Sutro Biopharma (STRO) 10-Q 14 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Nov. 09, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Trading Symbol	STRO
Entity Registrant Name	SUTRO BIOPHARMA, INC.
Entity Central Index Key	1,382,101
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		22,848,184

Condensed Balance Sheets (Unaud

Condensed Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 41,353	$ 22,020
Marketable securities	81,597
Accounts receivable, net (including amounts from related parties of $1,695 and $784 as of September 30, 2018 and December 31, 2017, respectively)	2,443	1,624
Prepaid expenses and other current assets	1,979	1,985
Total current assets	127,372	25,629
Property and equipment, net	11,673	13,997
Other long-term assets	5,966	1,128
Restricted cash	15	15
Total assets	145,026	40,769
Current liabilities:
Accounts payable	4,594	2,902
Accrued compensation	4,085	3,639
Deferred revenue—current	24,229	10,709
Debt—current	3,182	14,634
Other current liabilities	815	72
Total current liabilities	36,905	31,956
Deferred revenue, non-current	48,805	13,159
Deferred rent	473	428
Redeemable convertible preferred stock warrant liability	867	1,708
Debt—non-current	11,500
Other noncurrent liabilities	664	14
Total liabilities	99,214	47,265
Commitments and Contingencies
Redeemable convertible preferred stock, $0.001 par value — 498,070,991 and 177,082,393 shares authorized as of September 30, 2018 and December 31, 2017, respectively; 493,615,703 and 173,750,421 shares issued and outstanding as of September 30, 2018 and December 31, 2017, respectively; aggregate liquidation preference of $188,639 as of September 30, 2018	187,245	102,505
Stockholders’ deficit:
Note receivable from stockholder		(208)
Additional paid-in-capital	7,428	6,218
Accumulated other comprehensive loss	(27)
Accumulated deficit	(148,835)	(115,011)
Total stockholders’ deficit	(141,434)	(109,001)
Total liabilities, redeemable convertible preferred stock, and stockholders’ deficit	145,026	40,769
Redeemable Convertible Preferred Stock
Current liabilities:
Redeemable convertible preferred stock, $0.001 par value — 498,070,991 and 177,082,393 shares authorized as of September 30, 2018 and December 31, 2017, respectively; 493,615,703 and 173,750,421 shares issued and outstanding as of September 30, 2018 and December 31, 2017, respectively; aggregate liquidation preference of $188,639 as of September 30, 2018	$ 187,246	$ 102,505

Condensed Balance Sheets (Una_2

Condensed Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts receivable, related parties	$ 1,695	$ 784
Redeemable convertible preferred stock, shares authorized	498,070,991	177,082,393
Redeemable convertible preferred stock, aggregate liquidation preference	$ 188,639	$ 102,988
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	818,000,000	271,000,000
Common stock, shares issued	485,097	465,330
Common stock, shares outstanding	485,097	465,330
Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock, par value	$ 0.001	$ 0.001
Redeemable convertible preferred stock, shares authorized	498,070,991	177,082,393
Redeemable convertible preferred stock, shares issued	493,615,703	173,750,421
Redeemable convertible preferred stock, shares outstanding	493,615,703	173,750,421
Redeemable convertible preferred stock, aggregate liquidation preference	$ 188,639

Condensed Statements of Operati

Condensed Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue:
Total revenue	$ 7,836	$ 17,499	$ 19,333	$ 47,701
Operating expenses
Research and development	12,642	13,669	39,475	39,499
General and administrative	5,351	4,895	13,806	12,306
Total operating expenses	17,993	18,564	53,281	51,805
Loss from operations	(10,157)	(1,065)	(33,948)	(4,104)
Interest income	403	62	483	192
Interest expense	(415)	(235)	(1,199)	(235)
Other income (expense), net	(68)	(180)	840	(197)
Net loss	$ (10,237)	$ (1,418)	$ (33,824)	$ (4,344)
Net loss per share, basic and diluted	$ (21.26)	$ (3.14)	$ (71.06)	$ (9.77)
Weighted-average shares used in computing net loss per share	481,613	451,550	476,023	444,594
Collaboration Revenue
Revenue:
Total revenue	$ 6,924	$ 17,499	$ 13,955	$ 47,701
Other Revenue
Revenue:
Total revenue	$ 912	$ 0	$ 5,378	$ 0

Condensed Statements of Opera_2

Condensed Statements of Operations (Parenthetical) (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Revenue from related parties	$ 5,174	$ 15,754	$ 8,529	$ 42,292

Condensed Statements of Compreh

Condensed Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (10,237)	$ (1,418)	$ (33,824)	$ (4,344)
Other comprehensive income:
Unrealized gain (loss) on available-for-sale securities	(27)	5	(27)	16
Comprehensive loss	$ (10,264)	$ (1,413)	$ (33,851)	$ (4,328)

Condensed Consolidated Statemen

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net loss	$ (33,824)	$ (4,344)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and amortization	3,404	3,796
Amortization of premium (accretion of discount) on marketable securities	(166)	116
Stock-based compensation	802	1,126
Revaluation of SutroVax option liability	48	75
Revaluation of redeemable convertible preferred stock warrant liability	(839)	186
Accretion of debt discount	119	24
Loss on disposal of property and equipment	35	87
Other revenue	140	0
Changes in operating assets and liabilities:
Accounts receivable	(819)	(10,279)
Prepaid expenses and other current assets	(901)	(302)
Accounts payable	(135)	178
Accrued compensation	446	(178)
Other liabilities	1,097	106
Deferred rent	45	56
Deferred revenue	49,166	(22,881)
Net cash provided by (used in) operating activities	18,618	(32,234)
Cash flows from investing activities:
Purchases of marketable securities	(81,456)	(5,019)
Maturities of marketable securities	0	32,650
Sales of marketable securities	0	6,000
Purchases of property and equipment	(759)	(2,659)
Net cash provided by (used in) investing activities	(82,215)	30,972
Cash flows from financing activities:
Proceeds from issuance of debt	0	15,000
Payments of debt issuance fees	0	(171)
Payments of deferred offering costs	(2,413)	(74)
Proceeds from payment of note receivable by stockholder	208	0
Proceeds from issuances of common stock upon exercise of warrants	2	0
Proceeds from issuances of common stock upon exercise of stock options	394	61
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	84,739	0
Net cash provided by financing activities	82,930	14,816
Net increase in cash and cash equivalents	19,333	13,554
Cash, cash equivalents, and restricted cash—beginning of period	22,035	11,868
Cash, cash equivalents, and restricted cash—end of period	41,368	25,422
Supplemental disclosure of cash flow information:
Cash paid for interest	942	184
Supplemental disclosure of non-cash investing and financing information:
Vesting of early exercised shares	14	65
Purchase of property and equipment included in accounts payable	610	229
Deferred initial public offering costs included in accounts payable	$ 1,659	$ 85

Organization and Principal Acti

Organization and Principal Activities	9 Months Ended
	Sep. 30, 2018
Disclosure Text Block [Abstract]
Organization and Principal Activities	1. Organization and Principal Activities Description of Business Sutro Biopharma, Inc. (the “Company”) is a clinical stage drug discovery, development and manufacturing company focused on leveraging its integrated cell-free protein synthesis and site-specific conjugation platform, XpressCF+™, to create a broad variety of optimally designed, next-generation protein therapeutics for cancer and autoimmune disorders. The Company was incorporated on April 21, 2003, and was formerly known as Fundamental Applied Biology, Inc. The Company is headquartered in South San Francisco, California. The Company operates in one business segment, the development of biopharmaceutical products. Initial Public Offering On September 26, 2018, the Company’s registration statements on Form S-1 (File No. 333-227103 and 333-227548) relating to its initial public offering (“IPO”) of its common stock was declared effective by the Securities and Exchange Commission (“SEC”) and the shares of its common stock began trading on the Nasdaq Global Market on September 27, 2018. The public offering price of the shares sold in the IPO was $15.00 per share. The IPO closed on October 1, 2018, pursuant to which the Company sold 5,667,000 shares of common stock, for gross proceeds of approximately $85.0 million. The Company received net proceeds from the IPO of approximately $74.4 million, after underwriting discounts, commissions and estimated offering expenses. In addition to the shares of common stock sold in the IPO, the Company concurrently sold in a private placement to Merck Sharp & Dohme Corp., a subsidiary of Merck & Co., Inc., Kenilworth, NJ, USA (“Merck”), 666,666 shares of common stock at the IPO offering price of $15.00 per share, for proceeds of approximately $10.0 million. Immediately prior to the completion of the IPO, all outstanding shares of redeemable convertible preferred stock converted into common stock. Immediately prior to the completion of the IPO on October 1, 2018, all outstanding shares of redeemable convertible preferred stock were converted into 16,028,462 shares of common stock. Subsequent to the closing of the IPO, there were no shares of redeemable convertible preferred stock outstanding. The condensed financial statements as of September 30, 2018, including share and per share amounts, do not give effect to the IPO, or the conversion of the redeemable convertible preferred stock, as the IPO and such conversions were completed subsequent to September 30, 2018. Reverse Stock Split On September 14, 2018, the Company effected a reverse split of all shares of its common stock at a ratio of 36.3-for-1. Upon the effectiveness of the reverse stock split, (i) all shares of outstanding common stock were adjusted; (ii) the number of shares of common stock for which each outstanding option to purchase common stock is exercisable were adjusted; (iii) the exercise price of each outstanding option to purchase common stock were adjusted; (iv) the conversion ratio for each share of outstanding redeemable convertible preferred stock which is convertible into the Company’s common stock was proportionately reduced; (v) the number of shares of common stock for which each outstanding warrant to purchase common stock is exercisable was proportionally decreased; (vi) the conversion ratio for each outstanding warrant to purchase redeemable convertible preferred stock which is convertible into warrants to purchase the Company’s common stock after the offering was proportionally decreased; and (vii) the exercise price of each outstanding warrant was proportionally increased. All of the outstanding common stock share numbers (including shares of common stock subject to the Company’s options, as converted for the outstanding redeemable convertible preferred stock shares and warrants), share prices, exercise prices and per share amounts contained in the financial statements have been retroactively adjusted in the financial statements to reflect this reverse stock split for all periods presented. The par value per share and the authorized number of shares of common stock and redeemable convertible preferred stock were not adjusted as a result of the reverse stock split. Series E Redeemable Convertible Preferred Stock Split In July 2018, the Company’s board of directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a 1-for-1.1940912491 split (“Split”) of shares of the Company’s Series E redeemable convertible preferred stock, which was effected on July 26, 2018. The par value and authorized shares of redeemable convertible preferred stock and the other outstanding shares of redeemable convertible preferred stock were not adjusted as a result of the Split. All of the outstanding Series E redeemable convertible preferred shares and per share information included in the accompanying financial statements have been adjusted to reflect the Split. Liquidity The Company has incurred significant losses and has negative cash flows from operations. As of September 30, 2018, there was an accumulated deficit of $148.8 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 September 30, 2018, the Company had unrestricted cash, cash equivalents and marketable securities of $123.0 million, which is available to fund future operations. 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. The Company believes that its unrestricted cash, cash equivalents and marketable securities as of September 30, 2018 will be sufficient for the Company to continue as a going concern for at least one year from the issuance date of its unaudited interim condensed financial statements. In August 2017, the Company entered into a loan and security agreement with Oxford Finance LLC (“Oxford”) and Silicon Valley Bank (“SVB”) under which it borrowed $15.0 million (the “August 2017 Loan”) (see Note 6). The August 2017 Loan provides that an event of default will occur if, among other triggers, there occurs any circumstances that could reasonably be expected to result in a material adverse effect on the Company’s business, operations or condition, or on its ability to perform its obligations under the loan. The Company disclosed in its audited financial statements as of December 31, 2017 that the Company believed that there was substantial doubt about its ability to continue as a going concern given its continuing operating losses and its then current available capital resources, which could be deemed to be an event of default if such condition was considered to have a material adverse effect on the Company’s business, operations or condition. As a result, the Company classified the entire debt balance as a current liability as of December 31, 2017 given that a determination of such an event of default was outside of the Company’s control. Based on the available financial resources described above, as of September 30, 2018, the Company has classified $3.2 million of the outstanding debt balance as current and the remainder as non-current, which reflects the scheduled repayments under the August 2017 Loan.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation and Use of Estimates The accompanying financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”). The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. The Company bases its estimates on historical experience and market-specific or other relevant assumptions that it believes are reasonable under the circumstances. The amounts of assets and liabilities reported in the Company’s balance sheets and the amount of expenses and income reported for each of the periods presented are affected by estimates and assumptions, which are used for, but are not limited to, determining research and development periods under multiple element arrangements, stock-based compensation expense, fair value of redeemable convertible preferred stock and warrant liabilities, fair value of common stock, income taxes and certain accrued liabilities. Actual results could differ from such estimates or assumptions. Unaudited Interim Condensed Financial Statements The interim condensed balance sheet as of September 30, 2018, the condensed statements of operations and comprehensive loss for the three and nine months ended September 30, 2018 and 2017, and the condensed statements of cash flows for the nine months ended September 30, 2018 and 2017 are unaudited. The unaudited interim condensed financial statements have been prepared on the same basis as the annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s financial position as of September 30, 2018, its results of operations and comprehensive loss for the three and nine months ended September 30, 2018 and 2017, and cash flows for the nine months ended September 30, 2018 and 2017. The financial data and the other financial information contained in these notes to the condensed financial statements related to the three-month and nine-month periods are also unaudited. The results of operations for the three and nine months ended September 30, 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018 or for any other future annual or interim period. The condensed balance sheet as of December 31, 2017 included herein was derived from the audited financial statements as of that date. These condensed financial statements should be read in conjunction with the Company's audited financial statements included in the prospectus dated September 26, 2018 that forms a part of the Company's registration statements on Form S-1 (File Nos 333-227103 and 333-227548), as filed with the SEC pursuant to Rule 424(b)(4) promulgated under the Securities Act of 1933, as amended. Cash, Cash Equivalents, and Restricted Cash The Company considers all highly liquid investments with original maturities of 90 days or less from the date of purchase to be cash equivalents. Under certain lease and credit agreements, the Company has pledged cash and cash equivalents as collateral. Restricted cash related to such agreements was $15,000 as of both September 30, 2018 and December 31, 2017. 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 statements of cash flows. September 30, 2018 2017 (in thousands) Cash and cash equivalents $ 41,353 $ 25,407 Restricted cash 15 15 Total cash, cash equivalents, and restricted cash shown in the statements of cash flows $ 41,368 $ 25,422 Fair Value Measurements 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 establishes a fair value hierarchy that requires an entity to maximize the use of observable inputs, where available, and minimizes the use of unobservable inputs when measuring fair value. The Company determined the fair value of financial assets and liabilities using the fair value hierarchy that describes three levels of inputs that may be used to measure fair value, as follows: Level 1—Quoted prices in active markets for identical assets and liabilities; Level 2—Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities; 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. The carrying amounts of accounts receivable, prepaid expenses, accounts payable and accrued compensation approximate fair value due to the short-term nature of these items. The fair value of the Company’s financial assets and liabilities is measured on a recurring basis by level within the fair value hierarchy. See Note 3. The fair value of the Company’s outstanding loan (See Note 6) is estimated using the net present value of the payments, discounted at an interest rate that is consistent with market interest rate, which is a Level 2 input. The estimated fair value of the Company’s outstanding loan approximates the carrying amount, as the loan bears a floating rate that approximates the market interest rate. Deferred Offering Costs The Company has deferred offering costs consisting of legal, accounting and other fees and costs directly attributable to the Company’s IPO, which was completed on October 1, 2018. The deferred offering costs will be offset against the gross proceeds of the IPO. As of September 30, 2018 and December 31, 2017, $4.6 million and $0.5 million, respectively, of deferred offering costs were recorded within other long-term assets on the balance sheet. Redeemable Convertible Preferred Stock Warrants The Company accounts for its redeemable convertible preferred stock warrants as a liability, and they are recorded at their estimated fair value, because the warrants may conditionally obligate the Company to transfer assets at some point in the future. At the end of each reporting period, changes in the estimated fair value during the period are recorded in other income (expense), net in the statement of operations. The Company will continue to adjust the liability for changes in estimated fair value until the earlier of the expiration of the warrants, exercise of the warrants, or conversion of the redeemable convertible preferred stock warrants into common stock warrants upon the completion of a liquidation event, including the completion of an IPO, which occurred on October 1, 2018. Revenue Recognition Revenue is recognized when persuasive evidence of an arrangement exists, delivery has occurred or services have been rendered, the price is fixed or determinable and collectability is reasonably assured. For multiple-element arrangements, each deliverable within a multiple-deliverable revenue arrangement is accounted for as a separate unit of accounting if both of the following criteria are met: the delivered item or items has value to the customer on a stand-alone basis; and (ii) for an arrangement that includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in management’s control. The Company recognizes revenue from milestone payments when: (i) the milestone event is substantive and its achievability has substantive uncertainty at the inception of the agreement, and the Company has completed its performance obligations related to the achievement of the milestone. Milestone payments are considered substantive if all of the following conditions are met: the milestone payment (a) is commensurate with either the Company’s performance subsequent to the inception of the arrangement to achieve the milestone or the enhancement of the value of the delivered item or items as a result of a specific outcome resulting from the Company’s performance subsequent to the inception of the arrangement to achieve the milestone, (b) relates solely to past performance, and (c) is reasonable relative to all of the deliverables and payment terms (including other potential milestone consideration) within the arrangement. Determining whether and when these revenue recognition criteria have been satisfied often involves assumptions and judgments that can have a significant impact on the timing and amount of reported revenue. Changes in assumptions or judgments or changes to the elements in an arrangement could cause a material increase or decrease in the amount of revenue that is reported in a particular period. Under certain collaborative arrangements, the Company is entitled to payments for certain research and development activities and for providing product and other related materials. The Company’s policy is to account for such payments by its collaboration partners as collaboration revenue. Research and Development Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities: salaries, employee benefits, laboratory supplies, outsourced research and development expenses, professional services and allocated facilities-related costs. Amounts incurred in connection with collaboration arrangements are also included as a research and development expense. Nonrefundable advance payments for goods or services that will be used or rendered for future research and development activities are deferred and capitalized and recognized as an expense as the goods are delivered or the related services are performed. For outsourced research and development expenses, such as professional fees payable to third parties for preclinical studies, clinical trials and research services, and other consulting costs, the Company estimates the 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. If the actual timing of the performance of services or the level of effort varies from the original estimates, the Company will adjust the accrual accordingly. 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. 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 for the period, without consideration for potential dilutive common shares. Basic net loss per share is the same as diluted net loss per share as the inclusion of all potential dilutive common shares would have been anti-dilutive. Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (“FASB”), or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the impact of recently issued standards that are not yet effective will not have a material impact on the Company’s financial statements upon adoption. Under the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”), the Company meets the definition of an emerging growth company, and has elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act. New Accounting Pronouncements Not Yet Adopted In May 2014, the FASB issued ASU No. 2014-09 (Topic 606), Revenue from Contracts with Customers. In August 2015, the FASB issued ASU No. 2015-14 (Topic 606), Revenue from Contracts with Customers: Deferral of the Effective Date, which delayed the effective date of ASU 2014-09 by one year. ASU 2014-09, as amended, became effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. For entities other than public entities, the standard is effective for fiscal years beginning after December 15, 2018, and interim periods beginning after December 15, 2019. Early adoption is permitted. ASU 2014-09 also permits two methods of adoption: retrospectively to each prior reporting period presented (full retrospective method), or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application (the modified retrospective method). The Company is in the process of evaluating the effect this guidance will have on revenue recognition for its collaboration and license agreements. The core principle of ASU 2014-09 is that an entity should recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. ASU 2014-09 defines a five-step process to achieve this core principle and, in doing so, it is possible more judgment and estimates may be required within the revenue recognition process than required under existing U.S. generally accepted accounting pronouncements. All of the Company’s revenue is currently generated from up-front payments, research and development services, and milestone and contingent payments under its collaboration arrangements. The Company is currently evaluating its collaboration agreements to determine the impact of adopting ASU 2014-09, inclusive of available transitional methods, on its financial statements and related disclosures. In January 2016, the FASB issued ASU 2016-01 (Topic 825), Recognition and Measurement of Financial Assets and Financial Liabilities, which will change how to recognize, measure, present and make disclosures about certain financial assets and financial liabilities. Under ASU 2016-01, if an entity designates a financial liability under the fair value option (“FVO”) in accordance with ASC 825, the entity shall measure the financial liability at fair value with qualifying changes in fair value recognized in net income. The entity shall present separately in other comprehensive income the portion of the total change in the fair value of the liability that results from a change in the instrument-specific credit risk. For public business entities, ASU 2016-01 is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. For all other entities, the guidance is effective for fiscal years beginning after December 15, 2018, and interim periods within fiscal years beginning after December 15, 2019. All entities can early adopt the provision related to financial liabilities measured using the FVO in ASC 825 for financial statements of annual or interim periods that have not yet been issued or made available for issuance. The Company does not expect the adoption of this amendment will have a material impact on its financial statements. In February 2016, the FASB issued Accounting Standards Update (ASU) No. 2016-02, Leases Leases Leases In August 2016, the FASB issued ASU 2016-15 (“ASC Topic 230”), Classification of Certain Cash Receipts and Cash Payments . The new guidance clarifies the classification of certain cash receipts and cash payments in the statement of cash flows, including debt prepayment or extinguishment costs, settlement of contingent consideration arising from a business combination, insurance settlement proceeds, and distributions from certain equity method investees. ASU 2016-15 is effective for annual periods beginning after December 15, 2017. Early adoption is permitted. The Company is in the process of assessing the impact, if any, of this ASU on its financial statements. The Company does not expect that the adoption of this amendment will have a material impact on its financial statements. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808), Clarifying the interaction between Topic 808 and Topic 606

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The following table sets forth the fair value of the Company’s financial assets and liabilities measured on a recurring basis by level within the fair value hierarchy: September 30, 2018 Total Level 1 Level 2 Level 3 (in thousands) Assets: Money market funds $ 31,879 $ 31,879 $ – $ – Commercial paper 34,939 – 34,939 – Corporate debt securities 15,774 – 15,774 – Asset-backed securities 16,864 – 16,864 – U.S. government agency securities 23,767 – 23,767 – Total $ 123,223 $ 31,879 $ 91,344 $ – Liabilities: Redeemable convertible preferred stock warrant liability $ 867 $ – $ – $ 867 Total $ 867 $ – $ – $ 867 December 31, 2017 Total Level 1 Level 2 Level 3 (in thousands) Assets: Money market funds $ 6,578 $ 6,578 $ – $ – Commercial paper 7,689 – 7,689 – Corporate debt securities 800 – 800 – U.S. government agency securities 3,893 – 3,893 – Total $ 18,960 $ 6,578 $ 12,382 – Liabilities: Redeemable convertible preferred stock warrant liability $ 1,708 $ – $ – $ 1,708 Total $ 1,708 $ – $ – $ 1,708 Where applicable, the Company uses quoted market prices in active markets for identical assets to determine fair value. This pricing methodology applies to Level 1 investments, which are composed of money market funds. If quoted prices in active markets for identical assets are not available, then the Company uses quoted prices for similar assets or inputs other than quoted prices that are observable, either directly or indirectly. These investments are included in Level 2 and consist of commercial paper, corporate debt securities, and U.S. government agency securities. These assets are valued using market prices when available, adjusting for accretion of the purchase price to face value at maturity. A financial instrument’s categorization within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement. The Company’s assessment of the significance of a particular input to the fair value measurement in its entirety requires management to make judgments and consider factors specific to the asset or liability. In certain cases where there is limited activity or less transparency around inputs to valuation, securities are classified as Level 3 within the valuation hierarchy. Level 3 liabilities that are measured at estimated fair value on a recurring basis consist of the redeemable convertible preferred stock warrant liability. Refer to Note 8 for the valuation techniques used to measure fair value and a description of the inputs and the information used to develop the inputs to the valuation models. Generally, increases or decreases in the fair value of the underlying redeemable convertible preferred stock would result in a directionally similar impact in the fair value measurement of the associated warrant liability. There were no transfers within the hierarchy during the nine months ended September 30, 2018 and 2017. The following table sets forth a summary of the changes in the estimated fair value of the Company’s redeemable convertible preferred stock warrant liability: Redeemable Convertible Preferred Stock Warrant Liability (in thousands) Balance as of December 31, 2017 $ 1,708 Proceeds from issuances of common stock upon exercise of warrants (2 ) Changes in estimated fair value of warrant liability included in other income (expense), net (839 ) Balance as of September 30, 2018 $ 867

Cash Equivalents and Marketable

Cash Equivalents and Marketable Securities	9 Months Ended
	Sep. 30, 2018
Cash And Cash Equivalents [Abstract]
Cash Equivalents and Marketable Securities	4. Cash Equivalents and Marketable Securities Cash equivalents and marketable securities consisted of the following: September 30, 2018 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market funds $ 31,879 $ – $ – $ 31,879 Commercial paper 34,939 – – 34,939 Corporate debt securities 15,781 1 (8 ) 15,774 Asset-based securities 16,876 – (12 ) 16,864 U.S. government agencies 23,776 – (9 ) 23,767 Total 123,251 1 (29 ) 123,223 Less amounts classified as cash equivalents (41,626 ) – – (41,626 ) Total marketable securities $ 81,625 $ 1 $ (29 ) $ 81,597 December 31, 2017 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market funds $ 6,578 $ – $ – $ 6,578 Commercial paper 7,689 – – 7,689 Corporate debt securities 800 – – 800 U.S. government agencies 3,893 – – 3,893 Total 18,960 – – 18,960 Less amounts classified as cash equivalents (18,960 ) – – (18,960 ) Total marketable securities $ – $ – $ – $ – All marketable securities as of September 30, 2018 had maturities of less than one year.

Collaboration Agreements and Su

Collaboration Agreements and Supply Agreements	9 Months Ended
	Sep. 30, 2018
Collaboration Agreements And Supply Agreements [Abstract]
Collaboration Agreements and Supply Agreements	5. Collaboration Agreements and Supply Agreements The Company has recognized revenue as follows: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (in thousands) (in thousands) Collaboration revenue: Celgene Corporation (“Celgene”)—related party: Amortization of up-front payment $ 1,655 $ 2,642 $ 4,912 $ 16,355 Research and development services 5 – 103 – Milestones and contingent payments – 13,112 25,937 Total 1,660 15,754 5,015 42,292 Merck Sharp & Dohme Corporation (“Merck”)—related party: Amortization of up-front payment 2,818 – 2,818 – Research and development services 696 – 696 – Total 3,514 – 3,514 – Merck KGaA, Darmstadt, Germany (operating in the United States and Canada under the name “EMD Serono”): Amortization of up-front payment 1,038 1,030 3,104 3,090 Research and development services 712 715 2,322 2,319 Total 1,750 1,745 5,426 5,409 Total collaboration revenue $ 6,924 $ 17,499 $ 13,955 $ 47,701 Other revenue—related parties: Celgene Corporation: Development and manufacturing services and clinical product supply $ 330 $ – $ 3,894 $ – SutroVax: Supply and other 582 – 1,484 – Total other revenue—related parties $ 912 $ – $ 5,378 $ – Total revenue $ 7,836 $ 17,499 $ 19,333 $ 47,701 2014 Celgene Agreement In September 2014, the Company signed a Collaboration and License Agreement with Celgene (the “2014 Celgene Agreement”) to discover and develop bispecific antibodies and/or antibody-drug conjugates (“ADCs”), focused primarily on the field of immuno-oncology, using the Company’s proprietary integrated cell-free protein synthesis platform, XpressCF+™. Upon signing the 2014 Celgene Agreement, the Company received an up-front, nonrefundable payment totaling $83.1 million. In March 2015, the Company received a $15.0 million contingent payment (“March 2015 payment”) from Celgene under the 2014 Celgene Agreement that provided Celgene a right to access certain of the Company’s technology for use in conjunction with certain Celgene intellectual property. In June 2016, the Company received a $25.0 million milestone (“June 2016 payment”) upon completion of certain preclinical activities. The March 2015 and June 2016 payments were being recognized as revenue over the remaining portion of the estimated period of the research term prior to entering into the 2017 Celgene Agreement. 2017 Celgene Agreement In August 2017, the Company entered into the 2017 Celgene Agreement to refocus its 2014 Celgene Agreement on four programs that are advancing through preclinical development, including an ADC program targeting B cell maturation antigen. Upon signing of the 2017 Celgene Agreement, the Company received an option fee payment of $12.5 million in August 2017 and is entitled to receive a second option fee payment of $12.5 million following the first investigational new drug (“IND”) clearance, if any, for one of the four programs, if Celgene desires to maintain its option to acquire the U.S. rights to develop and commercialize a second collaboration program to reach IND status. If Celgene exercises its option to acquire from the Company U.S. rights to a second collaboration program, it will make an option exercise fee payment to the Company, the amount of which depends on which program reaches IND status. The Company determined that the initial $12.5 million payment should be deferred and recognized over the entire potential period during which Celgene has an option to acquire worldwide rights to a second collaboration program. Consequently, the Company is recognizing revenue from such payment ratably over an approximate three-year period starting in August 2017 and ending in September 2020. In September 2017, the Company earned a $10.0 million milestone for certain manufacturing accomplishments, which payment was received from Celgene in October 2017. The entire $10.0 million amount was recognized as revenue when earned, as the Company had completed its performance obligations related to the achievement of the substantive milestone. The Company evaluated the terms of the 2017 Celgene Agreement, relative to the 2014 Celgene Agreement, and determined the 2017 Celgene Agreement to be a material modification to the 2014 Celgene Agreement for financial reporting purposes. As a result, the Company determined that the remaining deferred revenue balance of $8.2 million as of the date of entering into the 2017 Celgene Agreement, related to Celgene payments to the Company under the 2014 Celgene Agreement, will also be recognized ratably over an approximate three-year period starting in August 2017 and ending in September 2020 (the “Celgene Agreements”). The Company has received and will be eligible to receive financial support for research and development services assigned to the Company by Celgene, based on an agreed-upon level of full-time equivalent personnel effort and related reimbursement rate, which will be recognized as revenue as the related reimbursable activities approved by Celgene and the Company are performed by the Company. Under the terms of the 2017 Celgene Agreement, the Company is entitled to earn development and regulatory contingent payments for each of the four programs under the collaboration, and royalties on sales of any commercial products that may result from the 2017 Celgene Agreement. As of September 30, 2018, the Company is eligible to receive a potential future payment for manufacturing activities of $10.0 million, which is considered to be a substantive milestone for which the related payment will be recognized as revenue upon achievement. In addition, for licensed products for which Celgene holds worldwide rights, the Company is eligible to receive aggregate milestone and option fee payments of up to $295.0 million for certain licensed products and up to $393.7 million for certain other licensed products under the collaboration, if approved in multiple indications, and, depending on the licensed product, tiered royalties ranging from mid-single digits to low teen percentages on worldwide sales of any commercial products that may result from the 2017 Celgene Agreement. Additionally, for licensed products for which Celgene holds ex-U.S. rights, the Company will also be eligible to receive pre-commercial contingent payments and tiered royalties ranging from mid to high single digit percentages. The contingent payments under the 2017 Celgene Agreement are not considered to be substantive milestones because the receipt of such payments is based solely on the performance of Celgene. Celgene may terminate the 2017 Celgene Agreement at any time with 120 days’ prior written notice. Either the Company or Celgene has the right to terminate the 2017 Celgene Agreement based on the other party’s uncured material breach, challenge of the validity and enforceability of intellectual property, or bankruptcy. As of September 30, 2018 and December 31, 2017, there was $13.1 million and $18.0 million, respectively, of deferred revenue related to payments received by the Company under the Celgene Agreements. As of September 30, 2018 and December 31, 2017, the Company had $0.3 million and $0.8 million, respectively, of receivables from Celgene related to the Celgene Agreements, which are included in accounts receivable on the balance sheet. 2018 Master Services Agreement In March 2018, the Company entered into a Master Development and Clinical Manufacturing Services Agreement (the “Master Services Agreement”) with Celgene, wherein Celgene requested the Company to provide development, manufacturing and supply chain management services, including clinical product supply. The consideration for the services is based on an agreed-upon level of full-time equivalent personnel effort and related reimbursement rate in addition to agreed-upon pricing for the clinical product supply. For the three and nine months ended September 30, 2018, the Company earned $0.3 million and $3.9 million, respectively, in other revenue-related parties under the Master Services Agreement 2018 Merck Agreement In July 2018, the Company entered into an Exclusive Patent License and Research Collaboration Agreement (the “2018 Merck Agreement”) with Merck to jointly develop up to three research programs focusing on cytokine derivatives for cancer and autoimmune disorders. Under the 2018 Merck Agreement, the Company received from Merck an upfront payment of $60.0 million in August 2018 for the identification of and the pre-clinical research and development of two target programs, with an option for Merck to engage the Company to continue these activities for a third program upon the payment of an additional amount. The Company identified multiple deliverables under the 2018 Merck Agreement, which include access to certain intellectual property rights, performance of research and development services, and joint project team participation. The Company considered the provisions of the multiple-element arrangement guidance in determining whether access to the intellectual property rights under the arrangement has stand-alone value. Based on the Company’s expertise in applying its proprietary technology, the Company concluded that there is no stand-alone value of the intellectual property rights accessed by Merck. Consequently, the Company determined that the identified deliverables comprise a single unit of accounting, and the up-front cash payment was deferred and will be recognized over the relevant estimated period during which the Company has significant obligations to perform research and development services and participate in joint project team activities for Merck. Consequently, the Company is recognizing revenues from the up-front payment ratably over an estimated four-year period starting in July 2018. Revenue for research and development services under the 2018 Merck Agreement will be recognized as the related activities are performed by the Company. The Company is also eligible to receive aggregate milestone payments of up to $1.6 billion, assuming the development and sale of all therapeutic candidates and all possible indications identified under the collaboration. If one or more products from each of the target programs are developed for non-oncology or a single indication, the Company will be eligible for reduced aggregate milestone payments. In addition, the Company is eligible to receive tiered royalties ranging from mid-single digit to low teen percentages on the worldwide sales of any commercial products that may result from the collaboration. Additionally, Merck purchased 74,794,315 shares of the Company’s Series E redeemable convertible preferred stock at a price per share of $0.2674, resulting in gross proceeds of $20.0 million. Concurrent with the Company’s IPO, which was completed on October 1, 2018, Merck purchased 666,666 shares of common stock at a price per share of $15.00, resulting in proceeds of approximately $10.0 million. Merck may terminate the Merck Agreement at any time with 60 days’ prior written notice. Either the Company or Merck has the right to terminate the Merck Agreement based on the other party’s uncured material breach or bankruptcy. As of September 30, 2018, there was $57.2 million of deferred revenue related to payment received by the Company under the 2018 Merck Agreement. As of September 30, 2018, the Company had $0.8 million receivable from Merck related to the Merck Agreement, which is included in accounts receivable on the balance sheet. EMD Serono Agreement The Company signed a Collaboration Agreement and a License Agreement with EMD Serono in May 2014 and September 2014, respectively, which were entered into in contemplation of each other and therefore treated as a single agreement for accounting purposes. The Collaboration Agreement was terminated upon execution of the License Agreement (the “MDA Agreement”), which agreement is to develop ADCs for multiple cancer targets. Upon signing the Collaboration Agreement, the Company received an up-front, nonrefundable, non-creditable payment totaling $10.0 million. Upon signing the MDA Agreement, the Company received an additional up-front, nonrefundable payment totaling $10.0 million and will receive financial support for research and development services to be provided by the Company, based on an agreed-upon level of full-time equivalent personnel effort and related reimbursement rate. The Company is recognizing revenues from the up-front payments ratably over an estimated five-year period starting in June 2014. Revenue for research and development services under the MDA Agreement will be recognized as revenue as the related reimbursable activities approved by EMD Serono and the Company are performed by the Company. The Company is eligible to receive up to $52.5 million for each product developed under the MDA Agreement, primarily from pre-commercial contingent payments. In addition, the Company is eligible to receive tiered royalties ranging from low-to-mid single digit percentages, along with certain additional one-time royalties, on worldwide sales of any commercial products that may result from the MDA Agreement. The MDA Agreement term expires on a product-by-product and country-by-country basis. Upon expiration, EMD Serono will have a fully paid-up, royalty-free, perpetual, and irrevocable non-exclusive license, with the right to grant sublicenses, under certain Company intellectual property rights. EMD Serono may terminate the MDA Agreement at any time with 90 days’ prior written notice or upon the inability of the Company to provide EMD Serono access to a specified number of cancer drug targets. Either the Company or EMD Serono has the right to terminate the MDA Agreement based on the other party’s uncured material breach or bankruptcy. As of September 30, 2018 and December 31, 2017, there was $2.8 million and $5.9 million, respectively, of deferred revenue related to payments received by the Company under the MDA Agreement. As of September 30, 2018 and December 31, 2017, the Company had $0.7 million and $0.8 million, respectively, of receivables from EMD Serono related to the MDA Agreement, which are included in accounts receivable on the balance sheet. SutroVax, Inc. Supply Agreement In May 2018, the Company entered into a Supply Agreement (the “Supply Agreement”) with SutroVax, Inc., (“SutroVax”), wherein SutroVax engaged the Company to supply extracts and custom reagents, as requested by SutroVax. The pricing is based on an agreed upon cost plus arrangement. For the three and nine months ended September 30, 2018, the Company recognized $0.6 million and $1.5 million, respectively, in other revenue-related parties under the Supply Agreement. As of September 30, 2018, the Company had $0.6 million receivable from SutroVax related to the Supply Agreement, which is included in accounts receivable on the balance sheet. The Leukemia & Lymphoma Society, Inc. In August 2018, the Company entered into a Research, Development and Commercialization Agreement (the “LLS Agreement”) with The Leukemia & Lymphoma Society (“LLS”), under which LLS has agreed to contribute up to $6.0 million in clinical development funding for STRO-001, the Company’s CD74-targeting ADC to treat relapsed and/or refractory multiple myeloma and non-Hodgkin lymphoma. The funding will be provided in installments based upon the achievement of funding milestones, with the initial payment of $0.5 million received by the Company upon execution of the LLS Agreement. As of September 30, 2018, the Company had received total payments from LLS of $1.0 million, which will be reflected as a reduction of research and development expenses over an estimated period ending in 2021, as eligible STRO-001 clinical development costs are incurred by the Company. In consideration for the funding to the Company under the LLS Agreement, the Company may be required to make payments to LLS based on pre-specified late-stage clinical development, regulatory and commercialization milestones and should the Company enter into certain transactions relating to STRO-001 with a third party. The Company will recognize such payments, if any, in the period they are incurred as the contingent payments are not an unconditional purchase obligation. The LLS Agreement terminates upon the earlier of (a) fulfillment of all payment obligations by both parties or (b) 12 years after the effective date. LLS may terminate the LLS Agreement at any time with 60 days’ prior written notice. Either the Company or LLS has the right to terminate the LLS Agreement based on the other party’s uncured material breach. As of September 30, 2018, there was approximately $1.0 million of other liabilities related to payments received by the Company under the LLS Agreement.

Loan and Security Agreement

Loan and Security Agreement	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Loan and Security Agreement	6. Loan and Security Agreement In August 2017, the Company entered into a loan and security agreement with Oxford and SVB under which it borrowed $15.0 million (the “August 2017 Loan”). The loan is due in 30 monthly installments from March 2019 through its repayment in August 2021, with interest-only monthly payments until March 2019. If certain qualified funding events occur, the loan will be due in 24 monthly installments from September 2019 through its repayment in August 2021, with interest-only payments until September 2019. While the aforementioned qualified funding events occurred during the quarter ended September 30, 2018, the Company intends currently to commence repayment of the loan in March 2019. The August 2017 Loan is secured by all assets of the Company, excluding intellectual property and certain other assets. The August 2017 Loan contains customary affirmative and restrictive covenants, including with respect to fundamental transactions, the incurrence of additional indebtedness, grant liens, pay any dividend or make any distributions to the Company’s holders, make investments, merge or consolidate with any other person, or engage in transactions with its affiliates, but does not include any financial covenants. The loan agreement provides that an event of default will occur if, among other triggers, there occurs any circumstances that could reasonably be expected to result in a material adverse effect on the Company’s business, operations or condition, or on its ability to perform its obligations under the loan. The loan agreement also includes customary representations and warranties, other events of default and termination provisions. The Company disclosed in its audited financial statements as of December 31, 2017 that there was substantial doubt about its ability to continue as a going concern given its continuing operating losses and its current available capital resources, which could be deemed to be an event of default if such condition was considered to have a material adverse effect on the Company’s business, operations or condition. As a result, the Company classified the entire debt balance as a current liability as of December 31, 2017 given that a determination of such an event of default is outside of the Company’s control. As of September 30, 2018, the Company has classified $3.2 million of the outstanding debt balance as current and $11.5 million as non-current, which reflects the scheduled repayment terms under the August 2017 Loan. The interest charges on the loan will be based on a floating rate that equals the greater of 7.39% or the sum of the 30-day U.S. Dollar London Interbank Offered Rate (“LIBOR”) plus 6.40%. For the nine months ended September 2018, the average interest rate was 8.28%. In addition, the Company will make a final payment equal to 3.83% of the original principal amount of the loan, or $574,500, which will be accrued over the term of the loan using the effective-interest method. As of September 30, 2018, total interest expense accrued was $0.2 million. In connection with the August 2017 Loan, the Company issued to Oxford and SVB a warrant to purchase 454,820 shares and 227,410 shares, respectively, of Series D-2 redeemable convertible preferred stock at an exercise price of $0.6596 per share (the “2017 Warrant”). If there is a subsequent convertible preferred stock or other senior equity securities financing with a per share price less than the Series D-2 redeemable convertible preferred per share price, then the warrant shall instead be to purchase such class of shares, based on the per share price of such. In May and July 2018, the Company raised a total of $85.4 million in funding through the sale and issuance of 319,305,718 shares of Series E redeemable convertible preferred stock, at $0.2674 per share. Given that the price per share of the Series E redeemable convertible preferred stock was less than the Series D-2 redeemable convertible preferred per share price, the 2017 Warrant converted into a warrant to purchase a total of 1,682,871 shares of Series E redeemable convertible preferred stock at an exercise price of $0.2674 per share. The warrants were exercisable from the date of issuance and have a 10-year term. The estimated fair value upon issuance of the 2017 Warrant based on Series D-2 redeemable convertible preferred stock was $329,000, which was recorded as redeemable convertible preferred stock warrant liability. The fair value of the warrant at the date of issuance was determined using an Option Pricing Method and was recorded as a redeemable convertible preferred stock warrant liability with an offset to debt discount on the associated borrowings on the Company’s balance sheet. The debt discount is being amortized to interest expense over the repayment period of the loan using the effective-interest method. During the three and nine months ended September 30, 2018, the Company recorded interest expense related to this loan and accretion of debt discount of $0.4 million and $1.2 million, respectively. During the three and nine months ended September 30, 2017, the Company recorded interest expense related to this loan and accretion of debt discount of $0.2 million and $0.2 million, respectively.

Related-Party Transactions

Related-Party Transactions	9 Months Ended
	Sep. 30, 2018
Related Party Transactions [Abstract]
Related Party Transactions	7. Related-Party Transactions Related party transactions with Celgene, which owned 10.5% and 15.4% of the Company’s outstanding equity interest as of September 30, 2018 and December 31, 2017, respectively, are described in Note 5. Related party transactions with Merck, which owned 12.5% and 0% of the Company’s outstanding equity interest as of September 30, 2018 and December 31, 2017, respectively, are described in Note 5. Three directors of the Company have performed consulting services for the Company, which consulting services were terminated concurrently with the Company’s IPO in September 2018. Subsequent to his appointment to the Company’s Board of Directors, the Company paid to one of the directors $10,000 and $40,000 during the three and nine months ended September 30, 2018, respectively, and $15,000 and $45,000 in during the three and nine months ended September 30, 2017, respectively. Additionally, such director was granted options to purchase 9,805 shares of the Company’s common stock from 2009 to 2015, at the then-current fair values of the common stock ranging from $4.36 to $11.98 per share, related to his consulting services, which vest ratably over four years. As of September 30, 2018, all of such shares were vested. There were zero and $250,000 in transaction advisory fees during the nine months ended September 30, 2018 and 2017, respectively, paid to a firm of which such director is a managing executive, related to the Celgene Agreements. Additional payments, based on a single digit percentage of any future payments, will be made to such transaction advisory firm upon receipt of future payments under the 2017 Celgene Agreement (see Note 5). In June 2018, the Company entered into a side letter to its consulting agreement with such director, pursuant to which the Company agreed to pay such director a one-time success fee of $400,000 within 30 days of the execution of a definitive collaboration agreement with a third-party pharmaceutical company. Following the execution of the 2018 Merck Agreement in July 2018, the Company paid such director $400,000. The Company terminated the consulting agreement and side letter with such director prior to the completion of the Company’s IPO. The Company paid to the second director $5,000 and $20,000 during the three and nine months ended September 30, 2018, respectively, and $7,500, and $22,500 during the three and nine months ended September 30, 2017, respectively. Additionally, such director was granted an option to purchase 3,269 shares of the Company’s common stock in September 2015 at the then-current fair value of the common stock, related to his consulting services, which vests ratably over four years. The Company paid to the third director $5,000 and $20,000 during the three and nine months ended September 30, 2018, respectively, and $7,500 and $17,500 during the three and nine months ended September 30, 2017, respectively. On August 30, 2010, the Company received a promissory note with recourse from its chief executive officer, which was used to purchase common stock. The principal amount of the note was approximately $0.2 million, which accrues interest at 0.53%, compounding semiannually. The note can be prepaid without penalty and is due on August 30, 2019. As of December 31, 2017, the outstanding balance was $0.2 million and the note and related interest receivable were recorded as a component of stockholders’ deficit. The promissory note was paid in full by the chief executive officer in August 2018. Investment in SutroVax, Inc. (“SutroVax”) In December 2013, the Company and Johnson & Johnson Innovation, through the Johnson & Johnson Development Corporation, provided initial co-funding for a new company, SutroVax. SutroVax leverages the Company’s proprietary integrated cell-free protein synthesis platform, XpressCF+™, to develop novel vaccines for a broad range of disease targets. The Company had $584,000 and $34,000 in receivables due from SutroVax as of September 30, 2018 and December 31, 2017, respectively, which were included in accounts receivable on the condensed balance sheet. As of September 30, 2018 and December 31, 2017, the Company held a 5.6% and 7.8% common stock ownership interest in SutroVax, respectively, on a fully-diluted basis, with a carrying value of $0 and was accounted for under the cost method. SutroVax qualifies as a variable interest entity. However, the Company maintains only shared power to direct the activities that most significantly impact the performance of SutroVax. Therefore, the Company is not considered the primary beneficiary and consolidation is not required. See Note 5, SutroVax, Inc. Supply Agreement for discussion of the supply arrangement entered into with SutroVax in May 2018 and related revenue recognized for the three months and nine months ended September 30, 2018. In May 2018, the Company entered into amendments to the license agreement with SutroVax, which primarily clarified under certain limited future circumstances SutroVax’s ability to manufacture extract pursuant to the license agreement. The Company received a warrant for the purchase of 100,000 shares of SutroVax preferred stock which was valued at $140,000. The value of warrants received has been recognized as other revenue-related parties during the nine months ended September 30, 2018 as there are no remaining deliverables under the license agreement.

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock	9 Months Ended
	Sep. 30, 2018
Temporary Equity Disclosure [Abstract]
Redeemable Convertible Preferred Stock	8. Redeemable Convertible Preferred Stock Redeemable Convertible Preferred Stock In May, June and July 2018, the Company raised an aggregate total of $85.4 million in funding through the sale and issuance of 319,305,718 shares of Series E redeemable convertible preferred stock, at $0.2674 per share. The Series E redeemable convertible preferred stock per share price was less than the conversion price per share in each of the Company’s prior redeemable convertible preferred stock financings, and therefore, each prior conversion price was lowered by applying a broad-based weighted average adjustment. With certain senior rights, preferences and privileges provided for the Series E redeemable convertible preferred stock, all prior series (Series A through Series D-2) of issued redeemable convertible preferred stock will be hereafter referred to collectively as the “Junior Preferred.” Redeemable convertible preferred stock, $0.001 par value, as of September 30, 2018 consisted of: Shares Authorized Shares Issued and Outstanding Original Issue Price Per Share Carrying Value Liquidation Preference (in thousands, except for share and per share amounts) Series A 3,503,692 3,503,692 $ 0.5900 $ 1,992 $ 2,067 Series B 24,515,966 24,345,936 0.8822 19,865 21,478 Series C 78,582,049 76,661,901 0.4797 38,036 36,775 Series C-2 8,338,892 8,338,892 0.5996 4,845 5,000 Series D 43,362,233 43,362,233 0.5996 25,900 26,000 Series D-2 18,779,561 18,097,331 0.6596 11,868 11,937 Series E 320,988,598 319,305,718 0.2674 84,739 85,382 Balance at September 30, 2018 498,070,991 493,615,703 $ 187,245 $ 188,639 Redeemable convertible preferred stock, $0.001 par value, as of December 31, 2017 consisted of: Shares Authorized Shares Issued and Outstanding Original Issue Price Per Share Carrying Value Liquidation Preference (in thousands, except for share and per share amounts) Series A 3,503,692 3,503,692 $ 0.5900 $ 1,992 $ 2,067 Series B 24,515,966 24,345,936 0.8822 19,865 21,478 Series C 78,582,049 76,102,337 0.4797 38,035 36,506 Series C-2 8,338,892 8,338,892 0.5996 4,845 5,000 Series D 43,362,233 43,362,233 0.5996 25,900 26,000 Series D-2 18,779,561 18,097,331 0.6596 11,868 11,937 Balance at December 31, 2017 177,082,393 173,750,421 $ 102,505 $ 102,988 The significant rights, preferences and privileges of the redeemable convertible preferred stock are as follows: Redemption At the election of the holders of a majority of the then-outstanding shares of preferred stock, voting together as a single class on an as-converted to common stock basis, the Company will redeem all outstanding shares of preferred stock in three equal annual installments commencing May 24, 2023, by paying in cash an amount per share equal to the original issuance prices of $0.59 per share of Series A redeemable convertible preferred stock, $0.8822 per share of Series B redeemable convertible preferred stock, $0.4797 per share of Series C redeemable convertible preferred stock, $0.5996 per share of Series C-2 redeemable convertible preferred stock, $0.5996 per share of Series D redeemable convertible preferred stock, $0.6596 per share of Series D-2 redeemable convertible preferred stock, and $0.2674 per share of Series E redeemable convertible preferred stock, plus 8% of the applicable original issuance prices per annum calculated from the original issuance date of each share of preferred stock. If funds legally available for redemption of the preferred stock are insufficient to pay such holders the full redemption prices, the Company will effect such redemption first to the holders of Series E redeemable convertible preferred stock, until the related redemption price has been paid in full, and second to the Junior Preferred holders, pro rata among such holders, based on a formula. Additionally, all shares of preferred stock are redeemable in the event of a change in control or sale of substantially all of the assets of the Company. As certain redemption events are outside the control of the Company, all preferred stock amounts have been presented outside of stockholders’ deficit. The carrying value of the redeemable convertible preferred stock has not been accreted up to its redemption value as no redemption events are considered probable as of September 30, 2018. Dividends The holders of preferred stock are entitled to receive, when and as declared by the Board of Directors, dividends at the per annum rate of $0.0472 per share of Series A redeemable convertible preferred stock, $0.07056 per share of Series B redeemable convertible preferred stock, $0.03838 per share of Series C redeemable convertible preferred stock, $0.048 per share of Series C-2 redeemable convertible preferred stock, $0.048 per share of Series D redeemable convertible preferred stock, $0.0528 per share of Series D-2 redeemable convertible preferred stock, and $0.0214 per share of Series E redeemable convertible preferred stock, prior and in preference to any declaration or payment of a dividend to the common stockholders. Additionally, the holders of Series E redeemable convertible preferred stock are entitled to receive dividends prior and in preference to Junior Preferred holders and holders of common stock of the Company. Payment of any dividends to the Junior Preferred holders shall be on a pro rata, pari passu basis in proportion to the dividend rates set forth above for each series of Junior Preferred stock. Such dividends are not cumulative, and no right to such dividends shall accrue to holders of the preferred stock unless declared by the Board of Directors. Following payment of these dividends to the preferred stockholders, any additional dividends will be payable to the holders of the Company’s common and preferred stock on an as-if-converted-to-common-stock basis. No dividends have been declared to date. Liquidation In the event of any liquidation, dissolution, or winding up of the Company, either voluntary or involuntary, the holders of Series E redeemable convertible preferred stock are entitled to receive any distribution of assets or surplus funds in an amount equal to the original issuance price of the Series E redeemable convertible preferred stock (as adjusted for stock splits, stock dividends or distributions, recapitalizations, and similar events) and all declared but unpaid dividends, if any, prior and in preference to Junior Preferred holders and holders of common stock of the Company. Junior Preferred holders shall be entitled to receive pro rata, prior and in preference to any distribution to the holders of the common stock, an amount equal to the original issuance prices of each series (in each case, as adjusted for stock splits, stock dividends or distributions, recapitalizations, and similar events) and all declared but unpaid dividends, if any. After giving effect to the liquidation preferences noted above, all of the remaining assets of the Company shall be distributed to the holders of preferred stock and common stock pro rata based on the number of shares of common stock held by each such holder, treating, for this purpose, all such securities as if they had been converted to common stock immediately prior to the liquidation event. However, if the aggregate amount that the holders of preferred stock are entitled to receive exceeds two times the applicable original issuance prices per share for such series of preferred stock plus any dividends declared but unpaid thereon (the “Maximum Participation Amount”), each holder of preferred stock shall be entitled to receive upon such liquidation the greater of (i) the Maximum Participation Amount and (ii) the amount such holder would have received if all shares of such series of preferred stock had been converted into common stock immediately prior to the liquidation event. Unless the holders of a majority of the then-outstanding shares of preferred stock, voting together as a single class on an as-converted to common stock basis, elect otherwise, any of the following events shall be treated as a liquidation: (i) any consolidation, merger, acquisition, or any other corporate reorganization in which the stockholders of the Company immediately prior to such event own less than 50% of the voting power of the surviving or successor entity or its parent immediately after such event; (ii) any transaction or series of related transactions in which in excess of 50% of the Company’s voting power is transferred; or (iii) any sale, lease, transfer, exclusive license, or other disposition of all or substantially all of the assets of the Company. Voting Each share of redeemable convertible preferred stock is entitled to voting rights equivalent to the number of shares of common stock into which each share can be converted. The holders of Series E redeemable convertible preferred stock are entitled to elect one director of the Company, the holders of Series C redeemable convertible preferred stock are entitled to elect two directors of the Company, and the holders of Series A redeemable convertible preferred stock and Series B redeemable convertible preferred stock are each entitled to elect one director of the Company. Additionally, holders of common stock are entitled to elect one director of the Company, and all stockholders can elect the balance of the total number of directors of the Company. Conversion The conversion price as of September 30, 2018 of each series of redeemable convertible preferred stock listed below is subject to adjustment upon certain dilutive events, including in the event the Company issues certain additional equity securities at a purchase price less than the current conversion price. Each share of Series E redeemable convertible preferred stock shall be convertible, at the option of the holder thereof, into such number of fully paid and nonassessable shares of common stock as is determined by dividing $0.2674 by the Series E redeemable convertible preferred stock conversion price in effect at the time of conversion. The Series E redeemable convertible preferred stock conversion price as of September 30, 2018 is $0.2674 per share of common stock. The Series E redeemable convertible preferred stock conversion price is subject to adjustment upon certain dilutive events. As of October 1, 2018, each share of Series E redeemable convertible preferred stock did convert into common stock on a 1-for-0.0275 basis. Each share of Series D-2 redeemable convertible preferred stock shall be convertible, at the option of the holder thereof, into such number of fully paid and nonassessable shares of common stock as is determined by dividing $0.6596 by the Series D-2 redeemable convertible preferred stock conversion price in effect at the time of conversion. The Series D-2 redeemable convertible preferred stock conversion price as of September 30, 2018 is $0.4336 per share of common stock. The Series D-2 redeemable convertible preferred stock conversion price is subject to adjustment upon certain dilutive events. As of October 1, 2018, each share of Series D-2 redeemable convertible preferred stock did convert into common stock on a 1-for-0.0419 basis. Each share of Series D redeemable convertible preferred stock shall be convertible, at the option of the holder thereof, into such number of fully paid and nonassessable shares of common stock as is determined by dividing $0.5996 by the Series D redeemable convertible preferred stock conversion price in effect at the time of conversion. The Series D redeemable convertible preferred stock conversion price as of September 30, 2018 is $0.4081 per share of common stock. The Series D redeemable convertible preferred stock conversion price is subject to adjustment upon certain dilutive events. As of October 1, 2018, each share of Series D redeemable convertible preferred stock did convert into common stock on a 1-for-0.0405 basis. Each share of Series C-2 redeemable convertible preferred stock shall be convertible, at the option of the holder thereof, into such number of fully paid and nonassessable shares of common stock as is determined by Each share of Series C redeemable convertible preferred stock shall be convertible, at the option of the holder thereof, into such number of fully paid and nonassessable shares of common stock as is determined by Each share of Series B redeemable convertible preferred stock shall be convertible, at the option of the holder thereof, into such number of fully paid and nonassessable shares of common stock as is determined by Each share of Series A redeemable convertible preferred stock shall be convertible, at the option of the holder thereof, into such number of fully paid and nonassessable shares of common stock as is determined by Warrants During the period from 2008 to 2012, the Company issued various warrants for the purchase of redeemable convertible preferred stock in connection with debt financings and the issuance of redeemable convertible preferred stock. In August 2017, the Company issued warrants to Oxford and SVB to purchase an aggregate of 682,230 shares of Series D-2 redeemable convertible preferred stock at an exercise price of $0.6596 per share in connection with the issuance of August 2017 Loan (see Note 6). If there is a subsequent convertible preferred stock or other senior equity securities financing with a per share price less than the Series D-2 redeemable convertible preferred per share price, then the warrant shall automatically convert to a warrant to purchase such class of shares, based on the per share price of such equity. Given that the price per share of the Series E redeemable convertible preferred stock described above was less than the price per share of the Series D-2 redeemable convertible preferred stock, the 2017 Warrant converted into a warrant to purchase a total of 1,682,871 shares of Series E redeemable convertible preferred stock at an exercise price of $0.2674 per share. The warrant is exercisable from the original date of issuance and has a 10-year term. As of September 30, 2018 and December 31, 2017, the warrants outstanding and exercisable were as follows: Shares as of Estimated Fair Value as of, Stock Expiration Date Exercise Price Per Share September 30, 2018 December 31, 2017 September 30, 2018 December, 31 2017 (in thousands except for share and per share amounts) Series B redeemable convertible preferred June 2018 $ 0.8822 – 170,030 $ – $ 116 Series C redeemable convertible preferred (1) July 2020 – November 2021 $ 0.4797 1,920,148 2,479,712 336 1,263 Series D-2 redeemable convertible preferred August 2027 $ 0.6596 – 682,230 – 329 Series E redeemable convertible preferred (1) August 2027 $ 0.2674 1,682,871 – 531 – Total 3,603,019 3,331,972 $ 867 $ 1,708 (1) On October 1, 2018, 1,232,220 shares of the Series C redeemable convertible preferred warrants will be canceled, and the remaining 687,928 shares will be converted to warrants to purchase common stock on a 1-for-0.0370 basis. All Series E redeemable convertible preferred warrants will be converted to warrants to purchase common stock at a on a 1-for-0.0275 basis. The warrants were valued using the Option Pricing Method and were estimated using the following assumptions: Nine Months Ended September 30, 2018 2017 Average expected life 3.1-8.8 years 2.5 years Expected volatility 62.42%-71.21% 64.00% Risk-free interest rate 2.88%-3.40% 1.55% Expected dividend – – Common Stock Holders of common stock are entitled to one vote per share on all matters to be voted upon by the stockholders of the Company. As of September 30, 2018, the Company had reserved common stock, on an if-converted basis, for issuance as follows: Redeemable convertible preferred stock 16,028,462 Common stock options issued and outstanding 3,383,756 Remaining shares reserved for issuance under 2018 Equity Incentive Plan 2,577,223 Warrants to purchase redeemable convertible preferred stock 117,400 Warrants to purchase common stock 942 Total 22,107,783

Employee Stock Purchase Plan an

Employee Stock Purchase Plan and Stock-Based Compensation	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Employee Stock Purchase Plan and Stock-Based Compensation	9. Employee Stock Purchase Plan and Stock-Based Compensation 2018 Employee Stock Purchase Plan In September 2018, the Company adopted the 2018 Employee Stock Purchase Plan (“ESPP”), which became effective on September 26, 2018, the day that the Form S-1 related to the IPO was declared effective, in order to enable eligible employees to purchase shares of the Company’s common stock. The Company initially reserved 230,000 shares of common stock for sale under the ESPP. The aggregate number of shares reserved for sale under the ESPP will increase automatically on January 1st of each of the first ten calendar years after the effective date by the number of shares equal to the lesser of 1% of the total outstanding shares of the Company’s common stock as of the immediately preceding December 31 (rounded to the nearest whole share) or a number of shares as may be determined by the Company’s board of directors. The aggregate number of shares issued over the term of the Company’s ESPP, subject to stock-splits, recapitalizations or similar events, may not exceed 2,300,000 shares of the Company’s common stock. 2004 Equity Incentive Plan and 2018 Equity Incentive Plan In September 2018, the Company adopted the 2018 Equity Incentive Plan (“2018 Plan”), which became effective on September 25, 2018. As a result, the Company will not grant any additional awards under the 2004 Equity Incentive Plan (“2004 Plan”). The terms of the 2004 Plan and applicable award agreements will continue to govern any outstanding awards thereunder. In addition to the shares of common stock reserved for future issuance under the 2004 Plan that were added to the 2018 Plan upon its effective date, the Company has initially reserved 2,300,000 shares of common stock for issuance under the 2018 Plan. In addition, the number of shares of common stock reserved for issuance under the 2018 Plan will automatically increase on the first day of January for a period of up to ten years, commencing on January 1, 2019, in an amount equal to 5% of the total number of shares of the Company’s capital stock outstanding on the last day of the preceding year, or a lesser number of shares determined by the Company’s board of directors. The following table summarizes option activity under the Company’s 2004 Plan and 2018 Plan: Shares Available for Grant Outstanding Options Weighted- Average Exercise Price Weighted- Average Remaining Contract Term (Years) Aggregate Intrinsic Value Balances at December 31, 2017 91,149 835,320 $ 10.31 6.84 $ 3,813 Increase in authorized shares 5,053,013 Granted (2,570,848 ) 2,570,848 $ 13.15 Exercised (19,691 ) $ 6.67 Canceled 3,721 (3,721 ) $ 13.58 Balances at September 30, 2018 2,577,035 3,382,756 8.43 $ 8,434 Exercisable at September 30, 2018 650,197 5.80 $ 3,486 Vested and expected to vest at September 30, 2018 3,172,528 8.39 $ 8,073 The aggregate intrinsic value was calculated as the difference between the exercise prices of the underlying stock option awards and the estimated fair value of the Company’s common stock on the date of exercise. For the three months and nine months ended September 30, 2018, the aggregate intrinsic value of stock options exercised was $18,275 and $153,414, respectively, determined at the date of the option exercise. For the three months and nine months ended September 30, 2017, the aggregate intrinsic value of stock options exercised was $8,431 and $51,107, respectively, determined at the date of the option exercise. Employee Stock Options Valuation For determining stock-based compensation expense, the fair-value-based measurement of each employee stock option was estimated as of the date of grant using the Black-Scholes option-pricing model with assumptions as follows: Nine Months Ended September 30, 2018 2017 Expected term (in years) 1.75-6.08 5.52-6.08 Expected volatility 48.86%-56.75% 56.52%-58.55% Risk-free interest rate 2.72%-2.97% 1.89%-2.10% Expected dividend – – Using the Black-Scholes option-valuation model, the weighted-average estimated grant-date fair value of employee stock options granted during the three months and nine months ended September 30, 2018, was $8.93 and $8.93 per share, respectively, and during the three months and nine months ended September 30, 2017, was $7.20 and $7.14 per share, respectively. The total fair value of options vested during the three and nine months ended September 30, 2018 was $0.3 million and $0.7 million, respectively, and for the three and nine months ended September 30, 2017 was $0.6 million and $1.4 million, respectively. Non-Employee Stock-Based Compensation Expense The Company remeasures the estimated fair value of the unvested portion of the award each period, until the award is fully vested. The Company believes that the fair value of the stock options is more reliably measurable than the fair value of services received. The fair value of options granted to non-employees was estimated using the Black-Scholes method. The stock-based compensation expense related to non-employees for the three and nine months ended September 30, 2018 was $1,019 and $20,795, respectively, and for the three and nine months ended September 30, 2017 was $19,055 and $58,212, respectively. Stock-Based Compensation Expense Total stock-based compensation expense recognized was as follows: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Research and development $ 65 $ 19 $ 160 $ 91 General and administrative 256 574 642 1,035 Total $ 321 $ 593 $ 802 $ 1,126 As of September 30, 2018, there was approximately $22.2 million of total unrecognized compensation cost related to the unvested stock options granted under the Company’s Plans. The remaining unrecognized compensation cost is expected to be recognized over a weighted-average period of 3.6 years. Early Exercise of Options Certain stock options granted under the Company’s stock option Plan provide option holders the right to elect to exercise unvested options in exchange for restricted common stock. A summary of the restricted stock shares issued under the Company’s Plan is as follows: Shares Balance as of December 31, 2017 2,374 Vested (2,374 ) Balance as of September 30, 2018 – The shares were subject to repurchase by the Company at the original exercise price in the event the optionee’s employment was terminated either voluntarily or involuntarily. The repurchase right to these shares generally lapsed 25% after one year, and the remainder lapsed ratably over three years thereafter. The Company treated cash received from the exercise of unvested options as a refundable deposit, shown as a liability in its balance sheets. As of September 30, 2018 and December 31, 2017, the Company included cash received for the early exercise of options of approximately zero 2017 Call Option Plan In February 2017, the Company adopted a 2017 Call Option Plan to grant selected employees, officers, directors and consultants (collectively, the “Participants”) options to purchase shares of the common stock of SutroVax, an unconsolidated investee of the Company (see Note 7). The Company has reserved 450,000 shares of SutroVax common stock as of September 30, 2018 for issuance under the program. The call options vest 25% on each of January 1, 2017, 2018, 2019, and 2020, and expire one year from the vesting date. Using the Black-Scholes option-valuation model, the call options are measured at fair value on grant date and at each reporting period prior to their vesting, with cost recognized over the requisite service period as compensation cost. Any changes in the fair value subsequent to the vesting date are recognized in other income (expense), net in the statement of operations. The amounts recognized as compensation expense related to the 2017 Call Option Plan were $19,000 and $50,000 for the three and nine months ended September 30, 2018, respectively, and $15,000 and $65,000 for the three and nine months ended September 30, 2017, respectively. The amounts recognized as other income (expense) related to the 2017 Call Option Plan were $6,000 and $10,000 for the three and nine months ended September 30, 2018, respectively, and $6,000 and ($10,000) for the three and nine months ended September 30, 2017, respectively.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Net Loss Per Share	10. Net Loss Per Share The following table sets forth the computation of the Company’s basic and diluted net loss per share. Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Numerator: Net loss, basic and diluted $ (10,237 ) $ (1,418 ) $ (33,824 ) $ (4,344 ) Net loss $ (10,237 ) $ (1,418 ) $ (33,824 ) $ (4,344 ) Denominator: Shares issued in computing net loss per share, basic and diluted 481,613 451,550 476,023 444,594 Net loss per share, basic and diluted $ (21.26 ) $ (3.14 ) $ (71.06 ) $ (9.77 ) The following common stock equivalents were excluded from the computation of diluted net loss per share for the periods ended September 30, 2018 and 2017, because including them would have been antidilutive: As of September 30, 2018 2017 Redeemable convertible preferred stock 16,028,462 5,063,404 Options to purchase common stock 3,383,756 840,160 Warrants to purchase redeemable convertible preferred stock 117,400 93,527 Warrants to purchase common stock 942 1,099 Early exercised shares of common stock – 5,914 Total 19,530,560 6,004,104 Shares of common stock subject to repurchase are excluded from the computation of weighted-average shares as the continued vesting of such shares is contingent upon the holders’ continued service to the Company. For the computation of net loss per share for the nine months ended September 30, 2018 and 2017, zero and 5,914 shares subject to repurchase, respectively, were excluded from the computation of net loss per share.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2018
Subsequent Events [Abstract]
Subsequent Events	11. Subsequent Events Initial Public Offering On September 26, 2018, the Company’s registration statement on Form S-1 (File Nos. 333-227103 and 333-227548) relating to its IPO of its common stock was declared effective by the SEC and the shares of its common stock began trading on the Nasdaq Global Market on September 27, 2018. The public offering price of the shares sold in the IPO was $15.00 per share. The IPO closed on October 1, 2018, pursuant to which the Company sold 5,667,000 shares of common stock, for gross proceeds of approximately $85.0 million. The Company received net proceeds from the IPO of approximately $74.4 million, after underwriting discounts, commissions and estimated offering expenses. In addition to the shares of common stock sold in the IPO, the Company concurrently sold in a private placement to Merck, 666,666 shares of common stock at the IPO offering price of $15.00 per share, for proceeds of approximately $10.0 million. • Immediately prior to the completion of the IPO on October 1, 2018, all outstanding shares of redeemable convertible preferred stock were converted into 16,028,462 shares of common stock. Subsequent to the closing of the IPO, there were no shares of redeemable convertible preferred stock outstanding. The condensed financial statements as of September 30, 2018, including share and per share amounts, do not give effect to the IPO, or the conversion of the redeemable convertible preferred stock, as the IPO and such conversions were completed subsequent to September 30, 2018; and • Upon completion of the IPO, all outstanding warrants to purchase 2,370,799 shares of redeemable convertible preferred stock automatically converted into warrants to purchase 71,812 shares of common stock.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation and Use of Estimates	Basis of Presentation and Use of Estimates The accompanying financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”). The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. The Company bases its estimates on historical experience and market-specific or other relevant assumptions that it believes are reasonable under the circumstances. The amounts of assets and liabilities reported in the Company’s balance sheets and the amount of expenses and income reported for each of the periods presented are affected by estimates and assumptions, which are used for, but are not limited to, determining research and development periods under multiple element arrangements, stock-based compensation expense, fair value of redeemable convertible preferred stock and warrant liabilities, fair value of common stock, income taxes and certain accrued liabilities. Actual results could differ from such estimates or assumptions.
Unaudited Interim Condensed Financial Statements	Unaudited Interim Condensed Financial Statements The interim condensed balance sheet as of September 30, 2018, the condensed statements of operations and comprehensive loss for the three and nine months ended September 30, 2018 and 2017, and the condensed statements of cash flows for the nine months ended September 30, 2018 and 2017 are unaudited. The unaudited interim condensed financial statements have been prepared on the same basis as the annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s financial position as of September 30, 2018, its results of operations and comprehensive loss for the three and nine months ended September 30, 2018 and 2017, and cash flows for the nine months ended September 30, 2018 and 2017. The financial data and the other financial information contained in these notes to the condensed financial statements related to the three-month and nine-month periods are also unaudited. The results of operations for the three and nine months ended September 30, 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018 or for any other future annual or interim period. The condensed balance sheet as of December 31, 2017 included herein was derived from the audited financial statements as of that date. These condensed financial statements should be read in conjunction with the Company's audited financial statements included in the prospectus dated September 26, 2018 that forms a part of the Company's registration statements on Form S-1 (File Nos 333-227103 and 333-227548), as filed with the SEC pursuant to Rule 424(b)(4) promulgated under the Securities Act of 1933, as amended.
Cash, Cash Equivalents, and Restricted Cash	Cash, Cash Equivalents, and Restricted Cash The Company considers all highly liquid investments with original maturities of 90 days or less from the date of purchase to be cash equivalents. Under certain lease and credit agreements, the Company has pledged cash and cash equivalents as collateral. Restricted cash related to such agreements was $15,000 as of both September 30, 2018 and December 31, 2017. 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 statements of cash flows. September 30, 2018 2017 (in thousands) Cash and cash equivalents $ 41,353 $ 25,407 Restricted cash 15 15 Total cash, cash equivalents, and restricted cash shown in the statements of cash flows $ 41,368 $ 25,422
Fair Value Measurements	Fair Value Measurements 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 establishes a fair value hierarchy that requires an entity to maximize the use of observable inputs, where available, and minimizes the use of unobservable inputs when measuring fair value. The Company determined the fair value of financial assets and liabilities using the fair value hierarchy that describes three levels of inputs that may be used to measure fair value, as follows: Level 1—Quoted prices in active markets for identical assets and liabilities; Level 2—Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities; 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. The carrying amounts of accounts receivable, prepaid expenses, accounts payable and accrued compensation approximate fair value due to the short-term nature of these items. The fair value of the Company’s financial assets and liabilities is measured on a recurring basis by level within the fair value hierarchy. See Note 3. The fair value of the Company’s outstanding loan (See Note 6) is estimated using the net present value of the payments, discounted at an interest rate that is consistent with market interest rate, which is a Level 2 input. The estimated fair value of the Company’s outstanding loan approximates the carrying amount, as the loan bears a floating rate that approximates the market interest rate.
Deferred Offering Costs	Deferred Offering Costs The Company has deferred offering costs consisting of legal, accounting and other fees and costs directly attributable to the Company’s IPO, which was completed on October 1, 2018. The deferred offering costs will be offset against the gross proceeds of the IPO. As of September 30, 2018 and December 31, 2017, $4.6 million and $0.5 million, respectively, of deferred offering costs were recorded within other long-term assets on the balance sheet.
Redeemable Convertible Preferred Stock Warrants	Redeemable Convertible Preferred Stock Warrants The Company accounts for its redeemable convertible preferred stock warrants as a liability, and they are recorded at their estimated fair value, because the warrants may conditionally obligate the Company to transfer assets at some point in the future. At the end of each reporting period, changes in the estimated fair value during the period are recorded in other income (expense), net in the statement of operations. The Company will continue to adjust the liability for changes in estimated fair value until the earlier of the expiration of the warrants, exercise of the warrants, or conversion of the redeemable convertible preferred stock warrants into common stock warrants upon the completion of a liquidation event, including the completion of an IPO, which occurred on October 1, 2018.
Revenue Recognition	Revenue Recognition Revenue is recognized when persuasive evidence of an arrangement exists, delivery has occurred or services have been rendered, the price is fixed or determinable and collectability is reasonably assured. For multiple-element arrangements, each deliverable within a multiple-deliverable revenue arrangement is accounted for as a separate unit of accounting if both of the following criteria are met: the delivered item or items has value to the customer on a stand-alone basis; and (ii) for an arrangement that includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in management’s control. The Company recognizes revenue from milestone payments when: (i) the milestone event is substantive and its achievability has substantive uncertainty at the inception of the agreement, and the Company has completed its performance obligations related to the achievement of the milestone. Milestone payments are considered substantive if all of the following conditions are met: the milestone payment (a) is commensurate with either the Company’s performance subsequent to the inception of the arrangement to achieve the milestone or the enhancement of the value of the delivered item or items as a result of a specific outcome resulting from the Company’s performance subsequent to the inception of the arrangement to achieve the milestone, (b) relates solely to past performance, and (c) is reasonable relative to all of the deliverables and payment terms (including other potential milestone consideration) within the arrangement. Determining whether and when these revenue recognition criteria have been satisfied often involves assumptions and judgments that can have a significant impact on the timing and amount of reported revenue. Changes in assumptions or judgments or changes to the elements in an arrangement could cause a material increase or decrease in the amount of revenue that is reported in a particular period. Under certain collaborative arrangements, the Company is entitled to payments for certain research and development activities and for providing product and other related materials. The Company’s policy is to account for such payments by its collaboration partners as collaboration revenue.
Research and Development	Research and Development Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities: salaries, employee benefits, laboratory supplies, outsourced research and development expenses, professional services and allocated facilities-related costs. Amounts incurred in connection with collaboration arrangements are also included as a research and development expense. Nonrefundable advance payments for goods or services that will be used or rendered for future research and development activities are deferred and capitalized and recognized as an expense as the goods are delivered or the related services are performed. For outsourced research and development expenses, such as professional fees payable to third parties for preclinical studies, clinical trials and research services, and other consulting costs, the Company estimates the 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. If the actual timing of the performance of services or the level of effort varies from the original estimates, the Company will adjust the accrual accordingly. 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.
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 for the period, without consideration for potential dilutive common shares. Basic net loss per share is the same as diluted net loss per share as the inclusion of all potential dilutive common shares would have been anti-dilutive.
Recent Accounting Pronouncements	Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (“FASB”), or other standard setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the impact of recently issued standards that are not yet effective will not have a material impact on the Company’s financial statements upon adoption. Under the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”), the Company meets the definition of an emerging growth company, and has elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act. New Accounting Pronouncements Not Yet Adopted In May 2014, the FASB issued ASU No. 2014-09 (Topic 606), Revenue from Contracts with Customers. In August 2015, the FASB issued ASU No. 2015-14 (Topic 606), Revenue from Contracts with Customers: Deferral of the Effective Date, which delayed the effective date of ASU 2014-09 by one year. ASU 2014-09, as amended, became effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. For entities other than public entities, the standard is effective for fiscal years beginning after December 15, 2018, and interim periods beginning after December 15, 2019. Early adoption is permitted. ASU 2014-09 also permits two methods of adoption: retrospectively to each prior reporting period presented (full retrospective method), or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application (the modified retrospective method). The Company is in the process of evaluating the effect this guidance will have on revenue recognition for its collaboration and license agreements. The core principle of ASU 2014-09 is that an entity should recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. ASU 2014-09 defines a five-step process to achieve this core principle and, in doing so, it is possible more judgment and estimates may be required within the revenue recognition process than required under existing U.S. generally accepted accounting pronouncements. All of the Company’s revenue is currently generated from up-front payments, research and development services, and milestone and contingent payments under its collaboration arrangements. The Company is currently evaluating its collaboration agreements to determine the impact of adopting ASU 2014-09, inclusive of available transitional methods, on its financial statements and related disclosures. In January 2016, the FASB issued ASU 2016-01 (Topic 825), Recognition and Measurement of Financial Assets and Financial Liabilities, which will change how to recognize, measure, present and make disclosures about certain financial assets and financial liabilities. Under ASU 2016-01, if an entity designates a financial liability under the fair value option (“FVO”) in accordance with ASC 825, the entity shall measure the financial liability at fair value with qualifying changes in fair value recognized in net income. The entity shall present separately in other comprehensive income the portion of the total change in the fair value of the liability that results from a change in the instrument-specific credit risk. For public business entities, ASU 2016-01 is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. For all other entities, the guidance is effective for fiscal years beginning after December 15, 2018, and interim periods within fiscal years beginning after December 15, 2019. All entities can early adopt the provision related to financial liabilities measured using the FVO in ASC 825 for financial statements of annual or interim periods that have not yet been issued or made available for issuance. The Company does not expect the adoption of this amendment will have a material impact on its financial statements. In February 2016, the FASB issued Accounting Standards Update (ASU) No. 2016-02, Leases Leases Leases In August 2016, the FASB issued ASU 2016-15 (“ASC Topic 230”), Classification of Certain Cash Receipts and Cash Payments . The new guidance clarifies the classification of certain cash receipts and cash payments in the statement of cash flows, including debt prepayment or extinguishment costs, settlement of contingent consideration arising from a business combination, insurance settlement proceeds, and distributions from certain equity method investees. ASU 2016-15 is effective for annual periods beginning after December 15, 2017. Early adoption is permitted. The Company is in the process of assessing the impact, if any, of this ASU on its financial statements. The Company does not expect that the adoption of this amendment will have a material impact on its financial statements. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808), Clarifying the interaction between Topic 808 and Topic 606

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 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 statements of cash flows. September 30, 2018 2017 (in thousands) Cash and cash equivalents $ 41,353 $ 25,407 Restricted cash 15 15 Total cash, cash equivalents, and restricted cash shown in the statements of cash flows $ 41,368 $ 25,422

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities Measured on Recurring Basis	The following table sets forth the fair value of the Company’s financial assets and liabilities measured on a recurring basis by level within the fair value hierarchy: September 30, 2018 Total Level 1 Level 2 Level 3 (in thousands) Assets: Money market funds $ 31,879 $ 31,879 $ – $ – Commercial paper 34,939 – 34,939 – Corporate debt securities 15,774 – 15,774 – Asset-backed securities 16,864 – 16,864 – U.S. government agency securities 23,767 – 23,767 – Total $ 123,223 $ 31,879 $ 91,344 $ – Liabilities: Redeemable convertible preferred stock warrant liability $ 867 $ – $ – $ 867 Total $ 867 $ – $ – $ 867 December 31, 2017 Total Level 1 Level 2 Level 3 (in thousands) Assets: Money market funds $ 6,578 $ 6,578 $ – $ – Commercial paper 7,689 – 7,689 – Corporate debt securities 800 – 800 – U.S. government agency securities 3,893 – 3,893 – Total $ 18,960 $ 6,578 $ 12,382 – Liabilities: Redeemable convertible preferred stock warrant liability $ 1,708 $ – $ – $ 1,708 Total $ 1,708 $ – $ – $ 1,708
Summary of Changes in Estimated Fair Value of Redeemable Convertible Preferred Stock Warrant Liability	The following table sets forth a summary of the changes in the estimated fair value of the Company’s redeemable convertible preferred stock warrant liability: Redeemable Convertible Preferred Stock Warrant Liability (in thousands) Balance as of December 31, 2017 $ 1,708 Proceeds from issuances of common stock upon exercise of warrants (2 ) Changes in estimated fair value of warrant liability included in other income (expense), net (839 ) Balance as of September 30, 2018 $ 867

Cash Equivalents and Marketab_2

Cash Equivalents and Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2018
Cash And Cash Equivalents [Abstract]
Schedule of Cash Equivalents and Marketable Securities	Cash equivalents and marketable securities consisted of the following: September 30, 2018 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market funds $ 31,879 $ – $ – $ 31,879 Commercial paper 34,939 – – 34,939 Corporate debt securities 15,781 1 (8 ) 15,774 Asset-based securities 16,876 – (12 ) 16,864 U.S. government agencies 23,776 – (9 ) 23,767 Total 123,251 1 (29 ) 123,223 Less amounts classified as cash equivalents (41,626 ) – – (41,626 ) Total marketable securities $ 81,625 $ 1 $ (29 ) $ 81,597 December 31, 2017 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market funds $ 6,578 $ – $ – $ 6,578 Commercial paper 7,689 – – 7,689 Corporate debt securities 800 – – 800 U.S. government agencies 3,893 – – 3,893 Total 18,960 – – 18,960 Less amounts classified as cash equivalents (18,960 ) – – (18,960 ) Total marketable securities $ – $ – $ – $ –

Collaboration Agreements and _2

Collaboration Agreements and Supply Agreements (Tables)	9 Months Ended
	Sep. 30, 2018
Collaboration Agreements And Supply Agreements [Abstract]
Summary of Recognized Revenue	The Company has recognized revenue as follows: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (in thousands) (in thousands) Collaboration revenue: Celgene Corporation (“Celgene”)—related party: Amortization of up-front payment $ 1,655 $ 2,642 $ 4,912 $ 16,355 Research and development services 5 – 103 – Milestones and contingent payments – 13,112 25,937 Total 1,660 15,754 5,015 42,292 Merck Sharp & Dohme Corporation (“Merck”)—related party: Amortization of up-front payment 2,818 – 2,818 – Research and development services 696 – 696 – Total 3,514 – 3,514 – Merck KGaA, Darmstadt, Germany (operating in the United States and Canada under the name “EMD Serono”): Amortization of up-front payment 1,038 1,030 3,104 3,090 Research and development services 712 715 2,322 2,319 Total 1,750 1,745 5,426 5,409 Total collaboration revenue $ 6,924 $ 17,499 $ 13,955 $ 47,701 Other revenue—related parties: Celgene Corporation: Development and manufacturing services and clinical product supply $ 330 $ – $ 3,894 $ – SutroVax: Supply and other 582 – 1,484 – Total other revenue—related parties $ 912 $ – $ 5,378 $ – Total revenue $ 7,836 $ 17,499 $ 19,333 $ 47,701

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock (Tables)	9 Months Ended
	Sep. 30, 2018
Schedule of Redeemable Convertible Preferred Stock	Redeemable convertible preferred stock, $0.001 par value, as of September 30, 2018 consisted of: Shares Authorized Shares Issued and Outstanding Original Issue Price Per Share Carrying Value Liquidation Preference (in thousands, except for share and per share amounts) Series A 3,503,692 3,503,692 $ 0.5900 $ 1,992 $ 2,067 Series B 24,515,966 24,345,936 0.8822 19,865 21,478 Series C 78,582,049 76,661,901 0.4797 38,036 36,775 Series C-2 8,338,892 8,338,892 0.5996 4,845 5,000 Series D 43,362,233 43,362,233 0.5996 25,900 26,000 Series D-2 18,779,561 18,097,331 0.6596 11,868 11,937 Series E 320,988,598 319,305,718 0.2674 84,739 85,382 Balance at September 30, 2018 498,070,991 493,615,703 $ 187,245 $ 188,639 Redeemable convertible preferred stock, $0.001 par value, as of December 31, 2017 consisted of: Shares Authorized Shares Issued and Outstanding Original Issue Price Per Share Carrying Value Liquidation Preference (in thousands, except for share and per share amounts) Series A 3,503,692 3,503,692 $ 0.5900 $ 1,992 $ 2,067 Series B 24,515,966 24,345,936 0.8822 19,865 21,478 Series C 78,582,049 76,102,337 0.4797 38,035 36,506 Series C-2 8,338,892 8,338,892 0.5996 4,845 5,000 Series D 43,362,233 43,362,233 0.5996 25,900 26,000 Series D-2 18,779,561 18,097,331 0.6596 11,868 11,937 Balance at December 31, 2017 177,082,393 173,750,421 $ 102,505 $ 102,988
Schedule of Warrants Outstanding And Exercisable	As of September 30, 2018 and December 31, 2017, the warrants outstanding and exercisable were as follows: Shares as of Estimated Fair Value as of, Stock Expiration Date Exercise Price Per Share September 30, 2018 December 31, 2017 September 30, 2018 December, 31 2017 (in thousands except for share and per share amounts) Series B redeemable convertible preferred June 2018 $ 0.8822 – 170,030 $ – $ 116 Series C redeemable convertible preferred (1) July 2020 – November 2021 $ 0.4797 1,920,148 2,479,712 336 1,263 Series D-2 redeemable convertible preferred August 2027 $ 0.6596 – 682,230 – 329 Series E redeemable convertible preferred (1) August 2027 $ 0.2674 1,682,871 – 531 – Total 3,603,019 3,331,972 $ 867 $ 1,708 (1) On October 1, 2018, 1,232,220 shares of the Series C redeemable convertible preferred warrants will be canceled, and the remaining 687,928 shares will be converted to warrants to purchase common stock on a 1-for-0.0370 basis. All Series E redeemable convertible preferred warrants will be converted to warrants to purchase common stock at a on a 1-for-0.0275 basis.
Schedule of Common Stock Reserved for Future Issuance	As of September 30, 2018, the Company had reserved common stock, on an if-converted basis, for issuance as follows: Redeemable convertible preferred stock 16,028,462 Common stock options issued and outstanding 3,383,756 Remaining shares reserved for issuance under 2018 Equity Incentive Plan 2,577,223 Warrants to purchase redeemable convertible preferred stock 117,400 Warrants to purchase common stock 942 Total 22,107,783
Warrants
Schedule of Warrants Valued Using Option Pricing Method	The warrants were valued using the Option Pricing Method and were estimated using the following assumptions: Nine Months Ended September 30, 2018 2017 Average expected life 3.1-8.8 years 2.5 years Expected volatility 62.42%-71.21% 64.00% Risk-free interest rate 2.88%-3.40% 1.55% Expected dividend – –

Employee Stock Purchase Plan _2

Employee Stock Purchase Plan and Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2018
Summary of Option Activity	The following table summarizes option activity under the Company’s 2004 Plan and 2018 Plan: Shares Available for Grant Outstanding Options Weighted- Average Exercise Price Weighted- Average Remaining Contract Term (Years) Aggregate Intrinsic Value Balances at December 31, 2017 91,149 835,320 $ 10.31 6.84 $ 3,813 Increase in authorized shares 5,053,013 Granted (2,570,848 ) 2,570,848 $ 13.15 Exercised (19,691 ) $ 6.67 Canceled 3,721 (3,721 ) $ 13.58 Balances at September 30, 2018 2,577,035 3,382,756 8.43 $ 8,434 Exercisable at September 30, 2018 650,197 5.80 $ 3,486 Vested and expected to vest at September 30, 2018 3,172,528 8.39 $ 8,073
Schedule of Stock-Based Compensation Expense Recognized	Total stock-based compensation expense recognized was as follows: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Research and development $ 65 $ 19 $ 160 $ 91 General and administrative 256 574 642 1,035 Total $ 321 $ 593 $ 802 $ 1,126
Summary of Restricted Stock Activity	A summary of the restricted stock shares issued under the Company’s Plan is as follows: Shares Balance as of December 31, 2017 2,374 Vested (2,374 ) Balance as of September 30, 2018 –
Employee Stock Options
Schedule of Warrants Valued Using Option Pricing Method	For determining stock-based compensation expense, the fair-value-based measurement of each employee stock option was estimated as of the date of grant using the Black-Scholes option-pricing model with assumptions as follows: Nine Months Ended September 30, 2018 2017 Expected term (in years) 1.75-6.08 5.52-6.08 Expected volatility 48.86%-56.75% 56.52%-58.55% Risk-free interest rate 2.72%-2.97% 1.89%-2.10% Expected dividend – –

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Summary of Computation of Company's Basic and Diluted Net Loss per Share	The following table sets forth the computation of the Company’s basic and diluted net loss per share. Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Numerator: Net loss, basic and diluted $ (10,237 ) $ (1,418 ) $ (33,824 ) $ (4,344 ) Net loss $ (10,237 ) $ (1,418 ) $ (33,824 ) $ (4,344 ) Denominator: Shares issued in computing net loss per share, basic and diluted 481,613 451,550 476,023 444,594 Net loss per share, basic and diluted $ (21.26 ) $ (3.14 ) $ (71.06 ) $ (9.77 )
Summary of Common Stock Equivalents of Antidilutive Securities Excluded from Computation of Diluted Net Loss per Share	The following common stock equivalents were excluded from the computation of diluted net loss per share for the periods ended September 30, 2018 and 2017, because including them would have been antidilutive: As of September 30, 2018 2017 Redeemable convertible preferred stock 16,028,462 5,063,404 Options to purchase common stock 3,383,756 840,160 Warrants to purchase redeemable convertible preferred stock 117,400 93,527 Warrants to purchase common stock 942 1,099 Early exercised shares of common stock – 5,914 Total 19,530,560 6,004,104

Organization and Principal Ac_2

Organization and Principal Activities - Additional Information (Details)	Oct. 01, 2018USD ($)$ / sharesshares	Sep. 14, 2018	Jul. 31, 2018	Sep. 30, 2018USD ($)Segmentshares	Aug. 31, 2018USD ($)	Dec. 31, 2017USD ($)shares	Aug. 31, 2017USD ($)
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Date of incorporation				Apr. 21, 2003
Headquartered in state, country name				South San Francisco, California
Number of operating segments | Segment				1
Reverse split of common stock				Company effected a reverse split of all shares of its common stock at a ratio of 36.3-for-1. Upon the effectiveness of the reverse stock split, (i) all shares of outstanding common stock were adjusted; (ii) the number of shares of common stock for which each outstanding option to purchase common stock is exercisable were adjusted; (iii) the exercise price of each outstanding option to purchase common stock were adjusted; (iv) the conversion ratio for each share of outstanding redeemable convertible preferred stock which is convertible into the Company’s common stock was proportionately reduced; (v) the number of shares of common stock for which each outstanding warrant to purchase common stock is exercisable was proportionally decreased; (vi) the conversion ratio for each outstanding warrant to purchase redeemable convertible preferred stock which is convertible into warrants to purchase the Company’s common stock after the offering was proportionally decreased; and (vii) the exercise price of each outstanding warrant was proportionally increased. All of the outstanding common stock share numbers (including shares of common stock subject to the Company’s options, as converted for the outstanding redeemable convertible preferred stock shares and warrants), share prices, exercise prices and per share amounts contained in the financial statements have been retroactively adjusted in the financial statements to reflect this reverse stock split for all periods presented. The par value per share and the authorized number of shares of common stock and redeemable convertible preferred stock were not adjusted as a result of the reverse stock split.
Accumulated deficit				$ 148,835,000		$ 115,011,000
Unrestricted cash, cash equivalents and marketable securities				$ 123,000,000
Substantial doubt about going concern, within one year				false
August 2017 Loan
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Debt, current				$ 3,200,000
Loan and Security Agreement | Oxford Finance LLC and Silicon Valley Bank | August 2017 Loan
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Borrowed amount					$ 15,000,000		$ 15,000,000
Minimum
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Going concern period from issuance date of unaudited interim condensed financial statements				1 year
Redeemable Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Redeemable convertible preferred stock, shares outstanding | shares				493,615,703		173,750,421
Series E Redeemable Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Reverse stock split conversion ratio			0.8374
Stock split description				Company’s board of directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a 1-for-1.1940912491 split (“Split”) of shares of the Company’s Series E redeemable convertible preferred stock, which was effected on July 26, 2018.
Common Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Reverse stock split conversion ratio		36.3
Subsequent Event | IPO | Redeemable Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Redeemable convertible preferred stock, shares outstanding | shares	0
Subsequent Event | IPO | Common Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Common stock price, per share | $ / shares	$ 15
Issuance of common stock, shares | shares	5,667,000
Gross proceeds from issuance of initial public offering	$ 85,000,000
Net proceeds from issuance of initial public offering	$ 74,400,000
Subsequent Event | IPO | Common Stock | Redeemable Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Convertible preferred stock converted into common stock | shares	16,028,462
Subsequent Event | Private Placement | Merck | Common Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Common stock price, per share | $ / shares	$ 15
Issuance of common stock, shares | shares	666,666
Gross proceeds from issuance of private placement	$ 10,000,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Accounting Policies [Abstract]
Restricted cash	$ 15	$ 15	$ 15
Deferred offering costs	$ 4,600	$ 500

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash and Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 41,353	$ 22,020	$ 25,407
Restricted cash	15	$ 15	15
Total cash, cash equivalents, and restricted cash shown in the statements of cash flows	$ 41,368		$ 25,422

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Redeemable Convertible Preferred Stock Warrant Liability
Liabilities:
Total	$ 867	$ 1,708
Fair Value Measurements Recurring
Assets:
Total	123,223	18,960
Liabilities:
Total	867	1,708
Fair Value Measurements Recurring | Redeemable Convertible Preferred Stock Warrant Liability
Liabilities:
Total	867	1,708
Fair Value Measurements Recurring | Level 1
Assets:
Total	31,879	6,578
Liabilities:
Total	0	0
Fair Value Measurements Recurring | Level 1 | Redeemable Convertible Preferred Stock Warrant Liability
Liabilities:
Total	0	0
Fair Value Measurements Recurring | Level 2
Assets:
Total	91,344	12,382
Liabilities:
Total	0	0
Fair Value Measurements Recurring | Level 2 | Redeemable Convertible Preferred Stock Warrant Liability
Liabilities:
Total	0	0
Fair Value Measurements Recurring | Level 3
Assets:
Total	0	0
Liabilities:
Total	867	1,708
Fair Value Measurements Recurring | Level 3 | Redeemable Convertible Preferred Stock Warrant Liability
Liabilities:
Total	867	1,708
Fair Value Measurements Recurring | Money Market Funds
Assets:
Total	31,879	6,578
Fair Value Measurements Recurring | Money Market Funds | Level 1
Assets:
Total	31,879	6,578
Fair Value Measurements Recurring | Money Market Funds | Level 2
Assets:
Total	0	0
Fair Value Measurements Recurring | Money Market Funds | Level 3
Assets:
Total	0	0
Fair Value Measurements Recurring | Commercial Paper
Assets:
Total	34,939	7,689
Fair Value Measurements Recurring | Commercial Paper | Level 1
Assets:
Total	0	0
Fair Value Measurements Recurring | Commercial Paper | Level 2
Assets:
Total	34,939	7,689
Fair Value Measurements Recurring | Commercial Paper | Level 3
Assets:
Total	0	0
Fair Value Measurements Recurring | Corporate Debt Securities
Assets:
Total	15,774	800
Fair Value Measurements Recurring | Corporate Debt Securities | Level 1
Assets:
Total	0	0
Fair Value Measurements Recurring | Corporate Debt Securities | Level 2
Assets:
Total	15,774	800
Fair Value Measurements Recurring | Corporate Debt Securities | Level 3
Assets:
Total	0	0
Fair Value Measurements Recurring | Asset-backed Securities
Assets:
Total	16,864
Fair Value Measurements Recurring | Asset-backed Securities | Level 1
Assets:
Total	0
Fair Value Measurements Recurring | Asset-backed Securities | Level 2
Assets:
Total	16,864
Fair Value Measurements Recurring | Asset-backed Securities | Level 3
Assets:
Total	0
Fair Value Measurements Recurring | U.S. Government Agency Securities
Assets:
Total	23,767	3,893
Fair Value Measurements Recurring | U.S. Government Agency Securities | Level 1
Assets:
Total	0	0
Fair Value Measurements Recurring | U.S. Government Agency Securities | Level 2
Assets:
Total	23,767	3,893
Fair Value Measurements Recurring | U.S. Government Agency Securities | Level 3
Assets:
Total	$ 0	$ 0

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Fair value assets transferred from level 1 to level 2	$ 0	$ 0
Fair value assets transferred from level 2 to level 1	0	0
Fair value liabilities transferred from level 1 to level 2	0	0
Fair value liabilities transferred from level 2 to level 1	0	0
Fair value assets transferred into level 3	0	0
Fair value assets transferred out of level 3	0	0
Fair value liabilities transferred into level 3	0	0
Fair value liabilities transferred out of level 3	$ 0	$ 0

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Changes in Estimated Fair Value of Redeemable Convertible Preferred Stock Warrant Liability (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Proceeds from issuances of common stock upon exercise of warrants	$ (2)	$ 0
Redeemable Convertible Preferred Stock Warrant Liability
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Balance as of December 31, 2017	1,708
Proceeds from issuances of common stock upon exercise of warrants	(2)
Changes in estimated fair value of warrant liability included in other income (expense), net	(839)
Balance as of September 30, 2018	$ 867

Cash Equivalents and Marketab_3

Cash Equivalents and Marketable Securities - Schedule of Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Cash And Cash Equivalents [Line Items]
Amortized Cost Basis	$ 123,251	$ 18,960
Unrealized Gains	1	0
Unrealized Losses	(29)	0
Fair Value	123,223	18,960
Less amounts classified as cash equivalents, Amortized Cost Basis	(41,353)	(22,020)	$ (25,407)
Marketable Securities
Cash And Cash Equivalents [Line Items]
Amortized Cost Basis	81,625	0
Unrealized Gains	1	0
Unrealized Losses	(29)	0
Fair Value	81,597	0
Cash Equivalents
Cash And Cash Equivalents [Line Items]
Less amounts classified as cash equivalents, Amortized Cost Basis	(41,626)	(18,960)
Less amounts classified as cash equivalents, Fair Value	(41,626)	(18,960)
Money Market Funds
Cash And Cash Equivalents [Line Items]
Amortized Cost Basis	31,879	6,578
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	31,879	6,578
Commercial Paper
Cash And Cash Equivalents [Line Items]
Amortized Cost Basis	34,939	7,689
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	34,939	7,689
Corporate Debt Securities
Cash And Cash Equivalents [Line Items]
Amortized Cost Basis	15,781	800
Unrealized Gains	1	0
Unrealized Losses	(8)	0
Fair Value	15,774	800
Asset-backed Securities
Cash And Cash Equivalents [Line Items]
Amortized Cost Basis	16,876
Unrealized Gains	0
Unrealized Losses	(12)
Fair Value	16,864
U.S. Government Agencies
Cash And Cash Equivalents [Line Items]
Amortized Cost Basis	23,776	3,893
Unrealized Gains	0	0
Unrealized Losses	(9)	0
Fair Value	$ 23,767	$ 3,893

Cash Equivalents and Marketab_4

Cash Equivalents and Marketable Securities - Additional Information (Details)	9 Months Ended
	Sep. 30, 2018
Maximum
Cash And Cash Equivalents [Line Items]
Marketable securities maturity period	1 year

Collaboration Agreements and _3

Collaboration Agreements and Supply Agreements - Summary of Recognized Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue	$ 7,836	$ 17,499	$ 19,333	$ 47,701
Collaboration Revenue
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue	6,924	17,499	13,955	47,701
Collaboration Agreements and Supply Agreements
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue	7,836	17,499	19,333	47,701
Total other revenue—related parties	912	0	5,378	0
Collaboration Agreements and Supply Agreements | Collaboration Revenue, Amortization of Up-front Payment | Celgene
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue	1,655	2,642	4,912	16,355
Collaboration Agreements and Supply Agreements | Collaboration Revenue, Amortization of Up-front Payment | Merck
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue	2,818	0	2,818	0
Collaboration Agreements and Supply Agreements | Collaboration Revenue, Amortization of Up-front Payment | Merck KGaA, Darmstadt, Germany “EMD Serono”)
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue	1,038	1,030	3,104	3,090
Collaboration Agreements and Supply Agreements | Collaboration Revenue, Research and Development Services | Celgene
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue	5	0	103	0
Collaboration Agreements and Supply Agreements | Collaboration Revenue, Research and Development Services | Merck
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue	696	0	696	0
Collaboration Agreements and Supply Agreements | Collaboration Revenue, Research and Development Services | Merck KGaA, Darmstadt, Germany “EMD Serono”)
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue	712	715	2,322	2,319
Collaboration Agreements and Supply Agreements | Collaboration Revenue, Milestones and Contingent Payments | Celgene
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue	0	13,112	0	25,937
Collaboration Agreements and Supply Agreements | Collaboration Revenue
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue	6,924	17,499	13,955	47,701
Collaboration Agreements and Supply Agreements | Collaboration Revenue | Celgene
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue	1,660	15,754	5,015	42,292
Collaboration Agreements and Supply Agreements | Collaboration Revenue | Merck
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue	3,514	0	3,514	0
Collaboration Agreements and Supply Agreements | Collaboration Revenue | Merck KGaA, Darmstadt, Germany “EMD Serono”)
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue	1,750	1,745	5,426	5,409
Collaboration Agreements and Supply Agreements | Development and Manufacturing Services and Clinical Product Supply | Celgene
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total other revenue—related parties	330	0	3,894	0
Collaboration Agreements and Supply Agreements | Supply and Other | SutroVax
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total other revenue—related parties	$ 582	$ 0	$ 1,484	$ 0

Collaboration Agreements and _4

Collaboration Agreements and Supply Agreements - Additional Information (Details)	Oct. 01, 2018USD ($)$ / sharesshares	Sep. 30, 2018USD ($)	Aug. 31, 2018USD ($)Program	Jul. 31, 2018Program	Aug. 31, 2017USD ($)Program	Jun. 30, 2016USD ($)	Mar. 31, 2015USD ($)	Sep. 30, 2014USD ($)	May 31, 2014USD ($)	Sep. 30, 2018USD ($)	Jul. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable, related parties		$ 1,695,000								$ 1,695,000		$ 1,695,000		$ 784,000
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs												84,739,000	$ 0
Accounts receivable		2,443,000								2,443,000		2,443,000		1,624,000
Series E Redeemable Convertible Preferred Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs											$ 85,400,000
Subsequent Event | Common Stock | IPO
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Issuance of common stock, shares | shares	5,667,000
Share issue price per share | $ / shares	$ 15
SutroVax
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable		584,000								584,000		584,000		34,000
2014 Celgene Agreement | Celgene
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front, nonrefundable payment received								$ 83,100,000
Revenue recognized from payments ratably over period								3 years
Contingent payment received							$ 15,000,000
Milestone payment received						$ 25,000,000
Deferred revenue					$ 8,200,000
2017 Celgene Agreement | Celgene
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from payments ratably over period					3 years
Number of programs advancing through preclinical development | Program					4
Option fee payment received					$ 12,500,000
Second option fee payment received					$ 12,500,000
Number of investigational new drug program | Program					1
Recognized of deferred revenue over the entire period					$ 12,500,000
Milestone revenue recognized		10,000,000
Deferred revenue		13,100,000								13,100,000		13,100,000		18,000,000
Potential milestone future payment received of manufacturing activities		10,000,000								10,000,000		10,000,000
Accounts receivable, related parties		$ 300,000								300,000		300,000		800,000
2017 Celgene Agreement | Celgene | Licensed Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum aggregate milestone and option fee payments eligible to receive												295,000,000
2017 Celgene Agreement | Celgene | Other Licensed Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum aggregate milestone and option fee payments eligible to receive												393,700,000
2018 Master Services Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue from other related parties										$ 300,000		$ 3,900,000
2018 Merck Agreement | Merck Sharp & Dohme Corp
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from payments ratably over period		4 years								4 years		4 years
Deferred revenue		$ 57,200,000								$ 57,200,000		$ 57,200,000
Number of research programs | Program				3
Upfront payment received			$ 60,000,000
Number of target programs | Program			2
Starting term of recognizing revenues from up front payment												2018-07
Revenue recognition aggregate milestone payments eligible to receive												$ 1,600,000,000
Revenue recognition milestone method description												If one or more products from each of the target programs are developed for non-oncology or a single indication, the Company will be eligible for reduced aggregate milestone payments. In addition, the Company is eligible to receive tiered royalties ranging from mid-single digit to low teen percentages on the worldwide sales of any commercial products that may result from the collaboration.
Accounts receivable		$ 800,000								$ 800,000		$ 800,000
2018 Merck Agreement | Merck Sharp & Dohme Corp | Subsequent Event | Common Stock | IPO
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Issuance of common stock, shares | shares	666,666
Share issue price per share | $ / shares	$ 15
Proceeds from issuance of common stock	$ 10,000,000
2018 Merck Agreement | Merck Sharp & Dohme Corp | Subsequent Event | Series E Redeemable Convertible Preferred Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Issuance of common stock, shares | shares	74,794,315
Share issue price per share | $ / shares	$ 0.2674
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	$ 20,000,000
Collaboration Agreement | EMD Serono
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front, nonrefundable and non-creditable payment received									$ 10,000,000
MDA Agreement | EMD Serono
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from payments ratably over period		5 years								5 years		5 years
Deferred revenue		$ 2,800,000								$ 2,800,000		$ 2,800,000		5,900,000
Starting term of recognizing revenues from up front payment												2014-06
Up-front, nonrefundable and non-creditable payment received								$ 10,000,000
Maximum amount eligible to receive for each product developed												$ 52,500,000
Accounts receivable		700,000								700,000		700,000		$ 800,000
Supply Agreement | SutroVax
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable, related parties		$ 600,000								600,000		600,000
Other revenue-related parties										$ 600,000		$ 1,500,000
Research Development and Commercialization Agreement | The Leukemia & Lymphoma Society, Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum contribution in clinical development					6,000,000
Initial milestone payment					500,000
Total milestone payments received					$ 1,000,000
Research development and commercialization agreement termination period					12 years
Research development and commercialization agreement termination description					The LLS Agreement terminates upon the earlier of (a) fulfillment of all payment obligations by both parties or (b) 12 years after the effective date. LLS may terminate the LLS Agreement at any time with 60 days’ prior written notice. Either the Company or LLS has the right to terminate the LLS Agreement based on the other party’s uncured material breach.
Other liabilities related to payments received					$ 1,000,000

Loan and Security Agreement - A

Loan and Security Agreement - Additional Information (Details)	1 Months Ended	3 Months Ended			9 Months Ended
	Aug. 31, 2017USD ($)Installment$ / sharesshares	Sep. 30, 2018USD ($)$ / shares	Jul. 31, 2018USD ($)$ / sharesshares	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)$ / shares	Sep. 30, 2017USD ($)	Aug. 31, 2018USD ($)	Dec. 31, 2017$ / shares
Debt Instrument [Line Items]
Debt, non-current		$ 11,500,000			$ 11,500,000
Funding through sale of stock					$ 84,739,000	$ 0
2017 Warrant
Debt Instrument [Line Items]
Estimated fair value portion of preferred stock			$ 329,000
Series D-2 Redeemable Convertible Preferred Stock
Debt Instrument [Line Items]
Exercise price per share of warrants | $ / shares		$ 0.6596			$ 0.6596			$ 0.6596
Series D-2 Redeemable Convertible Preferred Stock | 2017 Warrant
Debt Instrument [Line Items]
Exercise price per share of warrants | $ / shares	$ 0.6596
Series E Redeemable Convertible Preferred Stock
Debt Instrument [Line Items]
Exercise price per share of warrants | $ / shares		$ 0.2674			$ 0.2674			$ 0.2674
Funding through sale of stock			$ 85,400,000
Sale and issuance of shares | shares			319,305,718
Redeemable convertible preferred stock, par value | $ / shares			$ 0.2674
Series E Redeemable Convertible Preferred Stock | 2017 Warrant
Debt Instrument [Line Items]
Warrant to purchase stock | shares			1,682,871
Exercise price per share of warrants | $ / shares			$ 0.2674
Funding through sale of stock			$ 85,400,000
Sale and issuance of shares | shares			319,305,718
Redeemable convertible preferred stock, par value | $ / shares			$ 0.2674
Warrants exercisable, term			10 years
August 2017 Loan
Debt Instrument [Line Items]
Debt, current		$ 3,200,000			$ 3,200,000
Debt, non-current		11,500,000			11,500,000
Interest payable	$ 574,500	200,000			$ 200,000
Average interest rate					8.28%
Final payment fee percentage	3.83%
Loan and accretion of debt discount		$ 400,000		$ 200,000	$ 1,200,000	$ 200,000
August 2017 Loan | Floating Rate
Debt Instrument [Line Items]
Interest charges on loan	7.39%
August 2017 Loan | LIBOR
Debt Instrument [Line Items]
Interest charges on loan	6.40%
Description of interest charges on loan					the sum of the 30-day U.S. Dollar London Interbank Offered Rate (“LIBOR”) plus 6.40%.
Oxford | Series D-2 Redeemable Convertible Preferred Stock | 2017 Warrant
Debt Instrument [Line Items]
Warrant to purchase stock | shares	454,820
SVB | Series D-2 Redeemable Convertible Preferred Stock | 2017 Warrant
Debt Instrument [Line Items]
Warrant to purchase stock | shares	227,410
Loan and Security Agreement | Oxford Finance LLC and Silicon Valley Bank | August 2017 Loan
Debt Instrument [Line Items]
Borrowed amount	$ 15,000,000						$ 15,000,000
Loan and Security Agreement | Oxford Finance LLC and Silicon Valley Bank | August 2017 Loan | March 2019
Debt Instrument [Line Items]
Number of monthly installments | Installment	30
Payment terms, description					The loan is due in 30 monthly installments from March 2019 through its repayment in August 2021, with interest-only monthly payments until March 2019. If certain qualified funding events occur, the loan will be due in 24 monthly installments from September 2019 through its repayment in August 2021, with interest-only payments until September 2019.
Repayment installment, start date, month and year	2019-03
Repayment installment, end date, month and year	2021-08
Frequency of interest-only payments					monthly payments
Interest-only payments, maturity month and year	2019-03
Loan and Security Agreement | Oxford Finance LLC and Silicon Valley Bank | August 2017 Loan | September 2019
Debt Instrument [Line Items]
Number of monthly installments | Installment	24
Repayment installment, start date, month and year	2019-09
Repayment installment, end date, month and year	2021-08
Interest-only payments, maturity month and year	2019-09

Related-Party Transactions - Ad

Related-Party Transactions - Additional Information (Details)	Jul. 31, 2018USD ($)	Jun. 30, 2018USD ($)	May 31, 2018USD ($)shares	Aug. 30, 2010USD ($)	Sep. 30, 2018USD ($)Director	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)Director	Sep. 30, 2017USD ($)	Dec. 31, 2015$ / sharesshares	Dec. 31, 2017USD ($)
Related Party Transaction [Line Items]
Principal amount of promissory notes										$ 208,000
Accounts receivable					$ 2,443,000		$ 2,443,000			$ 1,624,000
Remaining deliverables under license agreement							0
Second Director Performing Consulting Services
Related Party Transaction [Line Items]
Payments for performed consulting services					5,000	$ 7,500	20,000	$ 22,500
Options granted to purchase common stock | shares									3,269
Second Director Performing Consulting Services | Minimum
Related Party Transaction [Line Items]
Fair values of the common stock price per share									4 years
Third Director Performing Consulting Services
Related Party Transaction [Line Items]
Payments for performed consulting services					$ 5,000	7,500	$ 20,000	17,500
Chief Executive Officer
Related Party Transaction [Line Items]
Principal amount of promissory notes				$ 200,000
Promissory note, accrue interest rate percentage				0.53%
Promissory note maturity date				Aug. 30, 2019
Collaboration Agreements
Related Party Transaction [Line Items]
Related party transaction agreed to pay one time success fee		$ 400,000
Merck Agreement
Related Party Transaction [Line Items]
Execution of directors fees paid	$ 400,000
Celgene
Related Party Transaction [Line Items]
Equity interest percentage					10.50%		10.50%			15.40%
Number of directors performed consulting services | Director					3		3
Celgene | First Director Performing Consulting Service
Related Party Transaction [Line Items]
Payments for performed consulting services					$ 10,000	$ 15,000	$ 40,000	45,000
Options granted to purchase common stock | shares									9,805
Advisory fees							$ 0	$ 250,000
Celgene | First Director Performing Consulting Service | Minimum
Related Party Transaction [Line Items]
Fair values of the common stock price per share | $ / shares									$ 4.36
Fair values of the common stock price per share									4 years
Celgene | First Director Performing Consulting Service | Maximum
Related Party Transaction [Line Items]
Fair values of the common stock price per share | $ / shares									$ 11.98
Merck Sharp & Dohme Corp
Related Party Transaction [Line Items]
Equity interest percentage					12.50%		12.50%			0.00%
SutroVax
Related Party Transaction [Line Items]
Accounts receivable					$ 584,000		$ 584,000			$ 34,000
Percentage common stock ownership interest					5.60%		5.60%			7.80%
Carrying value of common stock under the cost method							$ 0
Number of warrants to purchase | shares			100,000
Value of warrant to purchase of preferred stock			$ 140,000

Redeemable Convertible Prefer_3

Redeemable Convertible Preferred Stock - Additional Information (Details) $ / shares in Units, $ in Thousands		Oct. 01, 2018	Aug. 31, 2017$ / sharesshares	Jul. 31, 2018USD ($)$ / sharesshares	Sep. 30, 2018USD ($)Director$ / sharesshares	Sep. 30, 2017USD ($)	Dec. 31, 2017$ / sharesshares
Temporary Equity [Line Items]
Funding through sale of stock | $					$ 84,739	$ 0
Temporary equity, description					At the election of the holders of a majority of the then-outstanding shares of preferred stock, voting together as a single class on an as-converted to common stock basis, the Company will redeem all outstanding shares of preferred stock in three equal annual installments commencing May 24, 2023, by paying in cash an amount per share equal to the original issuance prices of $0.59 per share of Series A redeemable convertible preferred stock, $0.8822 per share of Series B redeemable convertible preferred stock, $0.4797 per share of Series C redeemable convertible preferred stock, $0.5996 per share of Series C-2 redeemable convertible preferred stock, $0.5996 per share of Series D redeemable convertible preferred stock, $0.6596 per share of Series D-2 redeemable convertible preferred stock, and $0.2674 per share of Series E redeemable convertible preferred stock, plus 8% of the applicable original issuance prices per annum calculated from the original issuance date of each share of preferred stock.
Preferred stock redemption commencing period					May 24, 2023
Percentage of issuance price calculated per annum added to original issuance price to determine redemption price					8.00%
Number of directors common shareholders are entitled to elect | Director					1
Voting rights per share					one vote per share
Series E Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Funding through sale of stock | $				$ 85,400
Sale and issuance of shares | shares				319,305,718
Redeemable convertible preferred stock, par value				$ 0.2674
Original issuance price, per share					$ 0.2674
Dividends declared per share rate					$ 0.0214
Number of directors preferred shareholders are entitled to elect | Director					1
Redeemable conversion price per share					$ 0.2674
Preferred stock, conversion basis		1-for-0.0275
Exercise price per share					$ 0.2674		$ 0.2674
Series E Redeemable Convertible Preferred Stock | Oxford Finance LLC and Silicon Valley Bank | Loan and Security Agreement
Temporary Equity [Line Items]
Issuance of warrants to purchase shares | shares			1,682,871
Exercise price per share			$ 0.2674
Term of warrant exercisable			10 years
Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Sale and issuance of shares | shares					493,615,703		173,750,421
Redeemable convertible preferred stock, par value					$ 0.001		$ 0.001
Temporary Equity liquidation percentage of voting power					Unless the holders of a majority of the then-outstanding shares of preferred stock, voting together as a single class on an as-converted to common stock basis, elect otherwise, any of the following events shall be treated as a liquidation: (i) any consolidation, merger, acquisition, or any other corporate reorganization in which the stockholders of the Company immediately prior to such event own less than 50% of the voting power of the surviving or successor entity or its parent immediately after such event; (ii) any transaction or series of related transactions in which in excess of 50% of the Company’s voting power is transferred; or (iii) any sale, lease, transfer, exclusive license, or other disposition of all or substantially all of the assets of the Company.
Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Original issuance price, per share					$ 0.5900		0.5900
Dividends declared per share rate					$ 0.0472
Number of directors preferred shareholders are entitled to elect | Director					1
Redeemable conversion price per share					$ 0.3756
Preferred stock, conversion basis		1-for-0.0433
Series B Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Original issuance price, per share					0.8822		0.8822
Dividends declared per share rate					$ 0.07056
Number of directors preferred shareholders are entitled to elect | Director					1
Redeemable conversion price per share					$ 0.4203
Preferred stock, conversion basis		1-for-0.0578
Exercise price per share					0.8822		0.8822
Series C Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Original issuance price, per share					0.4797		0.4797
Dividends declared per share rate					$ 0.03838
Number of directors preferred shareholders are entitled to elect | Director					2
Redeemable conversion price per share					$ 0.3573
Preferred stock, conversion basis		1-for-0.0370
Exercise price per share	[1]				0.4797		0.4797
Series C-2 Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Original issuance price, per share					0.5996		0.5996
Dividends declared per share rate					0.048
Redeemable conversion price per share					0.4081
Preferred stock, conversion basis		1-for-0.0405
Series D Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Original issuance price, per share					0.5996		0.5996
Dividends declared per share rate					0.048
Redeemable conversion price per share					0.4081
Preferred stock, conversion basis		1-for-0.0405
Series D-2 Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Original issuance price, per share					0.6596		0.6596
Dividends declared per share rate					0.0528
Redeemable conversion price per share					0.4336
Preferred stock, conversion basis		1-for-0.0419
Exercise price per share					$ 0.6596		$ 0.6596
Series D-2 Redeemable Convertible Preferred Stock | Oxford Finance LLC and Silicon Valley Bank | Loan and Security Agreement
Temporary Equity [Line Items]
Issuance of warrants to purchase shares | shares			682,230
Exercise price per share			$ 0.6596
[1]	On October 1, 2018, 1,232,220 shares of the Series C redeemable convertible preferred warrants will be canceled, and the remaining 687,928 shares will be converted to warrants to purchase common stock on a 1-for-0.0370 basis. All Series E redeemable convertible preferred warrants will be converted to warrants to purchase common stock at a on a 1-for-0.0275 basis.

Redeemable Convertible Prefer_4

Redeemable Convertible Preferred Stock - Schedule of Redeemable Convertible Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Shares Authorized	498,070,991	177,082,393
Shares Issued and Outstanding	493,615,703	173,750,421
Carrying Value	$ 187,245	$ 102,505
Liquidation Preference	$ 188,639	$ 102,988
Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized	3,503,692	3,503,692
Shares Issued and Outstanding	3,503,692	3,503,692
Original Issue Price Per Share	$ 0.5900	$ 0.5900
Carrying Value	$ 1,992	$ 1,992
Liquidation Preference	$ 2,067	$ 2,067
Series B Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized	24,515,966	24,515,966
Shares Issued and Outstanding	24,345,936	24,345,936
Original Issue Price Per Share	$ 0.8822	$ 0.8822
Carrying Value	$ 19,865	$ 19,865
Liquidation Preference	$ 21,478	$ 21,478
Series C Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized	78,582,049	78,582,049
Shares Issued and Outstanding	76,661,901	76,102,337
Original Issue Price Per Share	$ 0.4797	$ 0.4797
Carrying Value	$ 38,036	$ 38,035
Liquidation Preference	$ 36,775	$ 36,506
Series C-2 Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized	8,338,892	8,338,892
Shares Issued and Outstanding	8,338,892	8,338,892
Original Issue Price Per Share	$ 0.5996	$ 0.5996
Carrying Value	$ 4,845	$ 4,845
Liquidation Preference	$ 5,000	$ 5,000
Series D Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized	43,362,233	43,362,233
Shares Issued and Outstanding	43,362,233	43,362,233
Original Issue Price Per Share	$ 0.5996	$ 0.5996
Carrying Value	$ 25,900	$ 25,900
Liquidation Preference	$ 26,000	$ 26,000
Series D-2 Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized	18,779,561	18,779,561
Shares Issued and Outstanding	18,097,331	18,097,331
Original Issue Price Per Share	$ 0.6596	$ 0.6596
Carrying Value	$ 11,868	$ 11,868
Liquidation Preference	$ 11,937	$ 11,937
Series E Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized	320,988,598
Shares Issued and Outstanding	319,305,718
Original Issue Price Per Share	$ 0.2674
Carrying Value	$ 84,739
Liquidation Preference	$ 85,382

Redeemable Convertible Prefer_5

Redeemable Convertible Preferred Stock - Schedule of Warrants Outstanding and Exercisable (Details) - USD ($) $ / shares in Units, $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Class Of Warrant Or Right [Line Items]
Shares		3,603,019	3,331,972
Estimated fair value		$ 867	$ 1,708
Series B Redeemable Convertible Preferred Stock
Class Of Warrant Or Right [Line Items]
Expiration date		2018-06	2018-06
Exercise price per share		$ 0.8822	$ 0.8822
Shares			170,030
Estimated fair value		$ 0	$ 116
Series C Redeemable Convertible Preferred Stock
Class Of Warrant Or Right [Line Items]
Exercise price per share	[1]	$ 0.4797	$ 0.4797
Shares	[1]	1,920,148	2,479,712
Estimated fair value	[1]	$ 336	$ 1,263
Series C Redeemable Convertible Preferred Stock | Minimum
Class Of Warrant Or Right [Line Items]
Expiration date	[1]	2020-07
Series C Redeemable Convertible Preferred Stock | Maximum
Class Of Warrant Or Right [Line Items]
Expiration date	[1]	2021-11
Series D-2 Redeemable Convertible Preferred Stock
Class Of Warrant Or Right [Line Items]
Expiration date		2027-08	2027-08
Exercise price per share		$ 0.6596	$ 0.6596
Shares			682,230
Estimated fair value		$ 0	$ 329
Series E Redeemable Convertible Preferred Stock
Class Of Warrant Or Right [Line Items]
Expiration date		2027-08	2027-08
Exercise price per share		$ 0.2674	$ 0.2674
Shares		1,682,871	0
Estimated fair value		$ 531
[1]	On October 1, 2018, 1,232,220 shares of the Series C redeemable convertible preferred warrants will be canceled, and the remaining 687,928 shares will be converted to warrants to purchase common stock on a 1-for-0.0370 basis. All Series E redeemable convertible preferred warrants will be converted to warrants to purchase common stock at a on a 1-for-0.0275 basis.

Redeemable Convertible Prefer_6

Redeemable Convertible Preferred Stock - Schedule of Warrants Outstanding and Exercisable (Parenthetical) (Details)	Oct. 01, 2018shares
Series E Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Preferred stock, conversion basis	1-for-0.0275
Series C Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Preferred stock, conversion basis	1-for-0.0370
Series C Redeemable Convertible Preferred Stock | Subsequent Event
Temporary Equity [Line Items]
Cancellation of redeemable convertible preferred warrants	1,232,220
Convertion of warrants to purchase of shares of common stock	687,928

Redeemable Convertible Prefer_7

Redeemable Convertible Preferred Stock - Schedule of Warrants Valued Using Option Pricing Method (Details)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Valuation Technique [Extensible List]	us-gaap:ValuationTechniqueOptionPricingModelMember	us-gaap:ValuationTechniqueOptionPricingModelMember
Average Expected Life
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants, term		2 years 6 months
Average Expected Life | Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants, term	3 years 1 month 6 days
Average Expected Life | Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants, term	8 years 9 months 18 days
Expected Volatility
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants, measurement input		64.00%
Expected Volatility | Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants, measurement input	62.42%
Expected Volatility | Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants, measurement input	71.21%
Risk-free Interest Rate
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants, measurement input		1.55%
Risk-free Interest Rate | Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants, measurement input	2.88%
Risk-free Interest Rate | Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants, measurement input	3.40%
Expected Dividend
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants, measurement input	0.00%	0.00%

Redeemable Convertible Prefer_8

Redeemable Convertible Preferred Stock - Schedule of Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2018	Sep. 25, 2018
Temporary Equity [Line Items]
Redeemable convertible preferred stock	16,028,462
Common stock options issued and outstanding	3,383,756
Warrants to purchase redeemable convertible preferred stock	117,400
Warrants to purchase common stock	942
Total	22,107,783
2018 Equity Incentive Plan
Temporary Equity [Line Items]
Remaining shares reserved for issuance under 2018 Equity Incentive Plan	2,577,223
Total		2,300,000

Employee Stock Purchase Plan _3

Employee Stock Purchase Plan and Stock-Based Compensation - Additional Information (Details) - USD ($)	Sep. 26, 2018	Sep. 25, 2018	Feb. 28, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance				22,107,783		22,107,783
Aggregate intrinsic value of stock options exercised				$ 18,275	$ 8,431	$ 153,414	$ 51,107
Weighted-average estimated grant-date fair value of employee stock options granted				$ 8.93	$ 7.20	$ 8.93	$ 7.14
Total fair value of stock options vested				$ 300,000	$ 600,000	$ 700,000	$ 1,400,000
Stock-based compensation expense				321	593	802	1,126
Other income (expense), net				(68,000)	(180,000)	840,000	(197,000)
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense				1,019	19,055	20,795	58,212
ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance	230,000
Annual increase period of common stock reserved for issuance	10 years
Maximum number of shares issuable	2,300,000
ESPP | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase in stock reserved for issuance as percentage of capital stock outstanding on last day of preceding year	1.00%
2018 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance		2,300,000
Increase in stock reserved for issuance as percentage of capital stock outstanding on last day of preceding year		5.00%
2018 Equity Incentive Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Annual increase period of common stock reserved for issuance		10 years
2004 and 2018 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total unrecognized compensation cost related to unvested stock options granted				22,200,000		$ 22,200,000
Remaining unrecognized compensation cost expected to be recognized over weighted-average period						3 years 7 months 6 days
2004 and 2018 Equity Incentive Plan | Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment right to repurchase of shares lapse with respect to share percentage after year one						25.00%
Share-based compensation arrangement by share-based payment award ratable lapse remainder period						3 years
2004 and 2018 Equity Incentive Plan | Other Noncurrent Liabilities | Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Cash received for early exercise of options				0		$ 0		$ 14,000
2017 Call Option Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense				$ 19,000	15,000	$ 50,000	65,000
Call options expiration period			1 year
Share-based compensation arrangement by share-based payment award options, outstanding number of shares				315,000		315,000
Share-based compensation arrangement by share-based payment award options, unvested shares				210,000		210,000
Share-based compensation arrangement by share-based payment award options, shares granted								420,000
Share-based compensation arrangement by share based payment award options, exercise price per share								$ 0.76
Share-based compensation arrangement by share-based payment award options vested number of shares						105,000		105,000
Share-based compensation arrangement by share-based payment award options exercised shares						0		105,000
Other income (expense), net				$ 6,000	$ 6,000	$ 10,000	$ 10,000
2017 Call Option Plan | SutroVax
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance				450,000		450,000
2017 Call Option Plan | January 1 , 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Call options vesting percentage			25.00%
2017 Call Option Plan | January 1 , 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Call options vesting percentage			25.00%
2017 Call Option Plan | January 1 , 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Call options vesting percentage			25.00%
2017 Call Option Plan | January 1 , 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Call options vesting percentage			25.00%

Employee Stock Purchase Plan _4

Employee Stock Purchase Plan and Stock-Based Compensation - Summary of Option Activity (Details) - 2004 Plan and 2018 Equity Incentive Plan - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Available for Grant, Beginning Balance	91,149
Shares Available for Grant, Increase in authorized shares	5,053,013
Shares Available for Grant, Granted	(2,570,848)
Shares Available for Grant, Exercised	0
Shares Available for Grant, Canceled	3,721
Shares Available for Grant, Ending Balance	2,577,035	91,149
Outstanding Options, Beginning Balance	835,320
Options granted to purchase common stock	2,570,848
Outstanding Options Exercised	(19,691)
Outstanding Options Canceled	(3,721)
Outstanding Options, Ending Balance	3,382,756	835,320
Outstanding Options Exercisable	650,197
Outstanding Options Vested and expected to vest	3,172,528
Weighted - Average Exercise Price, Beginning Balance	$ 10.31
Weighted - Average Exercise Price, Granted	13.15
Weighted - Average Exercise Price, Exercised	6.67
Weighted - Average Exercise Price, Canceled	13.58
Weighted - Average Exercise Price, Ending Balance	0	$ 10.31
Weighted - Average Exercise Price, Exercisable	0
Weighted - Average Exercise Price, Vested and expected to vest	$ 0
Weighted - Average Remaining Contract Term	8 years 5 months 4 days	6 years 10 months 2 days
Weighted - Average Remaining Contract Term, Exercisable	5 years 9 months 18 days
Weighted - Average Remaining Contract Term, Vested and expected to vest	8 years 4 months 20 days
Aggregate Intrinsic Value, Balance	$ 8,434	$ 3,813
Aggregate Intrinsic Value, Exercisable	3,486
Aggregate Intrinsic Value, Vested and expected to vest	$ 8,073

Employee Stock Purchase Plan _5

Employee Stock Purchase Plan and Stock-Based Compensation - Schedule of Employee Stock Options Valuation (Details)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Methodology [Line Items]
Expected volatility, Minimum	48.86%	56.52%
Expected volatility, Maximum	56.75%	58.55%
Risk-free interest rate, Minimum	2.72%	1.89%
Risk-free interest rate, Maximum	2.97%	2.10%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Methodology [Line Items]
Expected term (in years)	1 year 9 months	5 years 6 months 7 days
Maximum
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Methodology [Line Items]
Expected term (in years)	6 years 29 days	6 years 29 days

Employee Stock Purchase Plan _6

Employee Stock Purchase Plan and Stock-Based Compensation - Schedule of Stock-Based Compensation Expense Recognized (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 321	$ 593	$ 802	$ 1,126
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	65	19	160	91
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 256	$ 574	$ 642	$ 1,035

Employee Stock Purchase Plan _7

Employee Stock Purchase Plan and Stock-Based Compensation - Summary of Restricted Stock Activity (Details) - 2004 and 2018 Equity Incentive Plan - Restricted Stock	9 Months Ended
	Sep. 30, 2018shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Balance as of December 31, 2017	2,374
Vested	(2,374)
Balance as of September 30, 2018	0

Net Loss Per Share - Summary of

Net Loss Per Share - Summary of Computation of Company's Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net loss, basic and diluted	$ (10,237)	$ (1,418)	$ (33,824)	$ (4,344)
Net loss	$ (10,237)	$ (1,418)	$ (33,824)	$ (4,344)
Denominator:
Shares issued in computing net loss per share, basic and diluted	481,613	451,550	476,023	444,594
Net loss per share, basic and diluted	$ (21.26)	$ (3.14)	$ (71.06)	$ (9.77)

Net Loss Per Share - Summary _2

Net Loss Per Share - Summary of Common Stock Equivalents of Antidilutive Securities Excluded from Computation of Diluted Net Loss per Share (Details) - shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of net loss per share	19,530,560	6,004,104
Redeemable Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of net loss per share	16,028,462	5,063,404
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of net loss per share	3,383,756	840,160
Warrants to Purchase Redeemable Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of net loss per share	117,400	93,527
Warrants to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of net loss per share	942	1,099
Early Exercised Shares of Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of net loss per share	0	5,914

Net Loss Per Share - Additional

Net Loss Per Share - Additional Information (Details) - shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of net loss per share	19,530,560	6,004,104
Early Exercised Shares of Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of net loss per share	0	5,914

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	Oct. 01, 2018	Sep. 30, 2018	Dec. 31, 2017
Redeemable Convertible Preferred Stock
Subsequent Event [Line Items]
Redeemable convertible preferred stock, shares outstanding		493,615,703	173,750,421
IPO | Subsequent Event | Redeemable Convertible Preferred Stock
Subsequent Event [Line Items]
Redeemable convertible preferred stock, shares outstanding	0
IPO | Common Stock | Subsequent Event
Subsequent Event [Line Items]
Common stock price, per share	$ 15
Issuance of common stock, shares	5,667,000
Gross proceeds from issuance of initial public offering	$ 85
Net proceeds from issuance of initial public offering	$ 74.4
IPO | Common Stock | Subsequent Event | Redeemable Convertible Preferred Stock
Subsequent Event [Line Items]
Convertible preferred stock shares issued upon conversion	16,028,462
Warrant to purchase stock	71,812
IPO | Warrants | Subsequent Event | Redeemable Convertible Preferred Stock
Subsequent Event [Line Items]
Convertible preferred stock shares issued upon conversion	2,370,799
Private Placement | Merck | Common Stock | Subsequent Event
Subsequent Event [Line Items]
Common stock price, per share	$ 15
Issuance of common stock, shares	666,666
Gross proceeds from issuance of private placement	$ 10