 31, 2018, the Company has recorded $12.0 million in deferred revenue, of which $7.8 million is classified as long-term and $4.2 million as short-term deferred revenue, in the accompanying balance sheet. As of December 31, 2017, the Company recorded $16.3 million in deferred revenue, of which $12.1 million was classified as long-term and $4.2 million as short-term deferred revenue. The Company determined that future contingent payments that may be received related to development and regulatory milestones under the Collaboration Agreement are based on the performance of AbbVie and are constrained due to the fact that it was not probable that a significant reversal of cumulative revenue would not occur, as their achievement is highly dependent on the successful completion of the research activities. Accordingly, revenue for the achievement of these milestones will be recognized in the period that it is deemed probable that the milestone will be achieved. Any consideration related to commercialization and sales milestones, and sales-based royalties will be recognized when the related sales occur as they were determined to relate predominantly to the license granted to AbbVie and have been excluded from the transaction price. The Company will re-evaluate the transaction price in each reporting period as uncertain events are resolved or other changes in circumstances occur. As of December 31, 2018, the Company has not recognized or earned any milestone payments under the Collaboration Agreement. The Company will recognize royalty revenue in the period of sale of the related products, based on the underlying contract terms. No such amounts were recognized during the year ended December 31, 2018. License Agreement with Werewolf Therapeutics, Inc. In March 2018, the Company entered into an assignment and license agreement (the “Werewolf Agreement” or “License Agreement”) with Werewolf Therapeutics, Inc. (“Werewolf”). Werewolf is affiliated with a holder of more than 5% of the Company’s capital stock, and one member of the Board is also the interim Chief Executive Officer of Werewolf and as such, Werewolf was deemed to be a related party. Pursuant to the Werewolf Agreement, the Company assigned certain patents and granted to Werewolf a non-exclusive, royalty-bearing, sublicensable license under certain other patents. In addition, Werewolf assigned certain patents to the Company. Under the Werewolf Agreement, the Company received an upfront fee of $0.5 million. If Werewolf commercializes products covered by the licensed patents, then beginning on the first sale of such products, the Company will be eligible to receive a royalty on net sales of such products by Werewolf, its affiliates and licensees at a percentage in the low single digits, subject to a minimum annual royalty payment at an amount in the low hundreds of thousands of dollars. The Werewolf Agreement will continue on a country-by-country basis until the expiration of the last to expire patent licensed under the Werewolf Agreement, and it may be terminated by either party for the other party’s uncured material breach. The Company may terminate the license grant to Werewolf upon Werewolf’s insolvency. In addition, Werewolf may terminate the Werewolf Agreement for convenience at any time upon advance notice to the Company. Upon any termination, the license granted to Werewolf under the Werewolf Agreement will immediately terminate. The Company assessed the License Agreement in accordance with Topic 606 and concluded that Werewolf is a customer, and there is only one promise and a performance obligation to deliver intellectual property license. The upfront fee of $0.5 million will be recognized upfront upon delivery of the license to Werewolf and royalties on net sales will be recognized when the underlying sales occur. For the year ended December 31, 2018, $0.5 million of revenue related to the upfront payment has been recognized in the accompanying condensed statement of operations and comprehensive loss. No royalty revenue was recognized under the Werewolf Agreement during the year ended December 31, 2018.

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Convertible Preferred Stock	8 . In May 2017, the Company entered into a Series B Preferred Stock Purchase Agreement (the “Series B Agreement”), pursuant to which the Company issued 3,128,540 shares of its Series B convertible preferred stock at a purchase price of $6.39 per share for net proceeds of $19.7 million, of which $7.5 million was sold to related party investors of the Company. In addition, as discussed in Note 5, the Company issued an aggregate of 811,103 shares of Series B convertible preferred stock upon the extinguishment of the 2016 Notes and the 2017 Notes in an aggregate of $5.2 million. Included in the terms of the Series B Agreement were certain rights (“Tranche Rights”) granted to the initial purchasers of Series B convertible preferred stock that became exercisable upon the achievement by the Company of certain regulatory milestones. The Tranche Rights provided the purchasers of Series B convertible preferred stock the right to purchase, and obligated the Company to sell, 3,128,540 additional shares of Series B convertible preferred stock at $6.39 per share upon the filing of an IND for HPN424. This milestone was achieved in July 2018, resulting in the Company issuing 3,128,540 shares of Series B convertible preferred stock to its existing Series B investors at a price per share of $6.39 for net proceeds of $20.0 million. The Company concluded the Tranche Rights did not meet the definition of a freestanding financial instrument, as the Tranche Rights were not legally detachable from the Series B convertible preferred stock. In November 2018, the Company entered into a Series C Preferred Stock Purchase Agreement, pursuant to which the Company issued and sold approximately 6,499,935 million shares of its Series C convertible preferred stock at a purchase price of $10.77 per share for net proceeds of approximately $69.7 million, of which approximately $29.0 million was sold to related party investors of the Company. Convertible preferred stock consists of the following: As of December 31, 2018 Shares Issued Aggregate Shares and Carrying Liquidation Authorized Outstanding Value Preference (In thousands, except share data) Series A 15,000,000 3,050,329 $ 14,926 $ 15,008 Series B 35,000,000 7,068,184 44,906 45,166 Series C 32,000,000 6,499,935 69,745 70,004 82,000,000 16,618,448 $ 129,577 $ 130,178 As of December 31, 2017 Shares Issued Aggregate Shares and Carrying Liquidation Authorized Outstanding Value Preference (In thousands, except share data) Series A 15,000,000 3,050,329 $ 14,926 $ 15,008 Series B 35,000,000 3,939,644 24,915 25,174 50,000,000 6,989,973 $ 39,841 $ 40,182 The Company classifies the convertible preferred stock outside of total stockholders’ deficit because, in the event of certain “liquidation events” that are not solely within the control of the Company (including a merger, acquisition or sale of all or substantially all of the Company’s assets), the shares would become redeemable at the option of the holders. The Company did not adjust the carrying values of the convertible preferred stock to the deemed liquidation values of such shares since a liquidation event was not probable at either of the reporting dates. Subsequent adjustments to increase or decrease the carrying values to the ultimate liquidation values will be made only if and when it becomes probable that such a liquidation event will occur. On the completion of the IPO (see Note 1), all outstanding shares of convertible preferred stock were automatically converted into 16,618,448 shares of common stock.

Equity

Equity	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Equity	9. Stock-Based Compensation In 2015, the Company adopted the 2015 Equity Incentive Plan (the “2015 Plan”). The 2015 Plan provides for the Company to sell or issue common stock or restricted common stock, or to grant incentive stock options or nonqualified stock options for the purchase of common stock, to employees, members of the Board and consultants of the Company under terms and provisions established by the Board. Under the terms of the 2015 Plan, options may be granted at an exercise price not less than fair market value. The Company generally grants stock-based awards with service conditions only. Options granted typically vest over a four-year period but may be granted with different vesting terms. In January 2019, the Company’s b oard of directors adopted and stockholders approved the Company’s 2019 Equity Incentive Plan (the “2019 Plan”), which became effective immediately prior to the execution of the underwriting agreement for the Company’s IPO in February 2019, at which point the 2015 Plan was terminated and no further grants were made under the Company’s 2015 Plan. However, all outstanding stock awards granted pursuant to the 2015 Plan will continue to be subject to the terms and conditions as set forth in the agreements evidencing such stock awards. As of December 31, 2018 and 2017, there were 132,394 shares and 237,182 shares reserved by the Company to grant under the 2015 Plan, respectively. The following summarizes option activity under the 2015 Plan: Weighted Weighted Average Number of Average Remaining Aggregate Outstanding Exercise Contractual Intrinsic Options Price Life Value (in years) (in thousands) Balance as of December 31, 2016 286,739 $ 0.59 9.35 $ 70 Options granted 1,686,997 1.16 Options exercised (97,373 ) 0.59 Options cancelled (85,064 ) 0.59 Balance as of December 31, 2017 1,791,299 1.13 9.24 $ 881 Options granted 1,762,147 2.06 Options exercised (198,943 ) 1.35 Options cancelled (30,515 ) 1.29 Balance as of December 31, 2018 3,323,988 1.61 9.07 $ 1,675 Vested and expected to vest as of December 31, 2018 3,323,988 1.61 9.07 $ 1,675 Exercisable as of December 31, 2018 548,332 0.93 8.11 $ 651 As of December 31, 2018, 3,902,175 of options were authorized for the grant of options. The aggregate intrinsic values of options outstanding, vested and exercisable, and vested and expected to vest were calculated as the difference between the exercise price of the options and the estimated fair value of the Company’s common stock, as determined by the Board of Directors. The intrinsic value of options exercised for the years ended December 31, 2018, 2017 and 2016 was $0.1 million, zero, and zero, respectively. There is no future tax benefit related to options exercised, as the Company has accumulated net operating losses at December 31, 2018, 2017 and 2016. During the years ended December 31, 2018, 2017 and 2016, the estimated weighted-average grant-date fair value of the employee options vested was $0.81, $0.72 and $0.68 per share, respectively, and the estimated weighted-average grant-date fair value of employee options granted was $1.41, $0.92 and $0.67 per share, respectively. As of December 31, 2018, there was $3.2 million of unrecognized stock-based compensation related to unvested stock options that is expected to be recognized over a weighted-average period of 3.4 years. The fair value of employee and director stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumptions: Year Ended December 31, 2018 2017 2016 Expected term (years) 6.06 6.33 6.03 Expected volatility 76.08% 73.39% 91.35% Risk-free interest rate 2.89% 2.03% 1.37% Expected dividend 0% 0% 0% The fair value of the shares of common stock underlying stock options has historically been determined by the Board. Because there has been no public market for the Company’s common stock, the Board has determined fair value of the common stock at the time of grant of the option by considering a number of objective and subjective factors including important developments in the Company’s operations, valuations performed by an independent third party, sales of convertible preferred stock, actual operating results and financial performance, the conditions in the biotechnology industry and the economy in general, the stock price performance and volatility of comparable public companies, and the lack of liquidity of the Company’s common stock, among other factors. The Black-Scholes option-pricing model requires the use of highly subjective assumptions which determine the fair value of stock-based awards. These assumptions include: Expected Term —The expected term represents the period that stock-based awards are expected to be outstanding. The expected term for option grants is determined using the simplified method. The simplified method deems the term to be the average of the time-to-vesting and the contractual life of the stock-based awards. Expected Volatility —Since the Company is privately held and does not have any trading history for its common stock, the expected volatility was estimated based on the average volatility for comparable publicly traded biotechnology companies over a period equal to the expected term of the stock option grants. The comparable companies were chosen based on their similar size, stage in the life cycle or area of specialty. Risk-Free Interest Rate —The risk-free interest rate is based on the U.S. Treasury zero coupon issues in effect at the time of grant for periods corresponding with the expected term of option. Expected Dividend —The Company has never paid dividends on its common stock and has no plans to pay dividends on its common stock. Therefore, the Company used an expected dividend yield of zero. Total stock-based compensation was as follows: Year Ended December 31, 2018 2017 2016 (in thousands) Operating Expenses Research and development $ 325 $ 145 $ 43 General and administrative 347 222 56 Total stock-based compensation $ 672 $ 367 $ 99 Stock-based compensation related to non-employee awards, which is included in the table above, was $0.1 million, $0.1 million and insignificant for the years ended December 31, 2018, 2017 and 2016, respectively. 2019 Equity Incentive Plan The board of directors adopted, and the Company’s stockholders approved the Company’s 2019 Plan in January 2019, which became effective as of immediately prior to the execution of the underwriting agreement for the Company’s IPO in February 2019, after which, no further grants were made under the Company’s 2015 Plan. No awards had been granted and no shares of our common stock had been issued under our 2019 Plan as of December 31, 2018. Initially, the aggregate number of shares of our common stock that may be issued pursuant to stock awards under our 2019 Plan is 5,656,381, which is the sum of (1) 2,200,000 shares plus (2) the number of shares reserved, and remaining available for issuance, under our 2015 Plan at the time our 2019 Plan becomes effective and (3) the number of shares subject to stock options or other stock awards granted under our 2015 Plan that would have otherwise returned to our 2015 Plan (such as upon the expiration or termination of a stock award prior to vesting. The number of shares of our common stock reserved for issuance under our 2019 Plan will automatically increase on January 1 of each year, beginning on January 1, 2020 and continuing through and including January 1, 2029, by 5% of the total number of shares of our capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by our board of directors. The maximum number of shares that may be issued upon the exercise of incentive stock options under our 2019 Plan is 8,000,000 shares. 2019 Employee Stock Purchase Plan The board of directors adopted, and the Company’s stockholders approved, the 2019 Employee Stock Purchase Plan, (the “2019 ESPP”) in January 2019. The 2019 ESPP became effective in February 2019. The initial reserve for purchase by participating employees under the 2019 ESPP an aggregate number of shares of common stock shall not exceed 250,000 shares. The maximum aggregate number of shares of common stock that may be issued under the 2019 ESPP is 750,000 shares. Additionally, the number of shares of common stock reserved for issuance under the 2019 ESPP will increase automatically each year, beginning on January 1, 2020 and continuing through and including January 1, 2029, in an amount equal to the lesser of (i) 1% of the total number of shares of the Registrant’s capital stock outstanding on December 31 of the preceding calendar year, (ii) 750,000 shares of Common Stock and (iii) a number of shares of Common Stock designated by action of the Registrant’s board of directors prior to the first day of any calendar year. The board of directors may act prior to the first day of any calendar year to provide that there will be no January 1 increase or that the increase will be for a lesser number of shares than would otherwise occur. Shares subject to purchase rights granted under the 2019 ESPP that terminate without having been exercised in full, the shares of Common Stock not purchased under such Purchase Right will again become available for issuance under the Plan. An employee may not be granted rights to purchase stock under the 2019 ESPP if such employee (i) immediately after the grant would own stock possessing 5% or more of the total combined voting power or value of all classes of stock of the Company or (ii) holds rights to purchase stock under the 2019 ESPP that would accrue at a rate that exceeds $25,000 worth of our stock for each calendar year that the rights remain outstanding. The administrator may approve offerings with a duration of not more than 27 months, and may specify one or more shorter purchase periods within each offering. Each offering will have one or more purchase dates on which shares of common stock will be purchased for the employees who are participating in the offering. The administrator, in its discretion, will determine the terms of offerings under the 2019 ESPP. The 2019 ESPP permits participants to purchase shares of our common stock through payroll deductions with up to 15% of their earnings. The purchase price of the shares will be not less than 85% of the lower of the fair market value of our common stock on the first day of an offering or on the date of purchase. Restricted Stock In 2015, the Company issued restricted stock awards to employees and directors under the 2015 Plan at a purchase price of $0.0005 per share. The shares related to restricted stock awards are subject to a lapsing repurchase right upon termination of employment at the original purchase price. In order to vest, the holders are required to provide continued service to the Company. For accounting purposes, unvested restricted stock awards are not considered issued and outstanding and therefore are not reflected as issued and outstanding in the accompanying statements of convertible preferred stock and stockholders’ deficit until the awards vest. A summary of restricted stock activity is shown in the below table: Number of of Restricted Stock Outstanding Restricted shares- December 31, 2016 316,836 Restricted stock awards vested (85,001 ) Unvested shares repurchased (118,678 ) Restricted shares- December 31, 2017 113,157 Restricted stock awards vested (81,623 ) Unvested shares repurchased (9,356 ) Restricted shares- December 31, 2018 22,178 As of December 31, 2018, the Company had reserved common stock, on an as-converted basis, for issuance as follows: Convertible preferred stock (as converted) 16,618,448 Common stock options issued and outstanding 3,323,988 Restricted Stock subject to future vesting 22,178 Early exercised stock options subject to future vesting 149,565 Warrants to purchase shares of common stock 565,270 Shares available for future grant under the 2015 Plan 132,394 Total 20,811,843 Early Exercised Stock Options The terms of the 2015 Plan permit option holders to exercise stock options before they are vested, subject to certain limitations. The shares related to early exercised stock options are subject to our lapsing repurchase right upon termination of employment at the original purchase price. In order to vest, the holders are required to provide continued service to the Company. The proceeds are initially recorded in other current liabilities and are reclassified to common stock and paid-in capital as the repurchase right lapses. As of December 31, 2018 and 2017, there was $188,000 and $78,000, respectively, recorded in other current liabilities relating to shares subject to repurchase. For accounting purposes, unvested early exercised shares are not considered issued and outstanding until the awards vest. As a result of early exercises under the 2015 Plan, 149,565 and 132,180 shares had not vested and were subject to repurchase as of December 31, 2018 and 2017, respectively. Note Receivable from Stockholder In August 2016, the Company received a recourse promissory note from our then CEO and President, in connection with this individual’s purchase of 152,516 shares of our common stock at a price of $0.59 per share. The principal amount of the note was approximately $90,000, and accrued simple interest at a rate of 1.22% per year. The note, along with accrued interest, can be prepaid without penalty and is due on the earlier of (i) August 29, 2022, (ii) the pricing of an IPO or the closing of an acquisition of the Company, in either case if the note’s existence would violate any applicable law, (iii) the date the Company determines that any change in the Company’s status or the individual’s status would cause the note to be deemed a prohibited extension of credit under Section 402 of the Sarbanes-Oxley Act of 2002, as amended, or any applicable law or (iv) on demand by the Company in certain circumstances. In 2016, upon the individual ceasing to be employed by the Company, the Company repurchased 105,067 shares of common stock at a price per share of $0.59 per share for a total cash payment of $62,000. As of December 31, 2016 and 2017, the outstanding loan balance was $28,000, which is recorded as a component of total stockholders’ deficit in the accompanying balance sheets. As of December 31, 2018 the outstanding balance of $28,000 was cancelled and the cancellation was recorded as stock-based compensation on the Company’s statement of operations and comprehensive loss.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	1 0 . Since inception, the Company has only generated pretax losses. For the years ended December 31, 2018 and 2017, the Company recorded no provision for income taxes due to the losses incurred. December 31, 2018 2017 (in thousands) Deferred tax assets: Net operating loss carry forwards $ 10,351 $ 6,494 Stock-based compensation 91 45 Deferred revenue $ 3,370 — Tax credits — 984 Other 469 165 Total deferred tax assets 14,281 7,688 Less: valuation allowance (14,173 ) (7,675 ) Net deferred tax assets 108 13 Deferred tax liability (108 ) (13 ) Net deferred tax assets $ — $ — The Company’s accounting for deferred taxes involves the evaluation of a number of factors concerning the realizability of its net deferred tax assets. The Company primarily considered such factors as its history of operating losses, the nature of the Company’s deferred tax assets, and the timing, likelihood and amount, if any, of future taxable income during the periods in which those temporary differences and carryforwards become deductible. At present, the Company does not believe that it is more likely than not that the deferred tax assets will be realized; accordingly, a full valuation allowance has been established and no deferred tax asset is shown in the accompanying balance sheets. The valuation allowance increased by approximately $6.5 million and 4.0 million during the years ended December 31, 2018 and 2017. The Company has net operating carryforwards for federal and California income tax purposes of approximately $74.0 million and $46.5 million as of December 31, 2018 and 2017. The federal net operating loss carryforwards of $23.1 million, if not utilized, will expire beginning in 2035 and $13.9 million is carryforward indefinitely with the yearly net operating loss utilization limited to 80 percent of taxable income. The state net operating loss carryforwards, if not utilized, will expire beginning in 2035. Federal and California tax laws impose significant restrictions on the utilization of net operating loss carryforwards in the event of a change in ownership of the Company, as defined by Internal Revenue Code Section 382 (“Section 382”). The Company believes a change in ownership, as defined by Section 382, has occurred but a formal study has not been completed. In addition, in the future the Company may experience ownership changes, which may limit the utilization of net operating loss carryforwards or other tax attributes. The Company has not reflected any benefit of such net operating loss carryforwards in the accompanying financial statements. The Company has established a full valuation allowance against its deferred tax assets due to the uncertainty surrounding the realization of such assets. December 31, 2018 2017 Computed expected tax benefit (at federal statutory income tax rate of 21%) $ (6,631 ) $ (7,088 ) Increase in valuation allowance 6,498 4,018 Other 133 329 Federal rate change (pursuant to the Tax Act) — 2,741 Total provision for income taxes $ — $ — The Tax Cuts and Jobs Act (the “Tax Act”) enacted in December 2017, among other changes, lowers our federal tax rate from 34% to 21%. Based on provisions of the Tax Act, the Company remeasured its deferred tax assets and liabilities to reflect the lower statutory tax rate. However, since the Company established a valuation allowance to offset its deferred tax assets, there was no impact to the effective tax rate, as any changes to deferred taxes would be offset by the valuation allowance. The deferred tax remeasurement was provisional and was subject to revision as the Company completes its analysis of the Tax Act, collects and prepares necessary data and interprets any additional guidance issued by standard-setting bodies. The Company has completed its analysis and determined no adjustment is required related to the tax effects of the Tax Act in 2018. Uncertain Tax Benefits The Company recognizes uncertain tax positions when it is more likely than not, based on the technical merits, that the position will not be sustained upon examination. No liability related to uncertain tax positions is recorded on the financial statements related to uncertain tax positions. A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows: December 31, 2018 2017 2016 (in thousands) Balance at beginning of year $ 422 $ 197 $ 46 Increases related to current year tax positions 3,016 225 151 Balance at end of year $ 3,438 $ 422 $ 197 The Company does not anticipate any significant changes to unrecognized tax benefits over the next 12 months. Income tax returns are filed in the U.S. and California. The years 2015 through 2018 remain open to examination by the domestic taxing jurisdictions to which the Company is subject. Net operating losses generated on a tax return basis by the Company for 2015 through 2018 remain open to examination by the domestic taxing jurisdictions.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Loss Per Share	1 1 . The following outstanding potentially dilutive common stock equivalents have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: As of December 31, 2018 2017 2016 Convertible preferred stock (as converted) 16,618,448 6,989,973 3,050,329 Common stock options issued and outstanding 3,323,988 1,791,299 286,739 Restricted Stock subject to future vesting 22,178 113,157 316,836 Early exercised stock options subject to future vesting 149,565 132,180 244,825 Warrants to purchase shares of common stock 565,270 565,270 470,753 Total 20,679,449 9,591,879 4,369,482 Neither the Company’s convertible preferred stock nor restricted stock subject to future vesting participates in losses. The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company (in thousands, except per share data): As of December 31, 2018 2017 2016 Net loss (27,366 ) (16,830 ) (11,406 ) Weighted-average shares used to compute basic and diluted net loss per share 1,066,877 894,901 527,931 Basic and diluted net loss per common share (25.65 ) (18.81 ) (21.61 )

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Event	12. Subsequent Event Initial Public Offering On February 7, 2019, the Company’s registration statement on Form S-1 (File No. 333-229040) relating to the IPO was declared effective by the Securities and Exchange Commission (the “SEC”). The shares began trading on The NASDAQ Global Select Market on February 8, 2019. The public offering price of the shares sold in the IPO was $14.00 per share. The IPO closed on February 12, 2019 and included 5,400,000 shares of common stock. Following the close of the offering, the underwriters exercised in part their option to purchase additional shares for a total of 369,201 shares at the IPO price. In aggregate, the shares issued in the offering generated approximately $70.7 million in net proceeds, which amount is net of $5.6 million in underwriters’ discount and offering costs of $4.5 million, Upon the closing of the IPO, all shares of preferred stock then outstanding were automatically converted into 16,618,448 shares of common stock and all warrants then outstanding were automatically net exercised for 563,043 shares of common stock. The table below shows, on a pro forma basis, the impact of the Company’s IPO on certain condensed balance sheet items as if all of the transactions occurred on December 31, 2018: Pro forma As of As of December 31, 2018 December 31, 2018 Cash and cash equivalents $ 89,493 $ 160,149 Deferred offering costs 2,969 — Convertible preferred stock 129,577 — Common stock 1 3 Additional paid-in capital 9,111 209,341 Total stockholders' deficit (equity) $ (53,479 ) $ 146,754 In January 2019, the board of directors adopted and the Company’s stockholders approved the Company’s 2019 Equity Incentive Plan, or the 2019 Plan, which took effect immediately prior to the execution of the underwriting agreement for the Company’s IPO in February 2019. The Plan is intended as the successor to and continuation of the 2015 Equity Incentive Plan. See footnote 9 for further discussion on the 2019 Plan. In January 2019, the board of directors adopted and the Company’s stockholders approved the 2019 ESPP Plan. The 2019 ESPP became effective in February 2019. The Company has initially reserved 250,000 shares of common stock for issuance under the 2019 ESPP. See footnote 9 for further discussion of the 2019 ESPP. Commitments and Contingencies From time to time, the Company may become involved in legal proceedings arising in the ordinary course of its business. On January 3, 2019, Maverick filed a complaint against the Company in the Delaware Court of Chancery and a motion for a temporary restraining order seeking to prohibit us from further developing our ProTriTAC platform. The complaint alleges various claims, including for breach of contract and misappropriation of trade secrets and seeks as relief, among other things, a declaration that the Company’s ProTriTAC technology impermissibly competes in the Maverick Field (as defined in the Asset Transfer Agreement), a preliminary and permanent injunction and unspecified damages. On January 18, 2019, the Court denied Maverick’s motion for a temporary restraining order. The Company disputes Maverick’s allegations and intends to vigorously defend the claims asserted against it. In view of the uncertainty regarding the possible outcome of this case and the nature of the relief sought, the Company does not believe that it is currently possible to determine a reasonable estimate of the possible loss or range of loss, or other possible adverse result, if any, that may be incurred with respect to this matter. The Company is not currently a party to any other material legal proceedings.

Quarterly Results (Unaudited)

Quarterly Results (Unaudited)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Quarterly Results (Unaudited)	13. Quarterly Results (Unaudited) The following table is in thousands, except per share amounts: Quarters Ended March 31, June 30, September 30, December 31, 2018 2018 2018 2018 Statement of operations data: Revenue Collaboration and license revenue $ 1,561 $ 1,063 $ 1,063 $ 1,063 Total revenue 1,561 1,063 1,063 1,063 Operating expenses Research and development 5,533 6,151 5,967 8,717 General and administrative 982 967 1,942 2,215 Total operating expenses 6,515 7,118 7,909 10,932 Loss from operations (4,954 ) (6,055 ) (6,846 ) (9,869 ) Interest income 73 66 108 148 Interest expense — — — — Other expense (2 ) (5 ) (22 ) (8 ) Net loss and comprehensive loss $ (4,883 ) $ (5,994 ) $ (6,760 ) $ (9,729 ) Net loss per share, basic and diluted (5.04 ) (5.89 ) (6.23 ) (8.15 ) Weighted-average common shares used in computing net loss per share, basic and diluted 969,235 1,017,336 1,084,477 1,193,797 Quarters Ended March 31, June 30, September 30, December 31, 2017 2017 2017 2017 Statement of operations data: Revenue Collaboration and license revenue $ — $ — $ — $ 708 Total revenue — — — 708 Operating expenses Research and development 2,154 2,801 3,876 4,791 General and administrative 843 847 1,012 912 Total operating expenses 2,997 3,648 4,888 5,703 Loss from operations (2,997 ) (3,648 ) (4,888 ) (4,995 ) Interest income 6 2 13 57 Interest expense (173 ) (112 ) — — Other expense (86 ) — (5 ) (4 ) Net loss and comprehensive loss $ (3,250 ) $ (3,758 ) $ (4,880 ) $ (4,942 ) Net loss per share, basic and diluted (3.86 ) (4.24 ) (5.35 ) (5.27 ) Weighted-average common shares used in computing net loss per share, basic and diluted 840,905 886,332 912,728 938,372

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The financial statements have been prepared in accordance with U.S. generally accepted accounting principles.
Reverse Stock Split	Reverse Stock Split On January 28, 2019, the Company filed an amendment to the Company’s amended and restated certificate of incorporation to effect a reverse split of shares of the Company’s common stock and convertible preferred stock on a 4.9175-for-one basis (the “Reverse Stock Split”). The par value and the authorized number of shares of the convertible preferred stock and common stock were not adjusted in connection with the Reverse Stock Split. All references to common stock, convertible preferred stock, warrants to purchase common stock, options to purchase common stock, early exercised options, restricted stock, share data, per share data and related information contained in the financial statements have been retrospectively adjusted to reflect the effect of the Reverse Stock Split for all periods presented.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of expenses during the reporting period. Significant estimates and assumptions made in the accompanying financial statements include, but are not limited to, the fair value of common stock, the fair value of stock options, the research period of the collaboration agreement with AbbVie, income tax uncertainties and certain accruals. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates.
Segment Reporting	Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the Company’s Chief Operating Decision Maker in making decisions regarding resource allocation and assessing performance. To date, the Company has viewed its operations and manages its business as one segment operating primarily in the United States.
Cash, Cash Equivalents and Restricted Cash	Cash, Cash Equivalents and Restricted Cash The Company considers all highly liquid investments purchased with original maturities of three months or less from the purchase date to be cash equivalents. Cash equivalents consist primarily of amounts invested in money market accounts and are stated at fair value. The Company maintained restricted cash of $0.5 million and zero as of December 31, 2018 and 2017, respectively. This amount as of December 31, 2018 is included within “Other assets” in the accompanying balance sheets and is comprised solely of a letter of credit required pursuant to the lease for the Company’s corporate headquarters entered into in August 2018 as discussed in Note 6. The following table provides a reconciliation of cash and cash equivalents and restricted cash reported within the balance sheets that sum to the total of the same amounts shown in the statement of cash flows. As of December 31, 2018 2017 2016 (in thousands) Balance Sheets Cash and cash equivalents $ 89,493 $ 29,423 $ 985 Restricted cash (included in other assets) 467 — — Cash, cash equivalents and restricted cash in Statements of Cash Flows $ 89,960 $ 29,423 $ 985
Concentration of Credit Risk	Concentration of Credit Risk Cash and cash equivalents consist of financial instruments that potentially subject the Company to a concentration of credit risk, to the extent of the amounts recorded on the balance sheets. The Company minimizes the amount of credit exposure by investing cash that is not required for immediate operating needs in money market funds.
Fair Value Measurement	Fair Value Measurement Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability, or an exit price, in the principal or most advantageous market for that asset or liability in an orderly transaction between market participants on the measurement date, and established a fair value hierarchy that requires an entity to maximize the use of observable inputs, where available, and minimize the use of unobservable inputs when measuring fair value. The Company measures fair value based on a three-level hierarchy of inputs, of which the first two are considered observable and the last unobservable. Unobservable inputs reflect the Company’s own assumptions about current market conditions. The three-level hierarchy of inputs is as follows: Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2—Inputs (other than quoted prices included in Level 1) are either directly or indirectly observable for the asset or liability. These include quoted prices for similar assets or liabilities in active markets and quoted prices for identical or similar assets or liabilities in markets that are not active; and Level 3— Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The carrying amounts reflected in the accompanying balance sheets for cash and cash equivalents, prepaid expenses, other current assets, accounts payable, accrued expenses and other current liabilities approximate their fair values, due to their short-term nature.
Property and Equipment, Net	Property and Equipment, Net Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation is computed on a straight-line basis over the estimated useful lives of the related assets, generally three to five years. Leasehold improvements are amortized using the straight-line method over the shorter of the assets’ estimated useful lives or the remaining term of the lease. Depreciation and amortization begin at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss is reflected in operations.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company evaluates its long-lived assets, including property and equipment, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets or group of assets may not be fully recoverable. If indicators of impairment exist and the undiscounted future cash flows that the assets are expected to generate are less than the carrying amount of the assets, the Company reduces the carrying amount of the assets through an impairment charge to their estimated fair values based on a discounted cash flow approach or, when available and appropriate, to comparable market values. There were no impairments of long-lived assets for any of the periods presented.
Revenue Recognition	Revenue Recognition Effective January 1, 2017, the Company early adopted Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers The Company enters into corporate collaborations under which it may obtain upfront license fees, research and development funding, and development, regulatory and commercial milestone payments and royalty payments. The Company’s performance obligations under these arrangements may include licenses of intellectual property, distribution rights, research and development services, delivery of manufactured product and/or participation on joint steering committees. Licenses of intellectual property: If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenue from upfront license fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. For licenses that are bundled with other promises, the Company utilizes judgement 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 proportional performance for purposes of recognizing revenue from non-refundable, up-front fees. The Company evaluates the measure of proportional performance each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Milestone payments: At the inception of each arrangement that includes development, regulatory or commercial milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price. Topic 606 suggests two alternatives to use when estimating the amount of variable consideration: the expected value method and the most likely amount method. Under the expected value method, an entity considers the sum of probability-weighted amounts in a range of possible consideration amounts. Under the most likely amount method, an entity considers the single most likely amount in a range of possible consideration amounts. Whichever method is used, it should be consistently applied throughout the life of the contract; however, it is not necessary for the Company to use the same approach for all contracts. The Company expects to use the most likely amount method for development and regulatory milestone payments. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s or the licensee’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The transaction price is then allocated to each performance obligation on a relative stand-alone selling price basis. The Company recognizes revenue as or when the performance obligations under the contract are satisfied. At the end of each subsequent reporting period, the Company re-evaluates the probability or achievement of each such milestone and any related constraint, and if necessary, adjusts its estimates of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. Commercial milestones and royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and in which the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue when the related sales occur. To date, the Company has not recognized any royalty revenue resulting from its collaboration arrangements. Upfront payments and fees are recorded as deferred revenue upon receipt or when due, and may require deferral of revenue recognition to a future period until the Company performs its obligations under these arrangements. Amounts payable to the Company are recorded as accounts receivable when the Company’s right to consideration is unconditional.
Research and Development Expenses and Accrued Research and Development Costs	Research and Development Expenses and Accrued Research and Development Costs The Company expenses research and development costs as incurred. Research and development expenses consist of personnel costs for the Company’s research and product development employees. Also included are non-personnel costs such as professional fees payable to third parties for preclinical studies, clinical trials, research services, laboratory supplies and equipment maintenance and depreciation, intellectual property licenses and other consulting costs. The Company estimates preclinical and clinical study and research expenses based on the services performed, pursuant to contracts with research institutions that conduct and manage preclinical studies, clinical trials and research services on its behalf. The Company estimates these expenses based on discussions with internal management personnel and external service providers as to the progress or stage of completion of services and the contracted fees to be paid for such services. The Company records the estimated costs of research and development activities based upon the estimated amount services provided but not yet invoiced, and includes these costs in development expenses. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service provides under the service agreements. The Company makes significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred. However, the status and timing of actual services performed may vary from the Company’s estimates, resulting in adjustments to expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations. Payments associated with licensing agreements to acquire exclusive license to develop, use, manufacture and commercialize products that have not reached technological feasibility and do not have alternate future use are expensed as incurred. Payments made to third parties under these arrangements in advance of the performance of the related services by the third parties are recorded as prepaid expenses until the services are rendered. Such payments are evaluated for current or long-term classification based on when such services are expected to be received.
Stock-Based Compensation	Stock-Based Compensation The Company maintains a stock-based compensation plan as a long-term incentive for employees, consultants and members of the Company’s board of directors (the “Board”). The plan allows for the issuance of non-statutory options (“NSOs”) and incentive stock options to employees and NSOs to nonemployees. Share-based payments are measured using fair-value-based measurements and recognized as compensation expense over the service period in which the awards are expected to vest. The Company’s fair-value-based measurements of awards to employees and directors as of the grant date utilize the single-option award-valuation approach, and the Company uses the straight-line method for expense attribution. The fair-value-based measurements of options granted to nonemployees are remeasured at each period end until the options vest and are amortized to expense as earned. The valuation model used for calculating the estimated fair value of stock awards is the Black-Scholes option-pricing model. The Black-Scholes model requires the Company to make assumptions and judgments about the variables used in the calculations, including the expected term (weighted-average period of time that the options granted are expected to be outstanding), the expected volatility of the Company’s common stock, the related risk-free interest rate and the expected dividend. The Company has elected to recognize forfeitures of share-based payment awards as they occur. Effective January 1, 2018, we early adopted ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting
Convertible Preferred Stock	Convertible Preferred Stock The Company records all shares of convertible preferred stock at their respective fair values less issuance costs on the dates of issuance. The convertible preferred stock is recorded outside of stockholders’ deficit because, in the event of certain deemed liquidation events considered not solely within the Company’s control, such as a merger, acquisition and sale of all or substantially all of the Company’s assets, the convertible preferred stock will become redeemable at the option of the holders. In the event of a change of control of the Company, proceeds received from the sale of such shares will be distributed in accordance with the liquidation preferences set forth in the Company’s Amended and Restated Certificate of Incorporation unless the holders of convertible preferred stock have converted their shares of convertible preferred stock into shares of common stock. The Company has determined not to adjust the carrying values of the convertible preferred stock to the liquidation preferences of such shares because of the uncertainty of whether or when such an event would occur.
Income Taxes	Income Taxes Income taxes are accounted for under the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates applied to taxable income in the years in which those temporary differences are expected to be realized. The effect on deferred tax assets and liabilities of a change in tax rates is recognized as income in the period that includes the enactment date. A valuation allowance is established when necessary to reduce deferred tax assets to the amount expected to be realized. Financial statement effects of uncertain tax positions are recognized when it is more-likely-than-not, based on the technical merits of the position, that it will be sustained upon examination. Interest and penalties related to unrecognized tax benefits are included as a component of other expense. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits. The Company accounts for uncertain tax positions in accordance with ASC 740-10, Accounting for Uncertainty in Income Taxes The Company includes any penalties and interest expense related to income taxes as a component of provision for income tax as necessary. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration for potentially dilutive securities. As discussed in Note 11, the unvested portion of early exercised stock options are excluded from the computation of weighted average shares as the continuing vesting of such shares is contingent on the holders’ continued service to the Company. Diluted net loss per share is the same as basic net loss per share for each period presented, since the effects of potentially dilutive securities are antidilutive given the net loss of the Company.
Comprehensive Income (Loss)	Comprehensive Income (Loss) The Company has no components of comprehensive income (loss) other than net loss. Thus, comprehensive loss is the same as net loss for all periods presented.
Deferred Offering Costs	Deferred Offering Costs The Company has deferred offering costs consisting of legal, accounting and other fees directly attributable to the Company’s IPO. The deferred offering costs were offset against the proceeds received upon completion of the IPO. As of December 31, 2018 and December 31, 2017, $3.0 million and zero of deferred offering costs were included in other assets on the condensed balance sheet, respectively.
Emerging Growth Company Status	Emerging Growth Company Status The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (the “JOBS Act”). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that we (i) are no longer an emerging growth company or (ii) affirmatively and irrevocably opt out of the extended transition period provided in the JOBS Act. As a result, these financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates. As described in “Recently Adopted Accounting Pronouncements” below, the Company early adopted Revenue from Contracts with Customers Stock Compensation—Improvements to Employee Share-Based Payment Accounting Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, as the JOBS Act does not preclude an emerging growth company from adopting a new or revised accounting standard earlier than the time that such standard applies to private companies. (Topic 842), Leases
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements On January 1, 2018, the Company early adopted ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting On January 1, 2018, the Company adopted ASU 2017-09, Stock Compensation – Scope of Modification Accounting The Company elected to adopt this guidance prospectively to awards modified on or after the adoption date. The adoption
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2016-02 (Topic 842), Leases In February 2018, the FASB issued ASU No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income 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

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Schedule of Reconciliation of Cash and Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash and cash equivalents and restricted cash reported within the balance sheets that sum to the total of the same amounts shown in the statement of cash flows. As of December 31, 2018 2017 2016 (in thousands) Balance Sheets Cash and cash equivalents $ 89,493 $ 29,423 $ 985 Restricted cash (included in other assets) 467 — — Cash, cash equivalents and restricted cash in Statements of Cash Flows $ 89,960 $ 29,423 $ 985

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Summary of Financial Assets Measured at Fair Value and Fair Value Hierarchy of Valuation	The following table presents information about the Company’s financial assets that are measured at fair value and indicates the fair value hierarchy of the valuation: Fair Value Measurements at December 31, 2018 Total Level 1 Level 2 Level 3 (in thousands) Assets Money market funds $ 60,396 $ 60,396 $ — $ — Total $ 60,396 $ 60,396 $ — $ — Fair Value Measurements at December 31, 2017 Total Level 1 Level 2 Level 3 (in thousands) Assets Money market funds $ 28,575 $ 28,575 $ — $ — Total $ 28,575 $ 28,575 $ — $ —

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
Property and Equipment, Net	Property and equipment, net consists of the following: December 31, 2018 2017 (in thousands) Laboratory equipment $ 2,402 $ 1,752 Furniture and fixtures 312 268 Computer equipment and software 32 32 Leasehold improvements 360 327 Construction in progress 906 78 4,012 2,457 Less: Accumulated depreciation and amortization (1,014 ) (370 ) Total property and equipment, net $ 2,998 $ 2,087
Accrued Liabilities	Accrued liabilities consist of the following: December 31, 2018 2017 Accrued research and development $ 504 $ 522 Accrued personnel costs 1,593 871 Accrued professional and consulting fees 333 — Accrued offering costs 709 — Other 202 99 Total accrued liabilities $ 3,341 $ 1,492

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Payments Under Non-Cancelable Operating Lease	As of December 31, 2018, future minimum payments under the Company’s non-cancelable operating lease are as follows (in thousands): Year Ended December 31, 2018 2019 $ 1,308 2020 2,762 2021 2,566 2022 2,647 2023 2,731 Thereafter 10,251 $ 22,265

Convertible Preferred Stock (Ta

Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Schedule of Convertible Preferred Stock	Convertible preferred stock consists of the following: As of December 31, 2018 Shares Issued Aggregate Shares and Carrying Liquidation Authorized Outstanding Value Preference (In thousands, except share data) Series A 15,000,000 3,050,329 $ 14,926 $ 15,008 Series B 35,000,000 7,068,184 44,906 45,166 Series C 32,000,000 6,499,935 69,745 70,004 82,000,000 16,618,448 $ 129,577 $ 130,178 As of December 31, 2017 Shares Issued Aggregate Shares and Carrying Liquidation Authorized Outstanding Value Preference (In thousands, except share data) Series A 15,000,000 3,050,329 $ 14,926 $ 15,008 Series B 35,000,000 3,939,644 24,915 25,174 50,000,000 6,989,973 $ 39,841 $ 40,182

Equity (Tables)

Equity (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Summary of Option Activity Under Equity Incentive Plan	The following summarizes option activity under the 2015 Plan: Weighted Weighted Average Number of Average Remaining Aggregate Outstanding Exercise Contractual Intrinsic Options Price Life Value (in years) (in thousands) Balance as of December 31, 2016 286,739 $ 0.59 9.35 $ 70 Options granted 1,686,997 1.16 Options exercised (97,373 ) 0.59 Options cancelled (85,064 ) 0.59 Balance as of December 31, 2017 1,791,299 1.13 9.24 $ 881 Options granted 1,762,147 2.06 Options exercised (198,943 ) 1.35 Options cancelled (30,515 ) 1.29 Balance as of December 31, 2018 3,323,988 1.61 9.07 $ 1,675 Vested and expected to vest as of December 31, 2018 3,323,988 1.61 9.07 $ 1,675 Exercisable as of December 31, 2018 548,332 0.93 8.11 $ 651
Summary of Fair Value of Employee and Director Stock Option Awards using a Black-Scholes Option-Pricing Model	The fair value of employee and director stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumptions: Year Ended December 31, 2018 2017 2016 Expected term (years) 6.06 6.33 6.03 Expected volatility 76.08% 73.39% 91.35% Risk-free interest rate 2.89% 2.03% 1.37% Expected dividend 0% 0% 0%
Summary of Stock-based Compensation	Total stock-based compensation was as follows: Year Ended December 31, 2018 2017 2016 (in thousands) Operating Expenses Research and development $ 325 $ 145 $ 43 General and administrative 347 222 56 Total stock-based compensation $ 672 $ 367 $ 99
Summary of Restricted Stock Activity	A summary of restricted stock activity is shown in the below table: Number of of Restricted Stock Outstanding Restricted shares- December 31, 2016 316,836 Restricted stock awards vested (85,001 ) Unvested shares repurchased (118,678 ) Restricted shares- December 31, 2017 113,157 Restricted stock awards vested (81,623 ) Unvested shares repurchased (9,356 ) Restricted shares- December 31, 2018 22,178
Schedule of Common Stock Reserved for Future Issuance on As-converted Basis	As of December 31, 2018, the Company had reserved common stock, on an as-converted basis, for issuance as follows: Convertible preferred stock (as converted) 16,618,448 Common stock options issued and outstanding 3,323,988 Restricted Stock subject to future vesting 22,178 Early exercised stock options subject to future vesting 149,565 Warrants to purchase shares of common stock 565,270 Shares available for future grant under the 2015 Plan 132,394 Total 20,811,843

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Components of Deferred Tax Assets and Liabilities	Significant components of the Company’s deferred tax assets and liabilities as of December 31, 2018 and 2017 consisted of the following: December 31, 2018 2017 (in thousands) Deferred tax assets: Net operating loss carry forwards $ 10,351 $ 6,494 Stock-based compensation 91 45 Deferred revenue $ 3,370 — Tax credits — 984 Other 469 165 Total deferred tax assets 14,281 7,688 Less: valuation allowance (14,173 ) (7,675 ) Net deferred tax assets 108 13 Deferred tax liability (108 ) (13 ) Net deferred tax assets $ — $ —
Income Taxes Rate Reconciliation	December 31, 2018 2017 Computed expected tax benefit (at federal statutory income tax rate of 21%) $ (6,631 ) $ (7,088 ) Increase in valuation allowance 6,498 4,018 Other 133 329 Federal rate change (pursuant to the Tax Act) — 2,741 Total provision for income taxes $ — $ —
Schedule of Reconciliation of Beginning and Ending Amounts of Unrecognized Tax Benefits	A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows: December 31, 2018 2017 2016 (in thousands) Balance at beginning of year $ 422 $ 197 $ 46 Increases related to current year tax positions 3,016 225 151 Balance at end of year $ 3,438 $ 422 $ 197

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Summary of Outstanding Potentially Dilutive Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share	The following outstanding potentially dilutive common stock equivalents have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: As of December 31, 2018 2017 2016 Convertible preferred stock (as converted) 16,618,448 6,989,973 3,050,329 Common stock options issued and outstanding 3,323,988 1,791,299 286,739 Restricted Stock subject to future vesting 22,178 113,157 316,836 Early exercised stock options subject to future vesting 149,565 132,180 244,825 Warrants to purchase shares of common stock 565,270 565,270 470,753 Total 20,679,449 9,591,879 4,369,482
Summary of Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders	The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company (in thousands, except per share data): As of December 31, 2018 2017 2016 Net loss (27,366 ) (16,830 ) (11,406 ) Weighted-average shares used to compute basic and diluted net loss per share 1,066,877 894,901 527,931 Basic and diluted net loss per common share (25.65 ) (18.81 ) (21.61 )

Subsequent Event (Tables)

Subsequent Event (Tables)	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Schedule of Impact of the Company's IPO on Condensed Balance Sheet Items	The table below shows, on a pro forma basis, the impact of the Company’s IPO on certain condensed balance sheet items as if all of the transactions occurred on December 31, 2018: Pro forma As of As of December 31, 2018 December 31, 2018 Cash and cash equivalents $ 89,493 $ 160,149 Deferred offering costs 2,969 — Convertible preferred stock 129,577 — Common stock 1 3 Additional paid-in capital 9,111 209,341 Total stockholders' deficit (equity) $ (53,479 ) $ 146,754

Quarterly Results (Unaudited) (

Quarterly Results (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Results	The following table is in thousands, except per share amounts: Quarters Ended March 31, June 30, September 30, December 31, 2018 2018 2018 2018 Statement of operations data: Revenue Collaboration and license revenue $ 1,561 $ 1,063 $ 1,063 $ 1,063 Total revenue 1,561 1,063 1,063 1,063 Operating expenses Research and development 5,533 6,151 5,967 8,717 General and administrative 982 967 1,942 2,215 Total operating expenses 6,515 7,118 7,909 10,932 Loss from operations (4,954 ) (6,055 ) (6,846 ) (9,869 ) Interest income 73 66 108 148 Interest expense — — — — Other expense (2 ) (5 ) (22 ) (8 ) Net loss and comprehensive loss $ (4,883 ) $ (5,994 ) $ (6,760 ) $ (9,729 ) Net loss per share, basic and diluted (5.04 ) (5.89 ) (6.23 ) (8.15 ) Weighted-average common shares used in computing net loss per share, basic and diluted 969,235 1,017,336 1,084,477 1,193,797 Quarters Ended March 31, June 30, September 30, December 31, 2017 2017 2017 2017 Statement of operations data: Revenue Collaboration and license revenue $ — $ — $ — $ 708 Total revenue — — — 708 Operating expenses Research and development 2,154 2,801 3,876 4,791 General and administrative 843 847 1,012 912 Total operating expenses 2,997 3,648 4,888 5,703 Loss from operations (2,997 ) (3,648 ) (4,888 ) (4,995 ) Interest income 6 2 13 57 Interest expense (173 ) (112 ) — — Other expense (86 ) — (5 ) (4 ) Net loss and comprehensive loss $ (3,250 ) $ (3,758 ) $ (4,880 ) $ (4,942 ) Net loss per share, basic and diluted (3.86 ) (4.24 ) (5.35 ) (5.27 ) Weighted-average common shares used in computing net loss per share, basic and diluted 840,905 886,332 912,728 938,372

Organization - Additional Infor

Organization - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Feb. 07, 2019	Dec. 31, 2018	Mar. 14, 2019	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Month and year of incorporation		2015-03
Common stock shares sold		1,383,221		1,193,629
Convertible shares of common stock		16,618,448
Accumulated deficit		$ 62,591		$ 35,225
Unrestricted cash and cash equivalents		89,493		$ 29,423	$ 985
Redeemable Convertible Preferred Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Convertible shares of common stock	17,181,491
Reclassified from temporary equity to additional paid in capital	$ 129,600
Redeemable Convertible Preferred Stock | Scenario Forecast
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Convertible preferred stock, outstanding			0
IPO
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Unrestricted cash and cash equivalents		$ 89,493
IPO | Subsequent Event
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Public offering price per share	$ 14
Common stock shares sold	5,769,201
Gross proceeds from issuance of common stock	$ 80,800
Net proceeds after deducting underwriting discounts, commissions and offering costs	$ 70,700

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details)	Jan. 28, 2019	Dec. 31, 2018USD ($)Segment	Dec. 31, 2017USD ($)
Summary Of Significant Accounting Policy [Line Items]
Highly liquid investments purchased with maximum original maturities		3 months
Restricted cash		$ 467,000	$ 0
Impairment of long-lived assets		0
Interest or penalties charged related to unrecognized tax benefits		0
Deferred offering costs		3,000,000	$ 0
ASC 740-10
Summary Of Significant Accounting Policy [Line Items]
Interest or penalties charged related to unrecognized tax benefits		$ 0
Minimum
Summary Of Significant Accounting Policy [Line Items]
Property and equipment, estimated useful life		3 years
Maximum
Summary Of Significant Accounting Policy [Line Items]
Property and equipment, estimated useful life		5 years
United States
Summary Of Significant Accounting Policy [Line Items]
Number of operating segments | Segment		1
Common Stock | Convertible Preferred Stock
Summary Of Significant Accounting Policy [Line Items]
Reverse split of shares, description		4.9175-for-one basis
Common Stock | Convertible Preferred Stock | Subsequent Event
Summary Of Significant Accounting Policy [Line Items]
Reverse split of shares	4.9175

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash and Cash Equivalents and Restricted Cash (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash and cash equivalents	$ 89,493,000	$ 29,423,000	$ 985,000
Restricted cash (included in other assets)	467,000	0
Cash, cash equivalents and restricted cash in Statements of Cash Flows	$ 89,960,000	$ 29,423,000	$ 985,000	$ 2,347,000

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value and Fair Value Hierarchy of Valuation (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets
Assets fair value	$ 60,396	$ 28,575
Money Market Funds
Assets
Assets fair value	60,396	28,575
Level 1
Assets
Assets fair value	60,396	28,575
Level 1 | Money Market Funds
Assets
Assets fair value	60,396	28,575
Level 2
Assets
Assets fair value	0	0
Level 3
Assets
Assets fair value	$ 0	$ 0

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value, assets	$ 60,396,000	$ 28,575,000
Fair value, assets, level 1 to level 2 transfers, amount	0	0
Assets, unrealized gains or losses	0	0
Assets, Realized gains or losses	0	0
Financial liabilities fair value measurements	0	0
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value, assets	0	0
Fair value, liabilities	0	0
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value, assets	0	0
Fair value, liabilities	$ 0	$ 0

Balance Sheet Components - Prop

Balance Sheet Components - Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment	$ 4,012	$ 2,457
Less: Accumulated depreciation and amortization	(1,014)	(370)
Total property and equipment, net	2,998	2,087
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment	2,402	1,752
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment	312	268
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Property and equipment	32	32
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment	360	327
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment	$ 906	$ 78

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Balance Sheet Related Disclosures [Abstract]
Depreciation and amortization expense	$ 0.7	$ 0.4

Balance Sheet Components - Accr

Balance Sheet Components - Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Balance Sheet Related Disclosures [Abstract]
Accrued research and development	$ 504	$ 522
Accrued personnel costs	1,593	871
Accrued professional and consulting fees	333
Accrued offering costs	709
Other	202	99
Total accrued liabilities	$ 3,341	$ 1,492

Convertible Notes - Additional

Convertible Notes - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	May 31, 2017	Nov. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 12, 2019
Convertible Notes [Line Items]
Gross proceeds from issuance of stock		$ 25,000,000
Fair value of warrants issued					$ 4,000
Total amortization expense relating to the discount created by the warrants and debt issuance costs			$ 0	$ 100,000
Unamortized debt discount			0	0
Convertible promissory notes outstanding			$ 0	0
IPO | Subsequent Event
Convertible Notes [Line Items]
Warrants exercise price						$ 14
Number of outstanding warrants for purchase of common stock						565,270
2016 Notes
Convertible Notes [Line Items]
Number of warrants issued for purchase of common stock			152,514
Fair value of warrants issued			$ 200,000
2017 Notes
Convertible Notes [Line Items]
Number of warrants issued for purchase of common stock			101,675
Fair value of warrants issued			$ 100,000
2016 Notes and 2017 Notes
Convertible Notes [Line Items]
Adjustment in additional paid-in-capital extinguishment				$ (204,000)
Number of warrants issued for purchase of common stock			565,270	565,270
Warrants exercise price			$ 0.05	$ 0.05
Anniversary period of issuance date of warrants			10 years
Warrants exercisable			565,270	565,270
Convertible Promissory Notes
Convertible Notes [Line Items]
Accrued interest		$ 5,000,000
Interest rate		8.00%
Convertible Promissory Notes | 2016 Notes
Convertible Notes [Line Items]
Face amount of debt		$ 2,500,000
Convertible Promissory Notes | 2017 Notes
Convertible Notes [Line Items]
Additional debt instrument face amount		$ 2,500,000
Convertible Preferred Stock | 2016 Notes and 2017 Notes | Minimum
Convertible Notes [Line Items]
Proceeds from issuance of warrants			$ 5,000,000
Convertible Preferred Stock | 2016 Notes and 2017 Notes | Series B
Convertible Notes [Line Items]
Accrued interest converted into shares	811,103
Adjustment in additional paid-in-capital extinguishment	$ 200,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Dec. 31, 2018	Aug. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 28, 2017
Commitments And Contingencies [Line Items]
Tenant improvement allowance receivable	$ 5,784		$ 5,784
Rent expense			1,200	$ 800	$ 800
Office and Laboratory Space
Commitments And Contingencies [Line Items]
Lease term		8 years
Lease term option to extend period		8 years
Lease commencement date		Jul. 1, 2019
Tenant improvement allowance entitled		$ 5,200
Additional tenant improvement allowance		1,400
Additional tenant improvement allowance exercised	1,400
Tenant improvement allowance receivable		6,600
Tenant improvement allowance utilized under lease recorded as construction in progress	$ 900		$ 900
Letter of credit for the benefit of the landlord		500
Initial annual base rent		$ 2,200
Percentage of increase in base rent		3.50%
Tizona Therapeutics, Inc.
Commitments And Contingencies [Line Items]
Lease term						36 months

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Payments Under the Company's Non-Cancelable Operating Lease (Details) $ in Thousands	Dec. 31, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 1,308
2020	2,762
2021	2,566
2022	2,647
2023	2,731
Thereafter	10,251
Future minimum payment under non-cancelable operating lease	$ 22,265

Collaboration & License Agree_2

Collaboration & License Agreements - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended					12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue		$ 1,063,000	$ 1,063,000	$ 1,063,000	$ 1,561,000	$ 708,000	$ 4,750,000	$ 708,000
Deferred revenue, noncurrent		7,792,000				12,042,000	7,792,000	12,042,000
Deferred revenue, current		4,250,000				4,250,000	$ 4,250,000	4,250,000
Collaborative Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration termination notice period							90 days
Research and Development Activities
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement upfront payment amount.		17,000,000					$ 17,000,000
Estimated period to recognize upfront payment							4 years
AbbVie Inc. | Development, Regulatory, and Commercial Sale Milestones For Licensed Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement upfront payment amount.		17,000,000					$ 17,000,000
AbbVie Inc. | Development, Regulatory, and Commercial Sale Milestones For Licensed Products | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement upfront payment amount.		600,000,000					$ 600,000,000
AbbVie Inc. | Collaborative Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration termination notice period							30 days
Transaction price							$ 17,000,000
Total revenue							4,300,000	700,000
Deferred revenue		12,000,000				16,300,000	12,000,000	16,300,000
Deferred revenue, noncurrent		7,800,000				12,100,000	7,800,000	12,100,000
Deferred revenue, current		$ 4,200,000				$ 4,200,000	4,200,000	$ 4,200,000
Milestone payments under collaboration agreement							0
Recognize royalty revenue							0
Werewolf Therapeutics, Inc. | Licensing Agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue							500,000
Recognize royalty revenue							0
Ownership interest in the company’s capital stock	5.00%
Upfront fee received	$ 500,000						$ 500,000

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
	Nov. 30, 2018	May 31, 2017	Nov. 30, 2016	Dec. 31, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Net proceeds from issuance of convertible preferred stock			$ 25
Convertible shares of common stock				16,618,448
Series B Preferred Stock
Class Of Stock [Line Items]
Stock price					$ 6.39
Series B Preferred Stock | Series B Agreement
Class Of Stock [Line Items]
Convertible preferred stock, shares issued		3,128,540
Stock price		$ 6.39
Net proceeds from issuance of convertible preferred stock		$ 19.7
Net proceeds from issuance of convertible preferred stock to related party investors		$ 7.5
Series B Preferred Stock | Series B Agreement | Tranche Rights
Class Of Stock [Line Items]
Convertible preferred stock, shares issued		3,128,540
Stock price		$ 6.39
Net proceeds from issuance of convertible preferred stock		$ 20
Series B Preferred Stock | 2016 Notes and 2017 Notes | Series B Agreement
Class Of Stock [Line Items]
Convertible preferred stock, shares issued		811,103
Net proceeds from issuance of convertible preferred stock		$ 5.2
Series C Preferred Stock
Class Of Stock [Line Items]
Stock price				$ 10.77
Series C Preferred Stock | Series C Agreement
Class Of Stock [Line Items]
Convertible preferred stock, shares issued	6,499,935
Stock price	$ 10.77
Net proceeds from issuance of convertible preferred stock	$ 69.7
Series C Preferred Stock | Series C Agreement | Related Party
Class Of Stock [Line Items]
Net proceeds from issuance of convertible preferred stock	$ 29

Convertible Preferred Stock - S

Convertible Preferred Stock - Schedule of Convertible Preferred Stock (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Convertible preferred stock, authorized	82,000,000	50,000,000
Shares Issued and Outstanding	16,618,448	6,989,973
Carrying Value	$ 129,577	$ 39,841
Convertible preferred stock, aggregate liquidation preference, value	$ 130,178	$ 40,182
Series A Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, authorized	15,000,000	15,000,000
Shares Issued and Outstanding	3,050,329	3,050,329
Carrying Value	$ 14,926	$ 14,926
Convertible preferred stock, aggregate liquidation preference, value	$ 15,008	$ 15,008
Series B Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, authorized	35,000,000	35,000,000
Shares Issued and Outstanding	7,068,184	3,939,644
Carrying Value	$ 44,906	$ 24,915
Convertible preferred stock, aggregate liquidation preference, value	$ 45,166	$ 25,174
Series C Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, authorized	32,000,000
Shares Issued and Outstanding	6,499,935
Carrying Value	$ 69,745
Convertible preferred stock, aggregate liquidation preference, value	$ 70,004

Equity - Additional Information

Equity - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Aug. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 28, 2019	Jan. 31, 2019
Class Of Stock [Line Items]
Shares reserved for future issuance		20,811,843
Aggregate intrinsic value of options exercisable		$ 100,000	$ 0	$ 0
Expected dividend yield		0.00%
Total stock-based compensation		$ 672,000	$ 367,000	99,000
Common Stock
Class Of Stock [Line Items]
Issuance of common stock, shares			3,050
Recourse Promissory Note
Class Of Stock [Line Items]
Principal amount	$ 90,000,000
Accrued simple interest	1.22%
Outstanding loans balance		$ 28,000,000	$ 28,000,000	28,000,000
Recourse Promissory Note | Common Stock
Class Of Stock [Line Items]
Issuance of common stock, shares	152,516
Shares issued, price per share	$ 0.59
Repurchase of common stock, shares	105,067
Repurchase of common stock	$ 62,000,000
Employee Stock Option
Class Of Stock [Line Items]
Shares reserved for future issuance		3,323,988
2015 Plan -non-employee awards
Class Of Stock [Line Items]
Total stock-based compensation		$ 100,000	100,000
Early Exercised Stock Options
Class Of Stock [Line Items]
Other current liabilities relating to shares subject to repurchase		$ 188,000	$ 78,000
Early exercised shares repurchased		149,565	132,180
2015 Plan
Class Of Stock [Line Items]
Stockbased award, vesting period					4 years
Shares reserved for future issuance		132,394	237,182
Number of shares authorized for grant		3,902,175
Future tax benefit related to options exercised		$ 0	$ 0	$ 0
Share-based compensation purchase price of restricted stock awards					$ 0.0005
2015 Plan | Employee Stock Option
Class Of Stock [Line Items]
Weighted average grant date fair value of options vested		$ 0.81	$ 0.72	$ 0.68
Weighted average grant date fair value of options granted		$ 1.41	$ 0.92	$ 0.67
Unrecognized stock based compensation expense		$ 3,200,000
Weighted-average period for unrecognized compensation cost to recognize		3 years 4 months 24 days
2015 Plan | Subsequent Event
Class Of Stock [Line Items]
Awards granted						0
2015 Notes | Subsequent Event
Class Of Stock [Line Items]
Awards granted						0
2019 Plan
Class Of Stock [Line Items]
Awards granted		0
Shares reserved for future issuance		2,200,000
Share-based compensation, common stock issued		0
Percentage of capital stock outstanding		5.00%
2019 Plan | Maximum
Class Of Stock [Line Items]
Shares reserved for future issuance		5,656,381
Share-based compensation, common stock issued		8,000,000
2019 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Shares reserved for future issuance		750,000
Percentage of capital stock outstanding		1.00%
Share-based compensation arrangement by share-based payment award, percentage of combined voting power owned		5.00%
Share-based compensation arrangement by share based payment award rights to purchase stock remains outstanding		$ 25,000,000
Share-based compensation arrangement by share based payment award offerings of purchase periods		27 months
Share-based compensation percentage of payroll deductions on purchase shares of common stock		15.00%
Share -based compensation arrangement by share based payment award stock price percentage of fair market value		85.00%
2019 Employee Stock Purchase Plan | Maximum
Class Of Stock [Line Items]
Shares reserved for future issuance		250,000
Share-based compensation, common stock issued		750,000
2019 Employee Stock Purchase Plan | Subsequent Event
Class Of Stock [Line Items]
Shares reserved for future issuance							250,000

Equity - Summary of Option Acti

Equity - Summary of Option Activity Under Equity Incentive Plan (Details) - 2015 Plan - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Outstanding Options
Outstanding Options, Beginning Balance	1,791,299	286,739
Options granted	1,762,147	1,686,997
Options exercised	(198,943)	(97,373)
Options cancelled	(30,515)	(85,064)
Outstanding Options, Ending Balance	3,323,988	1,791,299	286,739
Vested and expected to vest as of December 31, 2018	3,323,988
Exercisable as of December 31, 2018	548,332
Weighted Average Exercise Price
Outstanding Options, Weighted Average Exercise Price	$ 1.13	$ 0.59
Options granted, Weighted Average Exercise Price	2.06	1.16
Options exercised, Weighted Average Exercise Price	1.35	0.59
Options cancelled, Weighted Average Exercise Price	1.29	0.59
Outstanding Options, Weighted Average Exercise Price	1.61	$ 1.13	$ 0.59
Vested and expected to vest as of December 31, 2018	1.61
Exercisable as of December 31, 2018	$ 0.93
Weighted Average Remaining Contractual Life (in years)
Outstanding Options, Weighted Average Remaining Contractual Life	9 years 25 days	9 years 2 months 26 days	9 years 4 months 6 days
Vested and expected to vest as of December 31, 2018	9 years 25 days
Exercisable as of December 31, 2018	8 years 1 month 9 days
Aggregate Intrinsic Value
Options Outstanding, Aggregate Intrinsic Value	$ 1,675	$ 881	$ 70
Vested and expected to vest as of December 31, 2018	1,675
Exercisable as of December 31, 2018	$ 651

Equity - Summary of Fair Value

Equity - Summary of Fair Value of Employee and Director Stock Option Awards using a Black-Scholes Option-Pricing Model (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected dividend	0.00%
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	6 years 21 days	6 years 3 months 29 days	6 years 10 days
Expected volatility	76.08%	73.39%	91.35%
Risk-free interest rate	2.89%	2.03%	1.37%
Expected dividend	0.00%	0.00%	0.00%

Equity - Total Stock-based Comp

Equity - Total Stock-based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	$ 672	$ 367	$ 99
Research and development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	325	145	43
General and administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	$ 347	$ 222	$ 56

Equity - Summary of Restricted

Equity - Summary of Restricted Stock Activity (Details) - Restricted Stock - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted shares, Beginning Balance	113,157	316,836
Restricted stock awards vested	(81,623)	(85,001)
Unvested shares repurchased	(9,356)	(118,678)
Restricted shares, Ending Balance	22,178	113,157

Equity - Schedule of Common Sto

Equity - Schedule of Common Stock Reserved for Future Issuance on As-converted Basis (Details) - shares	Dec. 31, 2018	Dec. 31, 2017
Conversion Of Stock [Line Items]
Common stock reserved for future issuance on as-converted basis	20,811,843
Shares Available for Future Grant Under the 2015 Plan
Conversion Of Stock [Line Items]
Common stock reserved for future issuance on as-converted basis	132,394	237,182
Employee Stock Option
Conversion Of Stock [Line Items]
Common stock reserved for future issuance on as-converted basis	3,323,988
Restricted Stock
Conversion Of Stock [Line Items]
Common stock reserved for future issuance on as-converted basis	22,178
Early Exercised Stock Options Subject To Future Vesting
Conversion Of Stock [Line Items]
Common stock reserved for future issuance on as-converted basis	149,565
Warrants To Purchase Shares Of Common Stock
Conversion Of Stock [Line Items]
Common stock reserved for future issuance on as-converted basis	565,270
Convertible Preferred Stock
Conversion Of Stock [Line Items]
Common stock reserved for future issuance on as-converted basis	16,618,448

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Income tax provision	$ 0	$ 0
Deferred tax assets	0	0
Increase in valuation allowance	$ 6,500,000	$ 4,000,000
Federal statutory income tax rate	21.00%	34.00%
Liability for uncertain tax positions	$ 0
Unrecognized tax benefits	3,438,000	$ 422,000		$ 197,000	$ 46,000
Scenario Forecast
Income Taxes [Line Items]
Unrecognized tax benefits			$ 0
Federal and California
Income Taxes [Line Items]
Net operating carryforwards	74,000,000	$ 46,500,000
Federal
Income Taxes [Line Items]
Net operating loss carryforwards, subject to expiration	$ 23,100,000
Net operating loss carryforwards, expiration year	2035
State
Income Taxes [Line Items]
Net operating loss carryforwards, subject to expiration	$ 13,900,000
Net operating loss carryforwards, expiration year	2035
Limitation on operating losses carryforward, percentage	80.00%

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carry forwards	$ 10,351,000	$ 6,494,000
Stock-based compensation	91,000	45,000
Deferred revenue	3,370,000
Tax credits		984,000
Other	469,000	165,000
Total deferred tax assets	14,281,000	7,688,000
Less: valuation allowance	(14,173,000)	(7,675,000)
Net deferred tax assets	108,000	13,000
Deferred tax liability	(108,000)	(13,000)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Income Taxes Rat

Income Taxes - Income Taxes Rate Reconciliation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Computed expected tax benefit (at federal statutory income tax rate of 21%)	$ (6,631,000)	$ (7,088,000)
Increase in valuation allowance	6,498,000	4,018,000
Other	133,000	329,000
Federal rate change (pursuant to the Tax Act)		2,741,000
Total provision for income taxes	$ 0	$ 0

Income Taxes - Income Taxes R_2

Income Taxes - Income Taxes Rate Reconciliation (Parenthetical) (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	34.00%

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Beginning and Ending Amounts of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 422	$ 197	$ 46
Increases related to current year tax positions	3,016	225	151
Balance at end of year	$ 3,438	$ 422	$ 197

Net Loss Per Share - Summary of

Net Loss Per Share - Summary of Outstanding Potentially Dilutive Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Outstanding potentially dilutive common stock equivalents	20,679,449	9,591,879	4,369,482
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Outstanding potentially dilutive common stock equivalents	16,618,448	6,989,973	3,050,329
Common Stock Options Issued and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Outstanding potentially dilutive common stock equivalents	3,323,988	1,791,299	286,739
Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Outstanding potentially dilutive common stock equivalents	22,178	113,157	316,836
Early Exercised Stock Options Subject To Future Vesting
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Outstanding potentially dilutive common stock equivalents	149,565	132,180	244,825
Warrants To Purchase Shares Of Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Outstanding potentially dilutive common stock equivalents	565,270	565,270	470,753

Net Loss Per Share - Summary _2

Net Loss Per Share - Summary of Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net loss and comprehensive loss									$ (27,366)	$ (16,830)	$ (11,406)
Weighted-average common shares used in computing net loss per share, basic and diluted	1,193,797	1,084,477	1,017,336	969,235	938,372	912,728	886,332	840,905	1,066,877	894,901	527,931
Net loss per share, basic and diluted	$ (8.15)	$ (6.23)	$ (5.89)	$ (5.04)	$ (5.27)	$ (5.35)	$ (4.24)	$ (3.86)	$ (25.65)	$ (18.81)	$ (21.61)

Subsequent Event - Additional I

Subsequent Event - Additional Information (Details) - USD ($)	Feb. 12, 2019	Jan. 03, 2019	Jan. 31, 2019	Dec. 31, 2017	Feb. 07, 2019	Dec. 31, 2018
Subsequent Event [Line Items]
Proceeds from issuance of common stock				$ 2,000
Offering costs				$ 0		$ 3,000,000
Shares reserved of common stock for issuance						20,811,843
2019 Employee Stock Purchase Plan
Subsequent Event [Line Items]
Shares reserved of common stock for issuance						750,000
IPO
Subsequent Event [Line Items]
Offering costs						$ 2,969,000
Subsequent Event | ProTriTAC Platform
Subsequent Event [Line Items]
Maverick lawsuit filing date for temporary restraining order		January 3, 2019
Name of plaintiff		Maverick
Domicile of litigation		Delaware Court of Chancery
Description of allegation		The complaint alleges various claims, including for breach of contract and misappropriation of trade secrets and seeks as relief, among other things, a declaration that the Company’s ProTriTAC technology impermissibly competes in the Maverick Field (as defined in the Asset Transfer Agreement), a preliminary and permanent injunction and unspecified damages.
Description of default judgement		On January 18, 2019, the Court denied Maverick’s motion for a temporary restraining order. The Company disputes Maverick’s allegations and intends to vigorously defend the claims asserted against it. In view of the uncertainty regarding the possible outcome of this case and the nature of the relief sought, the Company does not believe that it is currently possible to determine a reasonable estimate of the possible loss or range of loss, or other possible adverse result, if any, that may be incurred with respect to this matter.
Subsequent Event | 2019 Employee Stock Purchase Plan
Subsequent Event [Line Items]
Purchase plan effective date			Feb. 28, 2019
Shares reserved of common stock for issuance			250,000
Subsequent Event | IPO
Subsequent Event [Line Items]
Shares issued, price per share					$ 14
Issuance of common stock, shares	5,400,000
Proceeds from issuance of common stock	$ 70,700,000
Underwriter's discount	5,600,000
Offering costs	$ 4,500,000
Subsequent Event | IPO | Underwriters
Subsequent Event [Line Items]
Issuance of common stock for exercise of stock options, shares	369,201
Subsequent Event | Preferred Stock | IPO
Subsequent Event [Line Items]
Conversion of equity shares into common stock	16,618,448
Subsequent Event | Warrant | IPO
Subsequent Event [Line Items]
Conversion of equity shares into common stock	563,043

Subsequent Event - Schedule of

Subsequent Event - Schedule of Impact of the Company's IPO on Condensed Balance Sheet Items (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsequent Event [Line Items]
Cash and cash equivalents	$ 89,493,000	$ 29,423,000	$ 985,000
Deferred offering costs	3,000,000	0
Convertible preferred stock	129,577,000	39,841,000
Common stock	1,000	1,000
Additional paid-in capital	9,111,000	8,309,000
Total stockholders' deficit (equity)	(53,479,000)	$ (26,943,000)	$ (10,444,000)	$ (6,897,000)
IPO
Subsequent Event [Line Items]
Cash and cash equivalents	89,493,000
Deferred offering costs	2,969,000
Convertible preferred stock	129,577,000
Common stock	1,000
Additional paid-in capital	9,111,000
Total stockholders' deficit (equity)	(53,479,000)
IPO | Pro Forma
Subsequent Event [Line Items]
Cash and cash equivalents	160,149,000
Common stock	3,000
Additional paid-in capital	209,341,000
Total stockholders' deficit (equity)	$ 146,754,000

Quarterly Results (Unaudited) -

Quarterly Results (Unaudited) - Schedule of Quarterly Results (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue
Collaboration and license revenue	$ 1,063	$ 1,063	$ 1,063	$ 1,561	$ 708				$ 4,750	$ 708
Total revenue	1,063	1,063	1,063	1,561	708				4,750	708
Operating expenses
Research and development	8,717	5,967	6,151	5,533	4,791	$ 3,876	$ 2,801	$ 2,154	26,368	13,622	$ 7,778
General and administrative	2,215	1,942	967	982	912	1,012	847	843	6,106	3,614	3,369
Total operating expenses	10,932	7,909	7,118	6,515	5,703	4,888	3,648	2,997	32,474	17,236	11,147
Loss from operations	(9,869)	(6,846)	(6,055)	(4,954)	(4,995)	(4,888)	(3,648)	(2,997)	(27,724)	(16,528)	(11,147)
Interest income	148	108	66	73	57	13	2	6	395	78	6
Interest expense							(112)	(173)		(285)	(261)
Other expense	(8)	(22)	(5)	(2)	(4)	(5)		(86)	(37)	(95)	(4)
Net loss and comprehensive loss	$ (9,729)	$ (6,760)	$ (5,994)	$ (4,883)	$ (4,942)	$ (4,880)	$ (3,758)	$ (3,250)	$ (27,366)	$ (16,830)	$ (11,406)
Net loss per share, basic and diluted	$ (8.15)	$ (6.23)	$ (5.89)	$ (5.04)	$ (5.27)	$ (5.35)	$ (4.24)	$ (3.86)	$ (25.65)	$ (18.81)	$ (21.61)
Weighted-average common shares used in computing net loss per share, basic and diluted	1,193,797	1,084,477	1,017,336	969,235	938,372	912,728	886,332	840,905	1,066,877	894,901	527,931