Jounce Therapeutics (JNCE) 10-Q 13 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 Period End Date	Sep. 30, 2018
Entity Registrant Name	JOUNCE THERAPEUTICS, INC.
Entity Central Index Key	1,640,455
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Small Business	true
Document Type	10-Q
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Amendment Flag	false
Trading Symbol	JNCE
Entity Common Stock, Shares Outstanding		32,700,495

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 53,283	$ 23,559
Short-term investments	158,476	212,093
Prepaid expenses and other current assets	2,606	19,945
Total current assets	214,365	255,597
Property and equipment, net	14,239	16,151
Long-term investments	2,969	22,199
Other non-current assets	2,663	2,713
Total assets	234,236	296,660
Current liabilities:
Accounts payable	2,910	2,849
Accrued expenses	7,779	8,454
Deferred rent and lease incentive, current	61	61
Deferred revenue, current—related party	58,947	51,142
Other current liabilities	57	45
Total current liabilities	69,754	62,551
Deferred rent and lease incentive, net of current portion	2,040	1,955
Deferred revenue, net of current portion—related party	59,025	65,018
Other non-current liabilities	7	27
Total liabilities	130,826	129,551
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.001 par value: 5,000 shares authorized at September 30, 2018 and December 31, 2017; no shares issued or outstanding at September 30, 2018 or December 31, 2017	0	0
Common stock, $0.001 par value: 160,000 shares authorized at September 30, 2018 and December 31, 2017; 32,700 and 32,265 shares issued at September 30, 2018 and December 31, 2017, respectively; 32,692 and 32,249 shares outstanding at September 30, 2018 and December 31, 2017, respectively	33	32
Additional paid-in capital	265,384	257,101
Accumulated other comprehensive loss	(140)	(409)
Accumulated deficit	(161,867)	(89,615)
Total stockholders’ equity	103,410	167,109
Total liabilities and stockholders’ equity	$ 234,236	$ 296,660

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	160,000,000	160,000,000
Common stock, shares issued (in shares)	32,700,000	32,265,000
Common stock, shares outstanding (in shares)	32,692,000	32,249,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue:
Collaboration revenue—related party	$ 14,528	$ 18,077	$ 45,101	$ 58,655
Operating expenses:
Research and development	16,751	17,094	53,408	49,241
General and administrative	6,517	5,371	19,842	17,077
Total operating expenses	23,268	22,465	73,250	66,318
Operating loss	(8,740)	(4,388)	(28,149)	(7,663)
Other income, net	1,103	721	2,810	2,105
Loss before provision for income taxes	(7,637)	(3,667)	(25,339)	(5,558)
Provision for income taxes	0	417	0	1,521
Net loss	(7,637)	(4,084)	(25,339)	(7,079)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(7,637)	(4,084)	(25,339)	(7,079)
Net loss attributable to common stockholders	$ (7,637)	$ (4,084)	$ (25,339)	$ (7,873)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.23)	$ (0.13)	$ (0.78)	$ (0.27)
Weighted-average common shares outstanding, basic and diluted (in shares)	32,641	32,182	32,462	29,321
Convertible preferred stock (Series A)
Reconciliation of net loss to net loss attributable to common stockholders:
Accrued dividends on convertible preferred stock	$ 0	$ 0	$ 0	$ (268)
Convertible preferred stock (Series B)
Reconciliation of net loss to net loss attributable to common stockholders:
Accrued dividends on convertible preferred stock	0	0	0	(318)
Convertible preferred stock (Series B-1)
Reconciliation of net loss to net loss attributable to common stockholders:
Accrued dividends on convertible preferred stock	$ 0	$ 0	$ 0	$ (208)

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Income (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 Comprehensive Income [Abstract]
Net loss	$ (7,637)	$ (4,084)	$ (25,339)	$ (7,079)
Other comprehensive income:
Unrealized gain on available-for-sale securities	76	208	269	89
Comprehensive loss	$ (7,561)	$ (3,876)	$ (25,070)	$ (6,990)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating activities:
Net loss	$ (25,339)	$ (7,079)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	6,963	3,463
Depreciation expense	2,867	3,500
Net amortization of premiums and discounts on investments	(609)	866
Loss on disposal of property and equipment	0	7
Changes in operating assets and liabilities:
Taxes receivable	16,737	0
Deferred taxes	0	(5,844)
Prepaid expenses and other current assets	602	(5,941)
Other non-current assets	50	(305)
Accounts payable	231	1,307
Accrued expenses and other current liabilities	(661)	2,071
Deferred revenue—related party	(45,101)	(58,655)
Deferred rent	85	(207)
Net cash used in operating activities	(44,175)	(66,817)
Investing activities:
Purchases of investments	(194,699)	(144,886)
Proceeds from maturities of investments	264,427	86,235
Proceeds from sales of investments	3,997	15,638
Purchases of property and equipment	(1,125)	(13,693)
Net cash provided by (used in) investing activities	72,600	(56,706)
Financing activities:
Proceeds from initial public offering of common stock, net of issuance costs	0	107,008
Proceeds from exercise of stock options	1,299	301
Net cash provided by financing activities	1,299	107,309
Net increase (decrease) in cash, cash equivalents and restricted cash	29,724	(16,214)
Cash, cash equivalents and restricted cash, beginning of period	24,829	46,368
Cash, cash equivalents and restricted cash, end of period	54,553	30,154
Non-cash investing and financing activities:
Purchases of property and equipment in accounts payable and accrued expenses	0	766
Stock option exercise receivables in prepaid expenses and other current assets	0	6
Supplemental cash flow information:
Cash paid for income taxes	$ 0	$ 11,500

Nature of Business

Nature of Business	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Business	Nature of Business Jounce Therapeutics, Inc. (the “Company”) is a clinical stage immunotherapy company dedicated to transforming the treatment of cancer by developing therapies that enable the immune system to attack tumors and provide long-lasting benefits to patients. The Company is subject to a number of risks similar to those of other clinical stage immunotherapy companies, including dependence on key individuals; the need to develop commercially viable products; competition from other companies, many of which are larger and better capitalized; and the need to obtain adequate additional financing to fund the development of its products. The Company’s most advanced product candidate, JTX-2011, is a clinical-stage monoclonal antibody that binds to and activates the I nducible T cell CO-S timulator (“ICOS”) a protein on the surface of certain T cells commonly found in many solid tumors. In June 2018, the Company presented preliminary efficacy data at the 2018 ASCO Annual Meeting. In the data presented at the 2018 ASCO Annual Meeting, JTX-2011 showed a favorable safety profile both alone and in combination with nivolumab in heavily pretreated patients with advanced cancer. Preliminary signals of clinical activity with JTX-2011 monotherapy and in combination with nivolumab were observed, accompanied by a potential pharmacodynamic biomarker. In June 2018, the Company also began enrollment of dose escalation cohorts evaluating the safety of JTX-2011 in combination with ipilimumab and in combination with pembrolizumab. On February 1, 2017, the Company closed its initial public offering (“IPO”) of 7,319,750 shares of the Company’s common stock at a public offering price of $16.00 per share, including 954,750 shares of common stock issued upon the full exercise by the underwriters of their option to purchase additional shares. The gross proceeds from the IPO were $117.1 million and net proceeds were $106.4 million , after deducting underwriting discounts and commissions and other offering expenses paid by the Company. Upon completion of the IPO, all outstanding preferred stock was automatically converted into an aggregate of 22,283,690 shares of common stock. In connection with the IPO, the Board of Directors and the stockholders of the Company approved a one-for-3.69 reverse stock split of the Company’s issued and outstanding common stock. The reverse stock split became effective on January 13, 2017. All share and per share amounts in the condensed consolidated financial statements have been retroactively adjusted for all periods presented to give effect to the reverse stock split, including reclassifying an amount equal to the reduction in par value to additional paid-in capital. As of September 30, 2018 , the Company had cash, cash equivalents, and investments of $214.7 million . The Company expects that its existing cash, cash equivalents, and investments will enable it to fund its expected operating expenses and capital expenditure requirements for at least 24 months from November 13, 2018 , the filing date of this Quarterly Report on Form 10-Q. The Company expects to finance its future cash needs through a combination of equity or debt financings and collaboration arrangements, including potential cash inflows from its Master Research and Collaboration Agreement (the “Celgene Collaboration Agreement”) with Celgene Corporation (“Celgene”).

Basis of Presentation and Summa

Basis of Presentation and Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation and Summary of Significant Accounting Policies	Basis of Presentation and Summary of Significant Accounting Policies Basis of Presentation and Consolidation The accompanying condensed consolidated financial statements as of September 30, 2018 and December 31, 2017 , and for the three and nine months ended September 30, 2018 and 2017 , have been prepared in accordance with the rules and regulations of the Securities and Exchange Commission (the “SEC”) and generally accepted accounting principles in the United States of America (“GAAP”) as found in the Accounting Standards Codification (“ASC”) of the Financial Accounting Standards Board (“FASB”) for condensed consolidated financial information. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. In the opinion of management, these condensed consolidated financial statements reflect all normal recurring adjustments which are necessary for a fair presentation of the Company’s financial position and results of its operations, as of and for the periods presented. These condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto contained in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017 filed with the SEC on March 8, 2018 (the “Annual Report on Form 10-K”). The information presented in the condensed consolidated financial statements and related notes as of September 30, 2018 , and for the three and nine months ended September 30, 2018 and 2017 , is unaudited. The December 31, 2017 condensed consolidated balance sheet included herein was derived from the audited financial statements as of that date, but does not include all disclosures, including notes, required by GAAP for complete financial statements. Interim results for the nine months ended September 30, 2018 are not necessarily indicative of the results that may be expected for the fiscal year ending December 31, 2018 , or any future period. The accompanying condensed consolidated financial statements include the accounts of Jounce Therapeutics, Inc. and its wholly-owned subsidiary, Jounce Mass Securities, Inc. All intercompany transactions and balances have been eliminated in consolidation. Summary of Significant Accounting Policies The significant accounting policies and estimates used in the preparation of the condensed consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2017 , and the notes thereto, which are included in the Company’s Annual Report on Form 10-K. There have been no material changes in the Company’s significant accounting policies during the nine months ended September 30, 2018 , except as discussed below with respect to the adoption of ASC Topic 606, Revenue from Contracts with Customers (“ASC 606”). Effective January 1, 2018, the Company adopted ASC 606. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. In applying ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the promises and performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies the performance obligations. The Company only applies the five-step model to contracts when it is probable that it will collect the consideration to which it is entitled in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract, determines those that are performance obligations and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. See Note 3, “Celgene Collaboration Agreement”, for further information on the application of ASC 606 to the Celgene Collaboration Agreement. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. On an ongoing basis, the Company’s management evaluates its estimates which include, but are not limited to, estimates related to revenue recognized under the Celgene Collaboration Agreement, accrued expenses, stock-based compensation expense and income taxes . The Company bases its estimates on historical experience and other market specific or other relevant assumptions it believes to be reasonable under the circumstances. Actual results could differ from those estimates. Recent Accounting Pronouncements In May 2014, the FASB issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (Topic 606) , which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. This guidance was originally effective for interim and annual periods beginning after December 15, 2016 and allowed for adoption using a full retrospective method, or a modified retrospective method. Subsequent to the issuance of ASU 2014-09, the FASB also issued the following updates related to ASC 606: • In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date , whereby the effective date for the new revenue standard was deferred by one year. As a result of ASU 2015-14, the new revenue standard is now effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2017, and early adoption was permitted for annual periods beginning after December 15, 2016, including interim periods within that annual period. • In March 2016, the FASB issued ASU 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net) , to clarify the implementation guidance on principal versus agent considerations. • In April 2016, the FASB issued ASU 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing , to clarify the principle for determining whether a good or service is “separately identifiable” from other promises in the contract and to clarify the categorization of licenses of intellectual property. • In May 2016, the FASB issued ASU 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Technical Expedients , to clarify guidance on transition, determining collectability, non-cash consideration and the presentation of sales and other similar taxes. • In December 2016, the FASB issued ASU 2016-20, Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers , that allows entities not to make qualitative disclosures about remaining performance obligations in certain cases, adds disclosure requirements for entities that elect certain optional exemptions and adds twelve additional technical corrections and improvements to the new revenue standard. The Company adopted ASC 606 effective January 1, 2018 under the modified retrospective method. The modified retrospective method requires that the cumulative effect of initially applying ASC 606 be recognized as an adjustment to the opening balance of retained earnings or accumulated deficit of the annual period that includes the date of initial application. Accordingly, during the first quarter of 2018, the Company recorded an increase to the opening balance of accumulated deficit and a corresponding increase to deferred revenue of $46.9 million related to the Celgene Collaboration Agreement. Additionally, the following tables present a summary of the amount by which each financial statement line item was affected as of and during the three and nine months ended September 30, 2018 by the application of ASC 606 as compared to ASC Topic 605, Revenue Recognition (“ASC 605”), the revenue recognition guidance that was in effect before this change in accounting principle (in thousands, except per share amounts): September 30, 2018 ASC 606 ASC 605 Difference Deferred revenue, current—related party $ 58,947 $ 43,804 $ 15,143 Deferred revenue, net of current portion—related party $ 59,025 $ 33,387 $ 25,638 Three Months Ended September 30, 2018 ASC 606 ASC 605 Difference Collaboration revenue—related party $ 14,528 $ 12,989 $ 1,539 Net loss $ (7,637 ) $ (9,176 ) $ 1,539 Net loss per share attributable to common stockholders, basic and diluted $ (0.23 ) $ (0.28 ) $ 0.05 Nine Months Ended September 30, 2018 ASC 606 ASC 605 Difference Collaboration revenue—related party $ 45,101 $ 38,968 $ 6,133 Net loss $ (25,339 ) $ (31,472 ) $ 6,133 Net loss per share attributable to common stockholders, basic and diluted $ (0.78 ) $ (0.97 ) $ 0.19 The application of ASC 606 did not have an impact on the Company’s net cash used in operating activities for the nine months ended September 30, 2018 , but did result in offsetting adjustments to net loss and the change in deferred revenue presented within the condensed consolidated statement of cash flows for that period. Both the cumulative adjustment of $46.9 million recorded upon the initial application of ASC 606 and the differences outlined above are primarily attributable to the transition from recognizing revenue on a straight-line basis over the estimated performance period for each unit of accounting, which was a permitted method of revenue recognition under ASC 605, to recognizing revenue based on the Company’s pattern of performance for each performance obligation under ASC 606. As part of the adoption of ASC 606, the Company implemented new processes to objectively measure the performance under the Celgene Collaboration Agreement. See Note 3, “Celgene Collaboration Agreement”, for further information on the application of ASC 606 to the Celgene Collaboration Agreement. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) , which requires a lessee to recognize assets and liabilities on the balance sheet for operating leases and changes many key definitions, including the definition of a lease. The new standard includes a short-term lease exception for leases with a term of 12 months or less, as part of which a lessee can make an accounting policy election not to recognize lease assets and lease liabilities. Lessees will continue to differentiate between finance leases (previously referred to as capital leases) and operating leases using classification criteria that are substantially similar to the previous guidance. In July 2018, the FASB also issued ASU 2018-11, Leases (Topic 842): Targeted Improvements , which permits entities to continue applying legacy guidance in ASC Topic 840, Leases , including its disclosure requirements, in the comparative periods presented in the year that the entity adopts the new leasing standard. The new standard will be effective for annual reporting periods beginning after December 15, 2018, including interim periods within those annual reporting periods, and early adoption is permitted for public entities. The Company will adopt this new standard on January 1, 2019 using the transition method permitted by ASU 2018-11, and it is currently evaluating the impact that ASU 2016-02 may have on the condensed consolidated financial statements. The Company has substantially completed an assessment of its contracts, and it expects to record lease assets and lease liabilities upon adoption of this guidance. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments , which is intended to reduce diversity in practice in how entities present certain types of cash transactions in the statement of cash flows. This guidance also clarifies how the predominance principle should be applied when classifying cash receipts and cash payments that have attributes of more than one class of cash flows. This guidance became effective for annual reporting periods beginning after December 15, 2017, including interim periods within those annual reporting periods. Accordingly, the Company adopted ASU 2016-15 effective January 1, 2018, and there was not a material impact to the condensed consolidated financial statements as a result of the adoption of this guidance. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash , which requires entities to show the change in the total of cash, cash equivalents, restricted cash and restricted cash equivalents within the statement of cash flows. As a result, entities no longer separately present transfers between unrestricted cash and restricted cash. This guidance became effective for annual reporting periods beginning after December 15, 2017, including interim periods within those annual reporting periods. Accordingly, the Company adopted ASU 2016-18 effective January 1, 2018 using a retrospective transition method, and there was not a material impact to the condensed consolidated financial statements as a result of the adoption of this guidance. See Note 7, “Restricted Cash”, for incremental disclosures associated with the adoption of ASU 2016-18. In May 2017, the FASB issued ASU 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting . This guidance is intended to provide clarity and reduce diversity in practice as to when changes to the terms or conditions of share-based payments are accounted for as modifications. Under this new guidance, entities are required to apply modification accounting if the fair value, vesting conditions or classification of the award changes. This guidance became effective for annual reporting periods beginning after December 15, 2017, including interim periods within those annual reporting periods, and early adoption was permitted. The guidance per ASU 2017-09 is to be adopted prospectively to an award modified on or after the adoption date. The Company adopted ASU 2017-09 effective January 1, 2018, and there was no impact to the condensed consolidated financial statements as a result of the adoption of this guidance. In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting . This guidance is intended to simplify the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. This guidance will be effective for annual reporting periods beginning after December 15, 2018, including interim periods within those annual reporting periods, and early adoption is permitted. The Company does not anticipate a material impact to the condensed consolidated financial statements as a result of the adoption of this guidance. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement, which eliminates, adds and modifies certain disclosure requirements for fair value measurements. This guidance will be effective for annual reporting periods beginning after December 15, 2019, including interim periods within those annual reporting periods, and early adoption is permitted. More specifically, an entity is permitted to early adopt any removed or modified disclosure requirements immediately and delay adoption of additional disclosure requirements until the effective date of this guidance. The Company does not anticipate a material impact to the condensed consolidated financial statements as a result of the adoption of this guidance. In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 , which clarifies that certain transactions between participants in a collaborative arrangement should be accounted for under ASC 606 when the counterparty is a customer. In addition, ASU 2018-18 adds unit-of-account guidance to ASC Topic 808, Collaborative Arrangements , in order to align this guidance with ASC 606 and also precludes an entity from presenting consideration from a transaction in a collaborative arrangement as revenue from contracts with customers if the counterparty is not a customer for that transaction. This guidance will be effective for annual reporting periods beginning after December 15, 2019, including interim periods within those annual reporting periods, and early adoption is permitted. The Company is currently evaluating the potential impact that ASU 2018-18 may have on the condensed consolidated financial statements.

Celgene Collaboration Agreement

Celgene Collaboration Agreement	9 Months Ended
	Sep. 30, 2018
Research and Development [Abstract]
Celgene Collaboration Agreement	Celgene Collaboration Agreement In July 2016, the Company entered into the Celgene Collaboration Agreement. The primary goal of the collaboration is to co-develop and co-commercialize innovative biologic immunotherapies that either activate or suppress the immune system by binding to targets identified by leveraging the Company’s Translational Science Platform. Under the Celgene Collaboration Agreement, the Company granted Celgene exclusive options to develop and commercialize the Company’s lead product candidate, JTX-2011, and up to four early-stage programs, consisting of targets to be selected from a pool of certain B cell, T regulatory cell and tumor-associated macrophage targets. Additionally, Celgene has an exclusive option to develop and commercialize the Company’s product candidate JTX-4014, an anti-PD-1 antibody, which, upon exercise of such option, will be a shared program that may be used by both parties in and outside of the collaboration. Prior to Celgene exercising any of its options, the Company is responsible for all research and development activities under the Celgene Collaboration Agreement. The Company received a non-refundable upfront cash payment of $225.0 million in July 2016 upon the execution of the Celgene Collaboration Agreement. The Company also received $36.1 million from the sale of 10,448,100 shares of Series B-1 convertible preferred stock upon the execution of a Series B-1 Preferred Stock Purchase Agreement with Celgene, which shares converted into 2,831,463 shares of common stock upon the completion of the IPO. If Celgene elects to exercise any of the program options, Celgene will pay the Company an option-exercise fee of $10.0 million to $60.0 million that varies by program, with an aggregate of $182.5 million if Celgene exercises all six program options. The initial research term of the collaboration is four years, which can be extended, at Celgene’s option, annually for up to three additional years for additional consideration that ranges from $30.0 million to $45.0 million per year, for an aggregate of $120.0 million if the term is extended for an additional three years. Worldwide Development Cost and U.S. Operating Profit and Loss Sharing Upon the exercise of each program option, the parties will enter into a co-development and co-commercialization agreement (“Co-Co Agreements”) or, in the case of JTX-4014, a license agreement (“JTX-4014 License Agreement”) that governs the development and commercialization of the applicable program. Although the agreements will not be executed unless and until Celgene exercises an option, the parties have agreed to the terms of the Co-Co Agreements and the JTX-4014 License Agreement as part of the Celgene Collaboration Agreement. Under the Co-Co Agreements and the JTX-4014 License Agreement, the Company will share with Celgene the U.S. profits or losses and development costs on such collaboration program as follows: • The Company will retain 60 percent of the U.S. operating profits or losses arising from commercialization of JTX-2011, with 40 percent allocated to Celgene. • The Company will retain 25 percent of the U.S. operating profits or losses arising from commercialization of the first program (the “Lead Program”), other than JTX-2011 or JTX-4014, for which an investigational new drug application (“IND”) is filed under the collaboration, with 75 percent allocated to Celgene. Celgene has a one-time right to substitute and swap the economics and governance of this program with that of another program for which it exercises an option (other than JTX-2011 and JTX-4014). • The Company and Celgene will equally share U.S. operating profits or losses arising from commercialization of up to three additional programs (other than JTX-2011, JTX-4014 or the Lead Program) (the “Other Programs”). • The Company and Celgene will share all development costs, other than for JTX-4014, in accordance with the applicable Co-Co Agreements, of which Celgene’s portion of the costs range from 67 percent to 85 percent. If Celgene exercises its option for a program other than JTX-4014, the Company will enter into a Co-Co Agreement, pursuant to which Celgene will have the exclusive right to develop and commercialize the products arising out of such collaboration program outside of the United States, and the Company will be eligible to receive tiered royalties ranging from a high single digit to mid-teen percentage rate on net product sales outside of the United States. Under each Co-Co Agreement, the Company will also have the right to opt out of profit sharing and instead receive milestones and royalties. Furthermore, if Celgene exercises its option for JTX-4014, the Company will enter into the JTX-4014 License Agreement, pursuant to which Celgene and the Company will each have equal rights to develop and commercialize JTX-4014 in combination with other proprietary molecules in their or the Company’s respective pipelines or in combination with products arising out of collaboration programs. Subject to terms specified in the license agreement for JTX-4014, the party owning the proprietary molecule that is combined with JTX-4014, if such molecule does not arise from a collaboration program with Celgene, will be solely responsible for all development and commercialization costs related to such combination. If JTX-4014 is combined with a product arising from a collaboration program, then the parties will share costs and, if co-packaged or co-formulated, profits or losses in accordance with the Co-Co Agreements for such other product. Milestones and Royalties Under the Co-Co Agreements and the JTX-4014 License Agreement, Celgene is required to pay the Company for specified development, regulatory and commercial milestones, if achieved, up to approximately $2.3 billion , across all collaboration programs. The development milestones are payable on initiation of certain clinical trials and range from $32.5 million to $105.0 million , per program, with an aggregate total of $290.0 million . The regulatory approval milestones are payable upon regulatory approval in the United States and outside the United States and range from $7.5 million to $50.0 million per milestone, with an aggregate total of $700.0 million . The commercial milestones are payable upon achievement of specified aggregate product sales outside the United States for each program and range from $40.0 million to $200.0 million per milestone, with an aggregate total of $1.270 billion . The Company is also eligible to receive royalties on product sales outside the United States ranging from high single digit to mid-teen royalties. Exercise of Options Celgene may exercise its option for a program at any time until the expiration of an option term for that program. For each program, the option term ends 45 to 60 days following Celgene’s receipt of a data package that includes certain information relating to the program’s research and development activities. The data package for a program may be delivered to Celgene after the applicable development milestone for such program has been achieved. Depending on the program, the applicable development milestone is (i) IND acceptance, (ii) availability of certain Phase 1a data or (iii) availability of certain Phase I/II data. If Celgene fails to exercise its option during the option term for a program, the Company will continue to retain all rights to such program. If Celgene exercises its option for a program other than JTX-4014, then the Company will enter into a Co-Co Agreement with Celgene for such program in substantially the form attached to the agreement as an exhibit. Under the co-development and co-commercialization agreement for JTX-2011 and one additional program for which Celgene opts in, other than JTX-4014, the Company will be responsible for leading development and commercialization activities in the United States and Celgene will be responsible for development and commercialization activities outside the United States. For all other additional programs for which Celgene opts in, other than JTX-4014, Celgene will lead development and commercialization activities worldwide. If Celgene exercises its option for JTX-4014, the Company and Celgene will enter into a license agreement, in substantially the form attached to the agreement as an exhibit, pursuant to which the Company and Celgene will both be able to equally access JTX-4014 for combinations within each other’s portfolios and with other molecules that are subject to the agreement, subject to joint governance. Once Celgene opts in with respect to a given program, Celgene and the Company must each use commercially reasonable efforts to develop and commercialize the corresponding product in the United States. Termination At any point during the Celgene Collaboration Agreement, including during the research, development and clinical trial process, or during the term of the applicable co-development and co-commercialization or license agreement, respectively, Celgene can terminate the applicable agreement with the Company in its entirety, or with respect to any program under the Celgene Collaboration Agreement, upon 120 days’ notice and can terminate the entire agreement with the Company in connection with a material breach of the agreement by the Company that remains uncured for 90 days. Exclusivity During the Celgene Collaboration Agreement’s research term (i.e., for four years plus up to three one -year extensions that Celgene may elect), the Company may not alone, or with a third party, research, develop, manufacture or commercialize a biologic that binds to ICOS or a defined pool of B cell, T regulatory cell or tumor-associated macrophage targets that meet certain criteria, each termed a “Collaboration Exclusive Target”, and inhibit, activate or otherwise modulate the activity of such Collaboration Exclusive Target. In addition, if Celgene exercises its option for a program within the Celgene Collaboration Agreement, other than JTX-4014, then until termination or expiration of the applicable Co-Co Agreement for such program, the Company may not directly or indirectly research, develop, manufacture or commercialize, outside of the Celgene Collaboration Agreement, any biologic with specified activity against that program’s Collaboration Exclusive Target. Accounting Analysis Identification of the Contract(s) The Company assessed the Celgene Collaboration Agreement and concluded that it represents a contract with a customer within the scope of ASC 606. The Company also concluded that each of the Co-Co Agreements and the JTX-4014 License Agreement, if executed in the future, would represent separate contracts apart from the Celgene Collaboration Agreement. Identification of Promises and Performance Obligations The Company determined that the Celgene Collaboration Agreement contains the following promises: (i) research and development services for the product candidate, JTX-2011 (“JTX-2011 Research Services”) (ii) research and development services for the product candidate, JTX-4014 (“JTX-4014 Research Services”) (iii) research and development services associated with the Lead Program and Other Programs (“Lead and Other Programs Research Services”), (iv) research services associated with target screening (“Target Screening Services”), (v) non-transferable, limited sub-licensable and non-exclusive licenses to use the Company’s intellectual property and the Company’s rights in the collaboration intellectual property to conduct certain activities, on a program-by-program basis (the “Research Licenses”), (vi) various record-keeping and reporting requirements on a program-by-program basis, (vii) exclusivity provisions with respect to each Collaboration Exclusive Target and biologics binding to such Collaboration Exclusive Targets and (viii) establishment of and participation in a joint steering committee (the “JSC”) and a joint patent committee (the “JPC”). The Company also evaluated the six program options as well as the research term extension options and concluded that none convey a material right to Celgene. Accordingly, neither the program options nor the research term extension options are considered to be promises within the Celgene Collaboration Agreement. The Company assessed the above promises and concluded that each of the JTX-2011 Research Services, JTX-4014 Research Services, Lead and Other Programs Research Services and Target Screening Services are both capable of being distinct and distinct within the context of the Celgene Collaboration Agreement. Therefore, the Company has concluded that each of the JTX-2011 Research Services, JTX-4014 Research Services, Lead and Other Programs Research Services and Target Screening Services represent separate performance obligations. The Company determined that the Research Licenses are not distinct within the context of the Celgene Collaboration Agreement as the Research Licenses allow Celgene to evaluate the results of the research and development services performed by the Company and the right to perform its duties under the Celgene Collaboration Agreement, but do not provide Celgene with any commercialization rights. Celgene can only benefit from the Research Licenses in conjunction with the related research and development services. Accordingly, the Research Licenses related to JTX-2011, JTX-4014 and the Lead and Other Programs have been combined with their respective research and development services performance obligations. Similarly, the Company also determined that the various record-keeping and reporting requirements related to each program and the exclusivity provisions with respect to each Collaboration Exclusive Target and biologics binding to such Collaboration Exclusive Targets are not distinct within the context of the Celgene Collaboration Agreement. Accordingly, the various record-keeping and reporting requirements on a program-by-program basis and the exclusivity provisions with respect to each Collaboration Exclusive Target and biologics binding to such Collaboration Exclusive Targets have been combined with their respective research and development services performance obligations. Finally, the Company assessed its participation in the JSC and the JPC and concluded that, while it does meet the definition of a performance obligation, it is both quantitatively and qualitatively immaterial in the context of the Celgene Collaboration Agreement. Accordingly, the Company has disregarded its participation in the JSC and the JPC as a performance obligation. Determination of Transaction Price As noted above, the Company received a non-refundable upfront cash payment of $225.0 million upon the execution of the Celgene Collaboration Agreement. This upfront payment represents an element of fixed consideration under the Celgene Collaboration Agreement. Celgene also purchased 10,448,100 shares of Series B-1 convertible preferred stock for gross proceeds of $36.1 million , which shares converted into 2,831,463 shares of common stock upon the completion of the IPO. The Company determined the shares of Series B-1 convertible preferred stock were sold at fair value. Therefore, the proceeds from the issuance of Series B-1 convertible preferred stock did not impact the transaction price to be allocated to the performance obligations. The Company evaluated as possible variable consideration the milestones, royalties, development cost sharing and profit sharing provisions discussed above. The Company concluded that none of these items represent variable consideration under the Celgene Collaboration Agreement as all such amounts are dependent upon the execution of a related Co-Co Agreement or the JTX-4014 License Agreement. The Co-Co Agreements and the JTX-4014 License Agreement, if executed in the future, would represent separate contracts apart from the Celgene Collaboration Agreement. The Company also considered the existence of any significant financing component within the Celgene Collaboration Agreement given its upfront payment structure. Based upon this assessment, the Company concluded that any difference between the promised consideration and the cash selling price of the services under the Celgene Collaboration Agreement arises for reasons other than the provision of financing, and the difference between those amounts is proportional to the reason for the difference. Accordingly, the Company has concluded that the upfront payment structure of the Celgene Collaboration Agreement does not result in the existence of a significant financing component. Based upon the above considerations, the Company has concluded that the transaction price associated with the Celgene Collaboration Agreement consists solely of the upfront payment of $225.0 million . Allocation of Transaction Price to Performance Obligations The Company has allocated the transaction price to each performance obligation on a relative standalone selling price basis. For all performance obligations, the Company determined the standalone selling price using estimates of the costs to perform the research and development services, including expected internal and external costs for services and supplies, adjusted to reflect a reasonable profit margin. The total estimated cost of the research and development services reflects the nature of the services to be performed and the Company’s best estimate of the length of time required to perform the services. Recognition of Revenue The Company recognizes revenue related to the Celgene Collaboration Agreement over time as the services related to each performance obligation are rendered. The Company has concluded that an input method under ASC 606 is a representative depiction of the transfer of services under the Celgene Collaboration Agreement. The method of measuring progress towards delivery of the services incorporates actual internal and external costs incurred, relative to total internal and external costs expected to be incurred to satisfy the performance obligations. The period over which total costs are estimated reflects the Company’s estimate of the period over which it will perform the research and development services to deliver a pre-defined data package to Celgene for each program subject to an option. The Company recognizes revenue for each performance obligation over periods ranging from twelve months to four years. For the three and nine months ended September 30, 2018 , the Company recognized collaboration revenue of $14.5 million and $45.1 million , respectively, under the Celgene Collaboration Agreement related to the $225.0 million upfront payment received in 2016. As of September 30, 2018 , the Company had $118.0 million of deferred revenue, which is classified as either “current” or “net of current portion” in the accompanying condensed consolidated balance sheets based on the period over which the revenue is expected to be recognized. This deferred revenue balance represents the aggregate amount of the transaction price allocated to the performance obligations that are partially unsatisfied as of September 30, 2018 . The Company expects to recognize revenue related to these performance obligations through July 2020. The following table presents changes in the Company’s contract liabilities during the nine months ended September 30, 2018 (in thousands): Balance as of Balance as of January 1, 2018 Additions Reductions September 30, 2018 Contract liabilities: Deferred revenue $ 163,073 $ — $ (45,101 ) $ 117,972 Totals $ 163,073 $ — $ (45,101 ) $ 117,972 The reductions to the deferred revenue contract liability during the nine months ended September 30, 2018 were comprised of revenue recognized for research and development services performed, as well as a cumulative catch-up adjustment of $4.1 million arising from changes in costs estimated to be incurred under the Celgene Collaboration Agreement. All revenue recognized during the nine months ended September 30, 2018 was included within the beginning balance of the deferred revenue contract liability.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements Fair value is an exit price, representing the amount that would be received from the sale of an asset or paid to transfer a liability in an orderly transaction between market participants. Fair value is determined based on observable and unobservable inputs. Observable inputs reflect readily obtainable data from independent sources, while unobservable inputs reflect certain market assumptions. As a basis for considering such assumptions, GAAP establishes a three-tier value hierarchy, which prioritizes the inputs used to develop the assumptions and for measuring fair value as follows: (Level 1) observable inputs such as quoted prices in active markets for identical assets; (Level 2) inputs other than the quoted prices in active markets that are observable either directly or indirectly; and (Level 3) unobservable inputs in which there is little or no market data, which requires the Company to develop its own assumptions. This hierarchy requires the Company to use observable market data, when available, and to minimize the use of unobservable inputs when determining fair value. The Company measures the fair value of money market funds, U.S. Treasuries and government agency securities based on quoted prices in active markets for identical securities. Investments also include corporate debt securities which are valued either based on recent trades of securities in inactive markets or based on quoted market prices of similar instruments and other significant inputs derived from or corroborated by observable market data. The carrying amounts reflected in the condensed consolidated balance sheets for cash, prepaid expenses and other current assets, accounts payable and accrued expenses approximate their fair values, due to their short-term nature. Assets measured at fair value on a recurring basis as of September 30, 2018 were as follows (in thousands): Total Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs Money market funds, included in cash equivalents $ 50,287 $ 50,287 $ — $ — Investments: Corporate debt securities 65,992 — 65,992 — U.S. Treasuries 63,688 63,688 — — Government agency securities 34,761 34,761 — — Totals $ 214,728 $ 148,736 $ 65,992 $ — Assets measured at fair value on a recurring basis as of December 31, 2017 were as follows (in thousands): Total Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs Money market funds, included in cash equivalents $ 21,059 $ 21,059 $ — $ — Investments: Corporate debt securities 65,173 — 65,173 — U.S. Treasuries 110,948 110,948 — — Government agency securities 58,171 58,171 — — Totals $ 255,351 $ 190,178 $ 65,173 $ — There were no changes in valuation techniques or transfers between the fair value measurement levels during the three and nine months ended September 30, 2018 or during the year ended December 31, 2017 . There were no liabilities measured at fair value on a recurring basis as of September 30, 2018 or December 31, 2017 .

Investments

Investments	9 Months Ended
	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Investments	Investments Short-term investments consist of investments with maturities greater than ninety days and less than one year from the balance sheet date. Long-term investments consist of investments with maturities of greater than one year that are not expected to be used to fund current operations. The Company classifies all of its investments as available-for-sale securities. Accordingly, these investments are recorded at fair value. Realized gains and losses, amortization and accretion of discounts and premiums are included in other income, net. Unrealized gains and losses on available-for-sale securities are included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Cash equivalents, short-term investments and long-term investments as of September 30, 2018 were comprised as follows (in thousands): September 30, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents and short-term investments: Money market funds, included in cash equivalents $ 50,287 $ — $ — $ 50,287 Corporate debt securities 66,033 — (41 ) 65,992 U.S. Treasuries 63,708 — (20 ) 63,688 Government agency securities 31,866 — (74 ) 31,792 Total cash equivalents and short-term investments $ 211,894 $ — $ (135 ) $ 211,759 Long-term investments: Government agency securities 2,974 — (5 ) 2,969 Total long-term investments 2,974 — (5 ) 2,969 Total cash equivalents and investments $ 214,868 $ — $ (140 ) $ 214,728 Cash equivalents, short-term investments and long-term investments as of December 31, 2017 were comprised as follows (in thousands): December 31, 2017 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents and short-term investments: Money market funds, included in cash equivalents $ 21,059 $ — $ — $ 21,059 Corporate debt securities 58,136 — (64 ) 58,072 U.S. Treasuries 111,049 — (101 ) 110,948 Government agency securities 43,204 — (131 ) 43,073 Total cash equivalents and short-term investments 233,448 — (296 ) 233,152 Long-term investments: Corporate debt securities 7,117 — (16 ) 7,101 Government agency securities 15,195 — (97 ) 15,098 Total long-term investments 22,312 — (113 ) 22,199 Total cash equivalents and investments $ 255,760 $ — $ (409 ) $ 255,351 As of September 30, 2018 and December 31, 2017 , the aggregate fair value of securities that were in an unrealized loss position for less than twelve months was $95.7 million and $113.9 million , respectively. As of September 30, 2018 and December 31, 2017 , the aggregate fair value of securities that were in an unrealized loss position for more than twelve months was $31.7 million and $107.9 million , respectively. As of September 30, 2018 , the Company did not intend to sell, and would not be more likely than not required to sell, the securities in an unrealized loss position before recovery of their amortized cost bases. Furthermore, the Company determined that there was no material change in the credit risk of these securities. As a result, the Company determined it did not hold any securities with any other-than-temporary impairment as of September 30, 2018 . There were immaterial realized gains and losses on available-for-sale securities during the three and nine months ended September 30, 2018 and 2017 .

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	9 Months Ended
	Sep. 30, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid Expenses and Other Current Assets	Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets as of September 30, 2018 and December 31, 2017 were comprised as follows (in thousands): September 30, December 31, 2018 2017 Prepaid expenses $ 2,175 $ 2,196 Taxes receivable — 16,737 Interest receivable on investments 431 969 Other current assets — 43 Total prepaid expenses and other current assets $ 2,606 $ 19,945 Taxes receivable decreased from December 31, 2017 to September 30, 2018 due to the Company’s receipt of $16.8 million in federal and state income tax refunds during the nine months ended September 30, 2018 .

Restricted Cash

Restricted Cash	9 Months Ended
	Sep. 30, 2018
Cash and Cash Equivalents [Abstract]
Restricted Cash	Restricted Cash As of both September 30, 2018 and December 31, 2017 , the Company maintained non-current restricted cash of $1.3 million . This amount is included within “Other non-current assets” in the accompanying condensed consolidated balance sheets and is comprised solely of a letter of credit required pursuant to the lease for the Company’s corporate headquarters. The following table provides a reconciliation of cash, cash equivalents and restricted cash that sums to the total of the same such amounts shown in the condensed consolidated statements of cash flows (in thousands): Nine Months Ended Nine Months Ended Beginning of Period End of Period Beginning of Period End of Period Cash and cash equivalents $ 23,559 $ 53,283 $ 44,848 $ 28,634 Restricted cash 1,270 1,270 1,520 1,520 Cash, cash equivalents and restricted cash $ 24,829 $ 54,553 $ 46,368 $ 30,154

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2018
Payables and Accruals [Abstract]
Accrued Expenses	Accrued Expenses Accrued expenses as of September 30, 2018 and December 31, 2017 were comprised as follows (in thousands): September 30, December 31, 2018 2017 Employee compensation and benefits $ 2,987 $ 3,683 External research and professional services 4,678 4,647 Lab consumables and other 114 124 Total accrued expenses $ 7,779 $ 8,454

Common Stock and Preferred Stoc

Common Stock and Preferred Stock	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Common Stock and Preferred Stock	Common Stock and Preferred Stock Common Stock The Company is authorized to issue 160,000,000 shares of common stock. Holders of common stock are entitled to one vote per share. Holders of common stock are entitled to receive dividends, if and when declared by the Board of Directors. Preferred Stock Upon completion of the Company’s IPO, all outstanding preferred stock was automatically converted into an aggregate of 22,283,690 shares of common stock. The Company is also authorized to issue 5,000,000 shares of undesignated preferred stock in one or more series. As of September 30, 2018 , no shares of preferred stock were issued or outstanding. Shares Reserved for Future Issuance As of September 30, 2018 and December 31, 2017 , the Company had reserved for future issuance the following number of shares of common stock (in thousands): September 30, December 31, 2018 2017 Shares reserved for vesting of restricted stock awards 8 16 Shares reserved for vesting of restricted stock units 382 — Shares reserved for exercises of outstanding stock options 5,387 4,868 Shares reserved for future issuance under the 2017 Stock Option and Incentive Plan 986 1,032 Total shares reserved for future issuance 6,763 5,916

Stock-based Compensation

Stock-based Compensation	9 Months Ended
	Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-based Compensation	Stock-based Compensation 2013 Stock Option and Grant Plan In February 2013, the Company’s Board of Directors adopted and the Company’s stockholders approved the 2013 Stock Option and Grant Plan (the “2013 Plan”), as amended and restated, under which it could grant incentive stock options (“ISOs”), non-qualified stock options and restricted stock awards to eligible employees, officers, directors, and consultants. The 2013 Plan was subsequently amended in January 2015, April 2015, July 2015, March 2016 and October 2016 to allow for the issuance of additional shares of common stock. 2017 Stock Option and Incentive Plan In January 2017, the Company’s Board of Directors adopted and the Company’s stockholders approved the 2017 Stock Option and Incentive Plan (the “2017 Plan”), which became effective immediately prior to the effectiveness of the Company’s IPO. Upon the adoption of the 2017 Plan, no further awards will be granted under the 2013 Plan. The 2017 Plan provides for the grant of ISOs, non-qualified stock options, restricted stock awards (“RSAs”), restricted stock units (“RSUs”), stock appreciation rights and other stock-based awards. The Company’s employees, officers, directors and consultants and advisors are eligible to receive awards under the 2017 Plan. The terms of awards, including vesting requirements, are determined by the Board of Directors, subject to the provisions of the 2017 Plan. The Company initially registered on a Registration Statement on Form S-8 1,753,758 shares of common stock under the 2017 Plan, which was comprised of (i) 1,510,000 shares of common stock reserved for issuance under the 2017 Plan, plus (ii) 243,758 shares of common stock originally reserved for issuance under the 2013 Plan that became available for issuance under the 2017 Plan upon the completion of the Company’s IPO. The 2017 Plan also provides that an additional number of shares will automatically be added to the shares authorized for issuance under the 2017 Plan on January 1, 2018 and each January 1 st thereafter. The number of shares added each year will be equal to the lesser of (i) 4% of the outstanding shares on the immediately preceding December 31 st or (ii) such amount as determined by the Compensation Committee of the Board of Directors. Effective January 1, 2018, 1,290,609 additional shares were automatically added to the shares authorized for issuance under the 2017 Plan and these shares were subsequently registered on a Registration Statement on Form S-8. As of September 30, 2018 , there were 985,939 shares available for future issuance under the 2017 Plan. 2017 Employee Stock Purchase Plan In January 2017, the Board of Directors adopted and the Company’s stockholders approved the 2017 Employee Stock Purchase Plan (the “2017 ESPP”), which became effective upon the closing of the Company’s IPO. The Company initially reserved 302,000 shares of common stock for future issuance under the 2017 ESPP. The 2017 ESPP also provides that an additional number of shares will automatically be added to the shares authorized for issuance under the 2017 ESPP on January 1, 2018 and each January 1 st thereafter through January 1, 2027. The number of shares added each year will be equal to the lesser of (i) 1% of the outstanding shares on the immediately preceding December 31 st , (ii) 603,000 shares or (iii) such amount as determined by the Compensation Committee of the Board of Directors. Effective January 1, 2018, 322,652 additional shares were automatically added to the shares authorized for issuance under the 2017 ESPP and these shares and were subsequently registered on a Registration Statement on Form S-8. No offering periods under the 2017 ESPP had been initiated as of September 30, 2018 . Stock-based Compensation Expense Total stock-based compensation expense recognized in the condensed consolidated statements of operations for the three and nine months ended September 30, 2018 and 2017 was as follows (in thousands): Three Months Ended Nine Months Ended 2018 2017 2018 2017 Research and development $ 1,055 $ 593 $ 3,417 $ 2,143 General and administrative 1,280 481 3,546 1,320 Total stock-based compensation expense $ 2,335 $ 1,074 $ 6,963 $ 3,463 RSA Activity Pursuant to RSA agreements originally issued under the terms of the 2013 Plan, the Company, at its discretion, has the option to repurchase unvested shares underlying RSAs at the initial purchase price if the employees or non-employees terminate their service relationships with the Company. The shares underlying RSAs are recorded in stockholders’ equity as they vest. The following table summarizes RSA activity for the nine months ended September 30, 2018 (in thousands, except per share amounts): RSAs Weighted-Average Grant Date Fair Value per Share Unvested as of December 31, 2017 16 $ — Issued — $ — Vested (8 ) $ — Repurchased — $ — Unvested as of September 30, 2018 8 $ — The aggregate fair value of RSAs that vested during the three months ended September 30, 2018 and 2017 , based upon the fair values of the stock underlying the RSAs on the day of vesting, was less than $0.1 million and $0.1 million , respectively. The aggregate fair value of RSAs that vested during the nine months ended September 30, 2018 and 2017 was $0.1 million and $1.2 million , respectively. RSU Activity The Company has also granted RSUs to its employees under the 2017 Plan. The following table summarizes RSU activity for the nine months ended September 30, 2018 (in thousands, except per share amounts): RSUs Weighted-Average Grant Date Fair Value per Share Unvested as of December 31, 2017 — $ — Issued 388 $ 8.02 Vested — $ — Cancelled (6 ) $ 8.02 Unvested as of September 30, 2018 382 $ 8.02 No RSUs vested during the three or nine months ended September 30, 2018 or 2017 . As of September 30, 2018 , there was unrecognized stock-based compensation expense related to unvested RSUs of $2.9 million , which the Company expects to recognize over a weighted-average period of approximately 1.9 years . Stock Option Activity The fair value of stock options granted during the three and nine months ended September 30, 2018 and 2017 was calculated on the date of grant using the following weighted-average assumptions: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Risk-free interest rate 2.9 % 1.9 % 2.7 % 2.0 % Expected dividend yield — % — % — % — % Expected term (in years) 6.1 6.1 6.0 6.1 Expected volatility 66.2 % 66.8 % 65.1 % 71.4 % Using the Black-Scholes option pricing model, the weighted-average grant date fair value of stock options granted during the three months ended September 30, 2018 and 2017 was $4.59 per share and $9.80 per share, respectively. The weighted-average grant date fair value of stock options granted during the nine months ended September 30, 2018 and 2017 was $13.12 and $11.30 , respectively. The following table summarizes stock option activity during the nine months ended September 30, 2018 (in thousands, except per share amounts): Options Weighted-Average Exercise Price Weighted Average Remaining Contractual Term (in years) Aggregate Intrinsic Value Outstanding at December 31, 2017 4,868 $ 6.28 7.9 $ 35,178 Granted 1,543 $ 21.55 Exercised (436 ) $ 2.98 Cancelled (588 ) $ 15.39 Outstanding at September 30, 2018 5,387 $ 9.93 7.3 $ 10,872 Exercisable at September 30, 2018 2,825 $ 5.21 6.1 $ 9,564 The aggregate intrinsic value of stock options exercised during the three months ended September 30, 2018 and 2017 was $0.6 million and $0.5 million , respectively. The aggregate intrinsic value of stock options exercised during the nine months ended September 30, 2018 and 2017 was $5.0 million and $1.5 million , respectively. As of September 30, 2018 , there was unrecognized stock-based compensation expense related to unvested stock options of $21.9 million , which the Company expects to recognize over a weighted-average period of approximately 2.5 years .

Related-party Transactions

Related-party Transactions	9 Months Ended
	Sep. 30, 2018
Related Party Transactions [Abstract]
Related-party Transactions	Related-party Transactions In July 2016, the Company entered into the Celgene Collaboration Agreement and a Series B-1 Preferred Stock Purchase Agreement with Celgene. Under the Celgene Collaboration Agreement, the Company received a non-refundable upfront payment of $225.0 million . Under the Series B-1 Preferred Stock Purchase Agreement, Celgene purchased 10,448,100 shares of Series B-1 convertible preferred stock for $36.1 million . These shares of Series B-1 convertible preferred stock converted into 2,831,463 shares of common stock upon the completion of the Company’s IPO. In addition, an affiliate of Celgene purchased 625,000 shares of the Company’s common stock in the IPO at the public offering price of $16.00 per share for a total of $10.0 million .

Net Loss per Share

Net Loss per Share	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Net Loss per Share	Net Loss per Share For purposes of the diluted loss per share calculation, outstanding stock options, unvested RSAs and unvested RSUs are considered to be potentially dilutive securities, however the following amounts were excluded from the calculation of diluted net loss per share because their effect would be anti-dilutive (in thousands): Three and Nine Months Ended 2018 2017 Outstanding stock options 5,387 4,780 Unvested RSAs 8 20 Unvested RSUs 382 — Total 5,777 4,800

Basis of Presentation and Sum_2

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	The accompanying condensed consolidated financial statements as of September 30, 2018 and December 31, 2017 , and for the three and nine months ended September 30, 2018 and 2017 , have been prepared in accordance with the rules and regulations of the Securities and Exchange Commission (the “SEC”) and generally accepted accounting principles in the United States of America (“GAAP”) as found in the Accounting Standards Codification (“ASC”) of the Financial Accounting Standards Board (“FASB”) for condensed consolidated financial information. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. In the opinion of management, these condensed consolidated financial statements reflect all normal recurring adjustments which are necessary for a fair presentation of the Company’s financial position and results of its operations, as of and for the periods presented. These condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto contained in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017 filed with the SEC on March 8, 2018 (the “Annual Report on Form 10-K”). The information presented in the condensed consolidated financial statements and related notes as of September 30, 2018 , and for the three and nine months ended September 30, 2018 and 2017 , is unaudited. The December 31, 2017 condensed consolidated balance sheet included herein was derived from the audited financial statements as of that date, but does not include all disclosures, including notes, required by GAAP for complete financial statements. Interim results for the nine months ended September 30, 2018 are not necessarily indicative of the results that may be expected for the fiscal year ending December 31, 2018 , or any future period.
Principles of Consolidation	The accompanying condensed consolidated financial statements include the accounts of Jounce Therapeutics, Inc. and its wholly-owned subsidiary, Jounce Mass Securities, Inc. All intercompany transactions and balances have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. On an ongoing basis, the Company’s management evaluates its estimates which include, but are not limited to, estimates related to revenue recognized under the Celgene Collaboration Agreement, accrued expenses, stock-based compensation expense and income taxes . The Company bases its estimates on historical experience and other market specific or other relevant assumptions it believes to be reasonable under the circumstances. Actual results could differ from those estimates.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the FASB issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (Topic 606) , which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. This guidance was originally effective for interim and annual periods beginning after December 15, 2016 and allowed for adoption using a full retrospective method, or a modified retrospective method. Subsequent to the issuance of ASU 2014-09, the FASB also issued the following updates related to ASC 606: • In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date , whereby the effective date for the new revenue standard was deferred by one year. As a result of ASU 2015-14, the new revenue standard is now effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2017, and early adoption was permitted for annual periods beginning after December 15, 2016, including interim periods within that annual period. • In March 2016, the FASB issued ASU 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net) , to clarify the implementation guidance on principal versus agent considerations. • In April 2016, the FASB issued ASU 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing , to clarify the principle for determining whether a good or service is “separately identifiable” from other promises in the contract and to clarify the categorization of licenses of intellectual property. • In May 2016, the FASB issued ASU 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Technical Expedients , to clarify guidance on transition, determining collectability, non-cash consideration and the presentation of sales and other similar taxes. • In December 2016, the FASB issued ASU 2016-20, Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers , that allows entities not to make qualitative disclosures about remaining performance obligations in certain cases, adds disclosure requirements for entities that elect certain optional exemptions and adds twelve additional technical corrections and improvements to the new revenue standard. The Company adopted ASC 606 effective January 1, 2018 under the modified retrospective method. The modified retrospective method requires that the cumulative effect of initially applying ASC 606 be recognized as an adjustment to the opening balance of retained earnings or accumulated deficit of the annual period that includes the date of initial application. Accordingly, during the first quarter of 2018, the Company recorded an increase to the opening balance of accumulated deficit and a corresponding increase to deferred revenue of $46.9 million related to the Celgene Collaboration Agreement. Additionally, the following tables present a summary of the amount by which each financial statement line item was affected as of and during the three and nine months ended September 30, 2018 by the application of ASC 606 as compared to ASC Topic 605, Revenue Recognition (“ASC 605”), the revenue recognition guidance that was in effect before this change in accounting principle (in thousands, except per share amounts): September 30, 2018 ASC 606 ASC 605 Difference Deferred revenue, current—related party $ 58,947 $ 43,804 $ 15,143 Deferred revenue, net of current portion—related party $ 59,025 $ 33,387 $ 25,638 Three Months Ended September 30, 2018 ASC 606 ASC 605 Difference Collaboration revenue—related party $ 14,528 $ 12,989 $ 1,539 Net loss $ (7,637 ) $ (9,176 ) $ 1,539 Net loss per share attributable to common stockholders, basic and diluted $ (0.23 ) $ (0.28 ) $ 0.05 Nine Months Ended September 30, 2018 ASC 606 ASC 605 Difference Collaboration revenue—related party $ 45,101 $ 38,968 $ 6,133 Net loss $ (25,339 ) $ (31,472 ) $ 6,133 Net loss per share attributable to common stockholders, basic and diluted $ (0.78 ) $ (0.97 ) $ 0.19 The application of ASC 606 did not have an impact on the Company’s net cash used in operating activities for the nine months ended September 30, 2018 , but did result in offsetting adjustments to net loss and the change in deferred revenue presented within the condensed consolidated statement of cash flows for that period. Both the cumulative adjustment of $46.9 million recorded upon the initial application of ASC 606 and the differences outlined above are primarily attributable to the transition from recognizing revenue on a straight-line basis over the estimated performance period for each unit of accounting, which was a permitted method of revenue recognition under ASC 605, to recognizing revenue based on the Company’s pattern of performance for each performance obligation under ASC 606. As part of the adoption of ASC 606, the Company implemented new processes to objectively measure the performance under the Celgene Collaboration Agreement. See Note 3, “Celgene Collaboration Agreement”, for further information on the application of ASC 606 to the Celgene Collaboration Agreement. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) , which requires a lessee to recognize assets and liabilities on the balance sheet for operating leases and changes many key definitions, including the definition of a lease. The new standard includes a short-term lease exception for leases with a term of 12 months or less, as part of which a lessee can make an accounting policy election not to recognize lease assets and lease liabilities. Lessees will continue to differentiate between finance leases (previously referred to as capital leases) and operating leases using classification criteria that are substantially similar to the previous guidance. In July 2018, the FASB also issued ASU 2018-11, Leases (Topic 842): Targeted Improvements , which permits entities to continue applying legacy guidance in ASC Topic 840, Leases , including its disclosure requirements, in the comparative periods presented in the year that the entity adopts the new leasing standard. The new standard will be effective for annual reporting periods beginning after December 15, 2018, including interim periods within those annual reporting periods, and early adoption is permitted for public entities. The Company will adopt this new standard on January 1, 2019 using the transition method permitted by ASU 2018-11, and it is currently evaluating the impact that ASU 2016-02 may have on the condensed consolidated financial statements. The Company has substantially completed an assessment of its contracts, and it expects to record lease assets and lease liabilities upon adoption of this guidance. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments , which is intended to reduce diversity in practice in how entities present certain types of cash transactions in the statement of cash flows. This guidance also clarifies how the predominance principle should be applied when classifying cash receipts and cash payments that have attributes of more than one class of cash flows. This guidance became effective for annual reporting periods beginning after December 15, 2017, including interim periods within those annual reporting periods. Accordingly, the Company adopted ASU 2016-15 effective January 1, 2018, and there was not a material impact to the condensed consolidated financial statements as a result of the adoption of this guidance. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash , which requires entities to show the change in the total of cash, cash equivalents, restricted cash and restricted cash equivalents within the statement of cash flows. As a result, entities no longer separately present transfers between unrestricted cash and restricted cash. This guidance became effective for annual reporting periods beginning after December 15, 2017, including interim periods within those annual reporting periods. Accordingly, the Company adopted ASU 2016-18 effective January 1, 2018 using a retrospective transition method, and there was not a material impact to the condensed consolidated financial statements as a result of the adoption of this guidance. See Note 7, “Restricted Cash”, for incremental disclosures associated with the adoption of ASU 2016-18. In May 2017, the FASB issued ASU 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting . This guidance is intended to provide clarity and reduce diversity in practice as to when changes to the terms or conditions of share-based payments are accounted for as modifications. Under this new guidance, entities are required to apply modification accounting if the fair value, vesting conditions or classification of the award changes. This guidance became effective for annual reporting periods beginning after December 15, 2017, including interim periods within those annual reporting periods, and early adoption was permitted. The guidance per ASU 2017-09 is to be adopted prospectively to an award modified on or after the adoption date. The Company adopted ASU 2017-09 effective January 1, 2018, and there was no impact to the condensed consolidated financial statements as a result of the adoption of this guidance. In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting . This guidance is intended to simplify the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. This guidance will be effective for annual reporting periods beginning after December 15, 2018, including interim periods within those annual reporting periods, and early adoption is permitted. The Company does not anticipate a material impact to the condensed consolidated financial statements as a result of the adoption of this guidance. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement, which eliminates, adds and modifies certain disclosure requirements for fair value measurements. This guidance will be effective for annual reporting periods beginning after December 15, 2019, including interim periods within those annual reporting periods, and early adoption is permitted. More specifically, an entity is permitted to early adopt any removed or modified disclosure requirements immediately and delay adoption of additional disclosure requirements until the effective date of this guidance. The Company does not anticipate a material impact to the condensed consolidated financial statements as a result of the adoption of this guidance. In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 , which clarifies that certain transactions between participants in a collaborative arrangement should be accounted for under ASC 606 when the counterparty is a customer. In addition, ASU 2018-18 adds unit-of-account guidance to ASC Topic 808, Collaborative Arrangements , in order to align this guidance with ASC 606 and also precludes an entity from presenting consideration from a transaction in a collaborative arrangement as revenue from contracts with customers if the counterparty is not a customer for that transaction. This guidance will be effective for annual reporting periods beginning after December 15, 2019, including interim periods within those annual reporting periods, and early adoption is permitted. The Company is currently evaluating the potential impact that ASU 2018-18 may have on the condensed consolidated financial statements.
Fair Value Measurements	Fair value is an exit price, representing the amount that would be received from the sale of an asset or paid to transfer a liability in an orderly transaction between market participants. Fair value is determined based on observable and unobservable inputs. Observable inputs reflect readily obtainable data from independent sources, while unobservable inputs reflect certain market assumptions. As a basis for considering such assumptions, GAAP establishes a three-tier value hierarchy, which prioritizes the inputs used to develop the assumptions and for measuring fair value as follows: (Level 1) observable inputs such as quoted prices in active markets for identical assets; (Level 2) inputs other than the quoted prices in active markets that are observable either directly or indirectly; and (Level 3) unobservable inputs in which there is little or no market data, which requires the Company to develop its own assumptions. This hierarchy requires the Company to use observable market data, when available, and to minimize the use of unobservable inputs when determining fair value. The Company measures the fair value of money market funds, U.S. Treasuries and government agency securities based on quoted prices in active markets for identical securities. Investments also include corporate debt securities which are valued either based on recent trades of securities in inactive markets or based on quoted market prices of similar instruments and other significant inputs derived from or corroborated by observable market data. The carrying amounts reflected in the condensed consolidated balance sheets for cash, prepaid expenses and other current assets, accounts payable and accrued expenses approximate their fair values, due to their short-term nature.

Basis of Presentation and Sum_3

Basis of Presentation and Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Schedule of New Accounting Pronouncements and Changes in Accounting Principles	Additionally, the following tables present a summary of the amount by which each financial statement line item was affected as of and during the three and nine months ended September 30, 2018 by the application of ASC 606 as compared to ASC Topic 605, Revenue Recognition (“ASC 605”), the revenue recognition guidance that was in effect before this change in accounting principle (in thousands, except per share amounts): September 30, 2018 ASC 606 ASC 605 Difference Deferred revenue, current—related party $ 58,947 $ 43,804 $ 15,143 Deferred revenue, net of current portion—related party $ 59,025 $ 33,387 $ 25,638 Three Months Ended September 30, 2018 ASC 606 ASC 605 Difference Collaboration revenue—related party $ 14,528 $ 12,989 $ 1,539 Net loss $ (7,637 ) $ (9,176 ) $ 1,539 Net loss per share attributable to common stockholders, basic and diluted $ (0.23 ) $ (0.28 ) $ 0.05 Nine Months Ended September 30, 2018 ASC 606 ASC 605 Difference Collaboration revenue—related party $ 45,101 $ 38,968 $ 6,133 Net loss $ (25,339 ) $ (31,472 ) $ 6,133 Net loss per share attributable to common stockholders, basic and diluted $ (0.78 ) $ (0.97 ) $ 0.19

Celgene Collaboration Agreeme_2

Celgene Collaboration Agreement (Tables)	9 Months Ended
	Sep. 30, 2018
Research and Development [Abstract]
Contract with Customer, Asset and Liability	The following table presents changes in the Company’s contract liabilities during the nine months ended September 30, 2018 (in thousands): Balance as of Balance as of January 1, 2018 Additions Reductions September 30, 2018 Contract liabilities: Deferred revenue $ 163,073 $ — $ (45,101 ) $ 117,972 Totals $ 163,073 $ — $ (45,101 ) $ 117,972

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Assets Measured at Fair Value	Assets measured at fair value on a recurring basis as of September 30, 2018 were as follows (in thousands): Total Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs Money market funds, included in cash equivalents $ 50,287 $ 50,287 $ — $ — Investments: Corporate debt securities 65,992 — 65,992 — U.S. Treasuries 63,688 63,688 — — Government agency securities 34,761 34,761 — — Totals $ 214,728 $ 148,736 $ 65,992 $ — Assets measured at fair value on a recurring basis as of December 31, 2017 were as follows (in thousands): Total Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs Money market funds, included in cash equivalents $ 21,059 $ 21,059 $ — $ — Investments: Corporate debt securities 65,173 — 65,173 — U.S. Treasuries 110,948 110,948 — — Government agency securities 58,171 58,171 — — Totals $ 255,351 $ 190,178 $ 65,173 $ —

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Schedule of Available-for-sale Securities by Security Type	Cash equivalents, short-term investments and long-term investments as of September 30, 2018 were comprised as follows (in thousands): September 30, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents and short-term investments: Money market funds, included in cash equivalents $ 50,287 $ — $ — $ 50,287 Corporate debt securities 66,033 — (41 ) 65,992 U.S. Treasuries 63,708 — (20 ) 63,688 Government agency securities 31,866 — (74 ) 31,792 Total cash equivalents and short-term investments $ 211,894 $ — $ (135 ) $ 211,759 Long-term investments: Government agency securities 2,974 — (5 ) 2,969 Total long-term investments 2,974 — (5 ) 2,969 Total cash equivalents and investments $ 214,868 $ — $ (140 ) $ 214,728 Cash equivalents, short-term investments and long-term investments as of December 31, 2017 were comprised as follows (in thousands): December 31, 2017 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash equivalents and short-term investments: Money market funds, included in cash equivalents $ 21,059 $ — $ — $ 21,059 Corporate debt securities 58,136 — (64 ) 58,072 U.S. Treasuries 111,049 — (101 ) 110,948 Government agency securities 43,204 — (131 ) 43,073 Total cash equivalents and short-term investments 233,448 — (296 ) 233,152 Long-term investments: Corporate debt securities 7,117 — (16 ) 7,101 Government agency securities 15,195 — (97 ) 15,098 Total long-term investments 22,312 — (113 ) 22,199 Total cash equivalents and investments $ 255,760 $ — $ (409 ) $ 255,351

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	9 Months Ended
	Sep. 30, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets as of September 30, 2018 and December 31, 2017 were comprised as follows (in thousands): September 30, December 31, 2018 2017 Prepaid expenses $ 2,175 $ 2,196 Taxes receivable — 16,737 Interest receivable on investments 431 969 Other current assets — 43 Total prepaid expenses and other current assets $ 2,606 $ 19,945

Restricted Cash (Tables)

Restricted Cash (Tables)	9 Months Ended
	Sep. 30, 2018
Cash and Cash Equivalents [Abstract]
Schedule of Restricted Cash	The following table provides a reconciliation of cash, cash equivalents and restricted cash that sums to the total of the same such amounts shown in the condensed consolidated statements of cash flows (in thousands): Nine Months Ended Nine Months Ended Beginning of Period End of Period Beginning of Period End of Period Cash and cash equivalents $ 23,559 $ 53,283 $ 44,848 $ 28,634 Restricted cash 1,270 1,270 1,520 1,520 Cash, cash equivalents and restricted cash $ 24,829 $ 54,553 $ 46,368 $ 30,154
Schedule of Cash and Cash Equivalents	The following table provides a reconciliation of cash, cash equivalents and restricted cash that sums to the total of the same such amounts shown in the condensed consolidated statements of cash flows (in thousands): Nine Months Ended Nine Months Ended Beginning of Period End of Period Beginning of Period End of Period Cash and cash equivalents $ 23,559 $ 53,283 $ 44,848 $ 28,634 Restricted cash 1,270 1,270 1,520 1,520 Cash, cash equivalents and restricted cash $ 24,829 $ 54,553 $ 46,368 $ 30,154

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2018
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses as of September 30, 2018 and December 31, 2017 were comprised as follows (in thousands): September 30, December 31, 2018 2017 Employee compensation and benefits $ 2,987 $ 3,683 External research and professional services 4,678 4,647 Lab consumables and other 114 124 Total accrued expenses $ 7,779 $ 8,454

Common Stock and Preferred St_2

Common Stock and Preferred Stock (Tables)	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Schedule of Stock by Class	As of September 30, 2018 and December 31, 2017 , the Company had reserved for future issuance the following number of shares of common stock (in thousands): September 30, December 31, 2018 2017 Shares reserved for vesting of restricted stock awards 8 16 Shares reserved for vesting of restricted stock units 382 — Shares reserved for exercises of outstanding stock options 5,387 4,868 Shares reserved for future issuance under the 2017 Stock Option and Incentive Plan 986 1,032 Total shares reserved for future issuance 6,763 5,916

Stock-based Compensation (Table

Stock-based Compensation (Tables)	9 Months Ended
	Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Stock-Based Compensation Expense	Total stock-based compensation expense recognized in the condensed consolidated statements of operations for the three and nine months ended September 30, 2018 and 2017 was as follows (in thousands): Three Months Ended Nine Months Ended 2018 2017 2018 2017 Research and development $ 1,055 $ 593 $ 3,417 $ 2,143 General and administrative 1,280 481 3,546 1,320 Total stock-based compensation expense $ 2,335 $ 1,074 $ 6,963 $ 3,463
Schedule of Restricted Stock Activity	The Company has also granted RSUs to its employees under the 2017 Plan. The following table summarizes RSU activity for the nine months ended September 30, 2018 (in thousands, except per share amounts): RSUs Weighted-Average Grant Date Fair Value per Share Unvested as of December 31, 2017 — $ — Issued 388 $ 8.02 Vested — $ — Cancelled (6 ) $ 8.02 Unvested as of September 30, 2018 382 $ 8.02 The following table summarizes RSA activity for the nine months ended September 30, 2018 (in thousands, except per share amounts): RSAs Weighted-Average Grant Date Fair Value per Share Unvested as of December 31, 2017 16 $ — Issued — $ — Vested (8 ) $ — Repurchased — $ — Unvested as of September 30, 2018 8 $ —
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions	The fair value of stock options granted during the three and nine months ended September 30, 2018 and 2017 was calculated on the date of grant using the following weighted-average assumptions: Three Months Ended Nine Months Ended 2018 2017 2018 2017 Risk-free interest rate 2.9 % 1.9 % 2.7 % 2.0 % Expected dividend yield — % — % — % — % Expected term (in years) 6.1 6.1 6.0 6.1 Expected volatility 66.2 % 66.8 % 65.1 % 71.4 %
Schedule of Stock Options, Activity	The following table summarizes stock option activity during the nine months ended September 30, 2018 (in thousands, except per share amounts): Options Weighted-Average Exercise Price Weighted Average Remaining Contractual Term (in years) Aggregate Intrinsic Value Outstanding at December 31, 2017 4,868 $ 6.28 7.9 $ 35,178 Granted 1,543 $ 21.55 Exercised (436 ) $ 2.98 Cancelled (588 ) $ 15.39 Outstanding at September 30, 2018 5,387 $ 9.93 7.3 $ 10,872 Exercisable at September 30, 2018 2,825 $ 5.21 6.1 $ 9,564

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Schedule of Antidilutive Securities Excluded from Computation of Net Loss per Share	For purposes of the diluted loss per share calculation, outstanding stock options, unvested RSAs and unvested RSUs are considered to be potentially dilutive securities, however the following amounts were excluded from the calculation of diluted net loss per share because their effect would be anti-dilutive (in thousands): Three and Nine Months Ended 2018 2017 Outstanding stock options 5,387 4,780 Unvested RSAs 8 20 Unvested RSUs 382 — Total 5,777 4,800

Nature of Business (Details)

Nature of Business (Details) $ / shares in Units, $ in Thousands	Nov. 13, 2018	Feb. 01, 2017USD ($)$ / sharesshares	Jan. 13, 2017	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from initial public offering of common stock, net of issuance costs				$ 0	$ 107,008
Reverse stock split			0.2710
Cash, cash equivalents, and marketable securities				$ 214,700
IPO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock (in shares) | shares		7,319,750
Public offering share price (in dollars per share) | $ / shares		$ 16
Proceeds from initial public offering of common stock, net of issuance costs		$ 117,100
Proceeds from IPO, net of discounts, commissions, and other offering expenses		$ 106,400
Stock converted upon completion of IPO (in shares) | shares		22,283,690
Over-Allotment Option
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock (in shares) | shares		954,750
Scenario, Forecast
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Duration of funding requirement	24 months

Basis of Presentation and Sum_4

Basis of Presentation and Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Accumulated deficit	$ (161,867)	$ (89,615)
Deferred revenue	$ 117,972	163,073
Cumulative adjustment		46,900
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Accumulated deficit		46,900
Deferred revenue		$ 46,900

Basis of Presentation and Sum_5

Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Revenue Recognition (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	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current—related party	$ 58,947		$ 58,947		$ 51,142
Deferred revenue, net of current portion—related party	59,025		59,025		$ 65,018
Collaboration revenue—related party	14,528	$ 18,077	45,101	$ 58,655
Net loss	$ (7,637)	$ (4,084)	$ (25,339)	$ (7,079)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.23)	$ (0.13)	$ (0.78)	$ (0.27)
Calculated under Revenue Guidance in Effect before Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current—related party	$ 43,804		$ 43,804
Deferred revenue, net of current portion—related party	33,387		33,387
Collaboration revenue—related party	12,989		38,968
Net loss	$ (9,176)		$ (31,472)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.28)		$ (0.97)
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current—related party	$ 15,143		$ 15,143
Deferred revenue, net of current portion—related party	25,638		25,638
Collaboration revenue—related party	1,539		6,133
Net loss	$ 1,539		$ 6,133
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ 0.05		$ 0.19

Celgene Collaboration Agreeme_3

Celgene Collaboration Agreement - Narrative (Details) $ in Thousands	Feb. 01, 2017shares	Jul. 31, 2016USD ($)extensiondeliverablecustomer_programshares	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue—related party			$ 14,528	$ 18,077	$ 45,101	$ 58,655
Deferred revenue			$ 117,972		117,972		$ 163,073
Cumulative catch-up adjustment from changes in costs estimated to be incurred					$ 4,100
Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of early-stage programs | customer_program		4
Non-refundable upfront payment received for research agreement		$ 225,000
Aggregate option-exercise fees from program options		$ 182,500
Number of program options | deliverable		6
Initial research term		4 years
Potential addition to research term		3 years
Aggregate consideration for additional years of research		$ 120,000
Potential milestone revenue		2,300,000
Aggregate development milestone revenue		290,000
Aggregate regulatory approval milestone revenue		700,000
Aggregate commercial milestone revenue		$ 1,270,000
Number of program extensions | extension		3
Program extension research term		1 year
Convertible preferred stock (Series B-1) | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Issuances of convertible preferred stock		$ 36,100
Issuances of convertible preferred stock (in shares) | shares		10,448,100
Minimum | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Option-exercise fee for program option		$ 10,000
Consideration for additional year of research		30,000
Development milestone revenue, per program		32,500
Regulatory approval milestone revenue, per program		7,500
Commercial milestone revenue, per program		$ 40,000
Program option term		45 days
Maximum | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Option-exercise fee for program option		$ 60,000
Consideration for additional year of research		45,000
Development milestone revenue, per program		105,000
Regulatory approval milestone revenue, per program		50,000
Commercial milestone revenue, per program		$ 200,000
Program option term		60 days
JTX-2011 | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percent of operating profit or loss retained		60.00%
JTX-2011 | Celgene Corporation | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percent of operating profit or loss retained		40.00%
Lead Program | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percent of operating profit or loss retained		25.00%
Lead Program | Celgene Corporation | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percent of operating profit or loss retained		75.00%
Other Programs | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percent of operating profit or loss retained		50.00%
Other Programs | Celgene Corporation | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percent of operating profit or loss retained		50.00%
All Programs, Excluding JTX-4014 | Celgene Corporation | Minimum | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percent of development costs		67.00%
All Programs, Excluding JTX-4014 | Celgene Corporation | Maximum | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Percent of development costs		85.00%
IPO
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Stock converted upon completion of IPO (in shares) | shares	22,283,690
Celgene Corporation | Celgene Collaboration Agreement | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Non-refundable upfront payment received for research agreement		$ 225,000
Celgene Corporation | Celgene Collaboration Agreement | Convertible preferred stock (Series B-1) | Celegene Collaborative Arrangement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Issuances of convertible preferred stock		$ 36,100
Issuances of convertible preferred stock (in shares) | shares		10,448,100
Celgene Corporation | Celgene Collaboration Agreement | IPO | Common Stock
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Stock converted upon completion of IPO (in shares) | shares	2,831,463

Celgene Collaboration Agreeme_4

Celgene Collaboration Agreement - Performance Obligations (Details) - Celegene Collaborative Arrangement - Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-04-01	Sep. 30, 2018
Minimum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period for unit of accounting	12 months
Maximum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period for unit of accounting	4 years

Celgene Collaboration Agreeme_5

Celgene Collaboration Agreement - Schedule of Contract Liabilities (Details) $ in Thousands	3 Months Ended
	Sep. 30, 2018USD ($)
Revenue, Performance Obligation, Contract Liability [Roll Forward]
Additions	$ 0
Reductions	(45,101)
Ending balance	$ 117,972

Fair Value Measurements - Asset

Fair Value Measurements - Assets Measured at Fair Value (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Measurements, Recurring
Investments:
Totals	$ 214,728,000	$ 255,351,000
Liabilities measured at fair value	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investments:
Totals	148,736,000	190,178,000
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Investments:
Totals	65,992,000	65,173,000
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Investments:
Totals	0	0
Money market funds, included in cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds, included in cash equivalents	50,287,000	21,059,000
Money market funds, included in cash equivalents | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds, included in cash equivalents	50,287,000	21,059,000
Money market funds, included in cash equivalents | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds, included in cash equivalents	50,287,000	21,059,000
Money market funds, included in cash equivalents | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds, included in cash equivalents	0	0
Money market funds, included in cash equivalents | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds, included in cash equivalents	0	0
Corporate debt securities | Fair Value, Measurements, Recurring
Investments:
Available-for-sale debt securities, fair value	65,992,000	65,173,000
Corporate debt securities | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investments:
Available-for-sale debt securities, fair value	0	0
Corporate debt securities | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Investments:
Available-for-sale debt securities, fair value	65,992,000	65,173,000
Corporate debt securities | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Investments:
Available-for-sale debt securities, fair value	0	0
U.S. Treasuries | Fair Value, Measurements, Recurring
Investments:
Available-for-sale debt securities, fair value	63,688,000	110,948,000
U.S. Treasuries | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investments:
Available-for-sale debt securities, fair value	63,688,000	110,948,000
U.S. Treasuries | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Investments:
Available-for-sale debt securities, fair value	0	0
U.S. Treasuries | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Investments:
Available-for-sale debt securities, fair value	0	0
Government agency securities | Fair Value, Measurements, Recurring
Investments:
Available-for-sale debt securities, fair value	34,761,000	58,171,000
Government agency securities | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investments:
Available-for-sale debt securities, fair value	34,761,000	58,171,000
Government agency securities | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Investments:
Available-for-sale debt securities, fair value	0	0
Government agency securities | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Investments:
Available-for-sale debt securities, fair value	$ 0	$ 0

Investments - Available-for-sal

Investments - Available-for-sale Securities by Security Type (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Unrealized Gains	$ 0	$ 0
Unrealized Losses	(140)	(409)
Cash equivalents, short-term and long-term investments, carrying value	214,868	255,760
Cash equivalents, short-term and long-term investments, fair vale disclosure	214,728	255,351
Money market funds, included in cash equivalents
Debt Securities, Available-for-sale [Line Items]
Cash equivalents at carrying value	50,287	21,059
Cash equivalents at fair value	50,287	21,059
Short-term Investments
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Unrealized Gains	0	0
Unrealized Losses	(135)	(296)
Total cash equivalents and short-term investments, carrying value	211,894	233,448
Total cash equivalents and short-term investments, fair value	211,759	233,152
Short-term Investments | Corporate debt securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Available-for-sale debt securities, amortized cost basis	66,033	58,136
Unrealized Gains	0	0
Unrealized Losses	(41)	(64)
Available-for-sale debt securities, fair value	65,992	58,072
Short-term Investments | U.S. Treasuries
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Available-for-sale debt securities, amortized cost basis	63,708	111,049
Unrealized Gains	0	0
Unrealized Losses	(20)	(101)
Available-for-sale debt securities, fair value	63,688	110,948
Short-term Investments | Government agency securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Available-for-sale debt securities, amortized cost basis	31,866	43,204
Unrealized Gains	0	0
Unrealized Losses	(74)	(131)
Available-for-sale debt securities, fair value	31,792	43,073
Long-term Investments
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Available-for-sale debt securities, amortized cost basis	2,974	22,312
Unrealized Gains	0	0
Unrealized Losses	(5)	(113)
Available-for-sale debt securities, fair value	2,969	22,199
Long-term Investments | Corporate debt securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Available-for-sale debt securities, amortized cost basis		7,117
Unrealized Gains		0
Unrealized Losses		(16)
Available-for-sale debt securities, fair value		7,101
Long-term Investments | U.S. Treasuries
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Available-for-sale debt securities, amortized cost basis		15,195
Unrealized Gains		0
Unrealized Losses		(97)
Available-for-sale debt securities, fair value		$ 15,098
Long-term Investments | Government agency securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Available-for-sale debt securities, amortized cost basis	2,974
Unrealized Gains	0
Unrealized Losses	(5)
Available-for-sale debt securities, fair value	$ 2,969

Investments - Narrative (Detail

Investments - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities in an unrealized loss position for less than twelve months	$ 95,700		$ 95,700		$ 113,900
Number of securities in unrealized loss position, greater than 12 months	31,700		31,700		107,900
Realized gains or losses on available-for-sale securities	0	$ 0	0	$ 0
Long-term Investments
Debt Securities, Available-for-sale [Line Items]
Long-term investments	$ 2,974		$ 2,974		$ 22,312

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid expenses	$ 2,175	$ 2,196
Taxes receivable	0	16,737
Interest receivable on investments	431	969
Other current assets	0	43
Total prepaid expenses and other current assets	$ 2,606	$ 19,945

Prepaid Expenses and Other Cu_4

Prepaid Expenses and Other Current Assets - Narrative (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Receipt of federal and state income tax refunds	$ 16.8

Restricted Cash - Narrative (De

Restricted Cash - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Abstract]
Non-current restricted cash	$ 1.3	$ 1.3

Restricted Cash - Schedule of C

Restricted Cash - Schedule of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 53,283	$ 23,559	$ 28,634	$ 44,848
Restricted cash	1,270	1,270	1,520	1,520
Cash, cash equivalents and restricted cash	$ 54,553	$ 24,829	$ 30,154	$ 46,368

Accrued Expenses (Details)

Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Employee compensation and benefits	$ 2,987	$ 3,683
External research and professional services	4,678	4,647
Lab consumables and other	114	124
Total accrued expenses	$ 7,779	$ 8,454

Common Stock and Preferred St_3

Common Stock and Preferred Stock - Narrative (Details)	Feb. 01, 2017shares	Sep. 30, 2018voteshares	Dec. 31, 2017shares
Class of Stock [Line Items]
Common stock, shares authorized (in shares)		160,000,000	160,000,000
Common stock, votes per share | vote		1
Preferred stock, shares authorized (in shares)		5,000,000	5,000,000
Preferred stock, shares issued (in shares)		0	0
Preferred stock, shares outstanding (in shares)		0	0
IPO
Class of Stock [Line Items]
Stock converted upon completion of IPO (in shares)	22,283,690

Common Stock and Preferred St_4

Common Stock and Preferred Stock - Shares Reserved for Future Issuance (Details) - shares	Sep. 30, 2018	Dec. 31, 2017	Jan. 31, 2017
Conversion of Stock [Line Items]
Common stock reserved for potential conversion (in shares)	6,763,000	5,916,000	1,753,758
Restricted Stock
Conversion of Stock [Line Items]
Common stock reserved for potential conversion (in shares)	8,000	16,000
Restricted Stock Units (RSUs)
Conversion of Stock [Line Items]
Common stock reserved for potential conversion (in shares)	382,000	0
Outstanding Employee Stock Options
Conversion of Stock [Line Items]
Common stock reserved for potential conversion (in shares)	5,387,000	4,868,000
Future Issuances from Employee Stock Options
Conversion of Stock [Line Items]
Common stock reserved for potential conversion (in shares)	986,000	1,032,000

Stock-based Compensation - Narr

Stock-based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	Jan. 01, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jan. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock eligible to be purchased (in shares)		6,763,000		6,763,000		5,916,000	1,753,758
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock eligible to be purchased (in shares)		8,000		8,000		16,000
Aggregate fair value of awards vested in period		$ 0.1	$ 0.1	$ 0.1	$ 1.2
Vested (in shares)				(8,000)
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock eligible to be purchased (in shares)		382,000		382,000		0
Vested (in shares)		0	0	0	0
Unrecognized stock-based compensation expense, RSUs		$ 2.9		$ 2.9
Remaining weighted average vesting period				1 year 10 months 13 days
Future Issuances from Employee Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for issuance (in shares)		985,939		985,939
Shares of common stock eligible to be purchased (in shares)		986,000		986,000		1,032,000
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value of options granted (in dollars per share)		$ 4.59	$ 9.80	$ 13.12	$ 11.30
Unrecognized stock-based compensation expense, options		$ 21.9		$ 21.9
Remaining weighted average vesting period				2 years 5 months 19 days
2013 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for issuance (in shares)							0
Shares of common stock eligible to be purchased (in shares)							243,758
2017 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock eligible to be purchased (in shares)							1,510,000
Percent of outstanding shares able to be added each year							4.00%
Number of additional shares authorized (in shares)	1,290,609
2013 Stock Option and Grant Plan, Employees | Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of stock options exercised		$ 0.6	$ 0.5	$ 5	$ 1.5
2017 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock eligible to be purchased (in shares)							302,000
Percent of outstanding shares able to be added each year							1.00%
Number of additional shares authorized (in shares)	322,652
Shares of common stock to determine number of additional shares (in shares)							603,000

Stock-based Compensation - Stoc

Stock-based Compensation - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 2,335	$ 1,074	$ 6,963	$ 3,463
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	1,055	593	3,417	2,143
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 1,280	$ 481	$ 3,546	$ 1,320

Stock-based Compensation - RSA

Stock-based Compensation - RSA and RSU Activity (Details) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Beginning unvested balance (in shares)			16,000
Issued (in shares)			0
Vested (in shares)			(8,000)
Repurchased (in shares)			0
Ending unvested balance (in shares)	8,000		8,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Beginning unvested balance (in dollars per share)			$ 0
Issued (in dollars per share)			0
Vested (in dollars per share)			0
Repurchased (in dollars per share)			0
Ending unvested balance (in dollars per share)	$ 0		$ 0
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Beginning unvested balance (in shares)			0
Issued (in shares)			388,000
Vested (in shares)	0	0	0	0
Cancelled (in shares)			(6,000)
Ending unvested balance (in shares)	382,000		382,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Beginning unvested balance (in dollars per share)			$ 0
Issued (in dollars per share)			8.02
Vested (in dollars per share)			0
Cancelled (in dollars per share)			8.02
Ending unvested balance (in dollars per share)	$ 8.02		$ 8.02

Stock-based Compensation - Weig

Stock-based Compensation - Weighted Average Assumptions (Details) - Employee Stock Option	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.90%	1.90%	2.70%	2.00%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Expected term (in years)	6 years 1 month 2 days	6 years 1 month 10 days	6 years 7 days	6 years 1 month 10 days
Expected volatility	66.20%	66.80%	65.10%	71.40%

Stock-based Compensation - St_2

Stock-based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning outstanding balance (in shares)	4,868
Granted (in shares)	1,543
Exercised (in shares)	(436)
Cancelled or forfeited (in shares)	(588)
Ending outstanding balance (in shares)	5,387	4,868
Exercisable (in shares)	2,825
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Beginning outstanding balance (in dollars per share)	$ 6.28
Granted (in dollars per share)	21.55
Exercised (in dollars per share)	2.98
Cancelled or forfeited (in dollars per share)	15.39
Ending outstanding balance (in dollars per share)	9.93	$ 6.28
Exercisable (in dollars per share)	$ 5.21
Remaining contractual life, outstanding	7 years 3 months 29 days	7 years 10 months 28 days
Remaining contractual life, exercisable	6 years 1 month 10 days
Aggregate intrinsic value, outstanding	$ 10,872	$ 35,178
Aggregate intrinsic value, exercisable	$ 9,564

Related-party Transactions (Det

Related-party Transactions (Details) - USD ($) $ / shares in Units, $ in Millions	Feb. 01, 2017	Jul. 31, 2016
Celegene Collaborative Arrangement
Related Party Transaction [Line Items]
Non-refundable upfront payment received for research agreement		$ 225
Convertible preferred stock (Series B-1) | Celegene Collaborative Arrangement
Related Party Transaction [Line Items]
Issuances of convertible preferred stock (in shares)		10,448,100
Issuances of convertible preferred stock		$ 36.1
IPO
Related Party Transaction [Line Items]
Stock converted upon completion of IPO (in shares)	22,283,690
Proceeds from IPO, net of discounts, commissions, and other offering expenses	$ 106.4
Celgene Corporation | Celgene Collaboration Agreement | Celegene Collaborative Arrangement
Related Party Transaction [Line Items]
Non-refundable upfront payment received for research agreement		$ 225
Celgene Corporation | Celgene Collaboration Agreement | Convertible preferred stock (Series B-1) | Celegene Collaborative Arrangement
Related Party Transaction [Line Items]
Issuances of convertible preferred stock (in shares)		10,448,100
Issuances of convertible preferred stock		$ 36.1
Celgene Corporation | IPO | Celgene Collaboration Agreement | Common Stock
Related Party Transaction [Line Items]
Stock converted upon completion of IPO (in shares)	2,831,463
Affiliated Entity | Celgene Corporation | IPO | Celgene Collaboration Agreement | Common Stock
Related Party Transaction [Line Items]
Sale of stock, number of shares issued (in shares)	625,000
Sale of stock, price per share (USD per share)	$ 16
Proceeds from IPO, net of discounts, commissions, and other offering expenses	$ 10

Net Loss per Share (Details)

Net Loss per Share (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	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 (in shares)	5,777	4,800	5,395	4,800
Outstanding stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of net loss per share (in shares)	5,387	4,780	5,387	4,780
Unvested RSAs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of net loss per share (in shares)	8	20	8	20
Unvested RSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of net loss per share (in shares)	382	0	382	0