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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2018	Jul. 31, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q2
Trading Symbol	UMRX
Entity Registrant Name	UNUM THERAPEUTICS INC.
Entity Central Index Key	0001622229
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		29,867,248

Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 46,732	$ 28,270
Marketable securities	47,715	12,691
Accounts receivable	1,202	830
Prepaid expenses and other current assets	1,412	513
Restricted cash	50	75
Total current assets	97,111	42,379
Property and equipment, net	3,938	4,108
Restricted cash	1,255	1,255
Deferred offering costs		1,373
Other assets	419
Total assets	102,723	49,115
Current liabilities:
Accounts payable	1,664	1,346
Accrued expenses and other current liabilities	2,787	2,953
Deferred revenue	20,363	6,891
Total current liabilities	24,814	11,190
Deferred rent	888	906
Deferred revenue, net of current portion	346	8,714
Total liabilities	26,048	20,810
Commitments and contingencies (Note 11)
Redeemable convertible preferred stock (Series A and B), $0.001 par value; no shares and 20,791,407 shares authorized at June 30, 2018 and December 31, 2017, respectively; no shares and 20,771,850 shares issued and outstanding at June 30, 2018 and December 31, 2017, respectively; liquidation preference of $77,297 at December 31, 2017		77,151
Stockholders' equity (deficit):
Preferred stock, $0.001 par value; 10,000,000 shares and no shares authorized at June 30, 2018 and December 31, 2017, respectively; no shares issued or outstanding at June 30, 2018 and December 31, 2017
Common stock, $0.001 par value; 150,000,000 shares and 60,040,000 shares authorized at June 30, 2018 and December 31, 2017, respectively; 29,867,248 shares and 10,201,690 shares issued and outstanding at June 30, 2018 and December 31, 2017, respectively	30	10
Additional paid-in capital	149,943	2,499
Accumulated other comprehensive loss	(13)	(16)
Accumulated deficit	(73,285)	(51,339)
Total stockholders' equity (deficit)	76,675	(48,846)
Total liabilities, redeemable convertible preferred stock and stockholders' equity (deficit)	$ 102,723	$ 49,115

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Redeemable convertible preferred stock (Series A and B), Par value	$ 0.001	$ 0.001
Redeemable convertible preferred stock (Series A and B), Shares authorized	0	20,791,407
Redeemable convertible preferred stock (Series A and B), Shares issued	0	20,771,850
Redeemable convertible preferred stock (Series A and B), Shares outstanding	0	20,771,850
Redeemable convertible preferred stock (Series A and B), Liquidation preference		$ 77,297
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, Par value	$ 0.001	$ 0.001
Common stock, Shares authorized	150,000,000	60,040,000
Common stock, Shares issued	29,867,248	10,201,690
Common stock, Shares outstanding	29,867,248	10,201,690

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Collaboration revenue	$ 1,666	$ 2,079	$ 3,886	$ 3,906
Operating expenses:
Research and development	9,126	7,141	17,268	14,093
General and administrative	1,979	971	3,043	1,915
Total operating expenses	11,105	8,112	20,311	16,008
Loss from operations	(9,439)	(6,033)	(16,425)	(12,102)
Other income (expense):
Interest income	259	97	340	187
Other income, net	157	73	327	113
Total other income, net	416	170	667	300
Net loss	(9,023)	(5,863)	(15,758)	(11,802)
Accretion of redeemable convertible preferred stock to redemption value		(17)	(16)	(33)
Net loss attributable to common stockholders	$ (9,023)	$ (5,880)	$ (15,774)	$ (11,835)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.31)	$ (0.58)	$ (0.80)	$ (1.16)
Weighted average common shares outstanding, basic and diluted	29,155,790	10,190,228	19,732,542	10,190,228
Comprehensive loss:
Net loss	$ (9,023)	$ (5,863)	$ (15,758)	$ (11,802)
Other comprehensive income (loss):
Unrealized gains (losses) on marketable securities, net of tax of $0	(6)	(7)	3	(19)
Total other comprehensive income (loss)	(6)	(7)	3	(19)
Comprehensive loss	$ (9,029)	$ (5,870)	$ (15,755)	$ (11,821)

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Unrealized gains (losses) on marketable securities, tax	$ 0	$ 0	$ 0	$ 0

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net loss	$ (15,758)	$ (11,802)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	639	564
Stock-based compensation expense	1,326	582
Premiums paid on marketable securities		(13)
Net amortization (accretion) of premiums (discounts) on marketable securities	(39)	5
Non-cash interest expense	11	9
Changes in operating assets and liabilities:
Accounts receivable	(372)	(330)
Prepaid expenses and other current assets	(910)	(306)
Other assets	(419)
Accounts payable	503	440
Accrued expenses and other current liabilities	227	(423)
Deferred rent	(18)	7
Deferred revenue	(1,084)	(1,762)
Net cash used in operating activities	(15,894)	(13,029)
Cash flows from investing activities:
Purchases of property and equipment	(463)	(537)
Purchases of marketable securities	(47,682)	(6,500)
Maturities and sales of marketable securities	12,700	10,000
Net cash provided by (used in) investing activities	(35,445)	2,963
Cash flows from financing activities:
Proceeds from initial public offering, net of underwriting discounts and commissions	66,793
Proceeds from private placement concurrent with initial public offering	5,000
Proceeds from issuance of common stock upon stock option exercises	39
Payments of initial public offering costs	(2,056)
Net cash provided by financing activities	69,776
Net increase (decrease) in cash, cash equivalents and restricted cash	18,437	(10,066)
Cash, cash equivalents and restricted cash at beginning of period	29,600	42,576
Cash, cash equivalents and restricted cash at end of period	48,037	32,510
Supplemental disclosure of noncash investing and financing information:
Conversion of redeemable convertible preferred stock into common stock	77,167
Purchases of property and equipment included in accounts payable and accrued expenses	81	70
Accretion of redeemable convertible preferred stock to redemption value	$ 16	$ 33

Nature of the Business and Basis of Presentation	6 Months Ended
Nature of the Business and Basis of Presentation	Jun. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Unum Therapeutics Inc. (“Unum” or the “Company”) is a clinical-stage biopharmaceutical company focused on the development and commercialization of novel immunotherapy products designed to harness the power of a patient’s immune system to cure cancer. The Company’s proprietary technology, called antibody-coupled T cell receptor (“ACTR”), is a universal, engineered cell therapy that is intended to be used in combination with a wide range of tumor-specific antibodies to target different tumor types. Unum was incorporated in March 2014 under the laws of the State of Delaware. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s drug development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. On March 16, 2018, the Company effected a one-for-1.5701314513884 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s redeemable convertible preferred stock (see Note 7). Accordingly, all share and per share amounts for all periods presented in the accompanying consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and adjustment of the redeemable convertible preferred stock conversion ratios. On April 3, 2018, the Company completed an initial public offering (“IPO”) of its common stock and issued and sold 5,770,000 shares of common stock at a public offering price of $12.00 per share, resulting in net proceeds of $61.5 million after deducting underwriting discounts and commissions and other offering costs. In addition, Seattle Genetics, Inc. (“Seattle Genetics”) purchased from the Company, concurrently with the IPO in a private placement, $5.0 million of shares of common stock at a price per share equal to the initial public offering price, or 416,666 shares (the “concurrent private placement”). The shares of common stock sold to Seattle Genetics in the concurrent private placement are not registered under the Securities Act of 1933, as amended, and are subject to a 180-day lock-up agreement with the underwriters of the IPO. Upon closing of the IPO, the Company’s outstanding redeemable convertible preferred stock automatically converted into shares of common stock (see Note 7). Upon conversion of the redeemable convertible preferred stock, the Company reclassified the carrying value of the redeemable convertible preferred stock to common stock and additional paid-in capital. On April 25, 2018, the Company issued and sold an additional 215,000 shares of its common stock at the IPO price of $12.00 per share pursuant to the underwriters’ partial exercise of their option to purchase additional shares of common stock, resulting in additional net proceeds of $2.4 million after deducting underwriting discounts and commissions. The accompanying consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. Since inception, the Company has funded its operations with the sales of redeemable convertible preferred stock, payments received in connection with a collaboration agreement, and most recently, with proceeds from the IPO and concurrent private placement completed in April 2018. The Company has incurred recurring losses since inception, including net losses attributable to the Company of $15.8 million for the six months ended June 30, 2018. As of June 30, 2018, the Company had an accumulated deficit of $73.3 million. The Company expects to continue to generate operating losses in the foreseeable future. As of August 13, 2018, the issuance date of the interim consolidated financial statements, the Company expects that its cash, cash equivalents and marketable securities will be sufficient to fund its operating expenses and capital expenditure requirements through at least 12 months from the issuance date of the interim consolidated financial statements, without considering available borrowings under the Company’s loan and security agreement. The Company will ultimately need to seek additional funding through equity offerings, debt financings, collaborations, licensing arrangements and other marketing and distribution arrangements, partnerships, joint ventures, combinations or divestitures of one or more of its businesses. The Company may not be able to obtain financing on acceptable terms, or at all, and the Company may not be able to enter into collaborative arrangements or divest its assets. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. Arrangements with collaborators or others may require the Company to relinquish rights to certain of its technologies or product candidates. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate its research and development programs or commercialization efforts, which could adversely affect its business prospects. The Company’s consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiary. All intercompany accounts and transactions have been eliminated in consolidation.

Summary of Significant Accounting Policies

6 Months Ended

Jun. 30, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Unaudited Interim Financial Information

The consolidated balance sheet at December 31, 2017 was derived from audited financial statements but does not include all disclosures required by GAAP. The accompanying unaudited consolidated financial statements as of June 30, 2018 and for the three and six months ended June 30, 2018 and 2017 have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. Certain information and footnote disclosures normally included in the financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. These consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2017 included in the Company’s Registration Statement on Form S-1, File No. 333-223414 on file with the SEC. In the opinion of management, all adjustments, consisting only of normal recurring adjustments necessary for a fair statement of the Company’s financial position as of June 30, 2018 and results of operations for the three and six months ended June 30, 2018 and 2017 and cash flows for the six months ended June 30, 2018 and 2017 have been made. The Company’s results of operations for the three and six months ended June 30, 2018 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2018.

Concentrations of Credit Risk and of Significant Suppliers

Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. The Company maintains most of its cash and cash equivalents at three accredited financial institutions. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships.

The Company is dependent on third-party vendors for its product candidates. In particular, the Company relies, and expects to continue to rely, on a small number of vendors to manufacture supplies and process its product candidates for its development programs. These programs could be adversely affected by a significant interruption in the manufacturing process.

Cash Equivalents

The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents.

Fair Value Measurements

Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:

•

Level 1—Quoted prices in active markets for identical assets or liabilities.

•

Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.

•

Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.

The Company’s cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts receivable, accounts payable and accrued expenses approximate their fair values due to the short-term nature of these assets and liabilities.

Marketable Securities

The Company’s marketable securities are classified as available-for-sale and are carried at fair value, with the unrealized gains and losses reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity (deficit). Realized gains and losses and declines in value determined to be other than temporary are based on the specific identification method and are included as a component of other income (expense), net in the consolidated statements of operations and comprehensive loss. The Company classifies its marketable securities with maturities beyond one year as short-term, based on their highly liquid nature and because such marketable securities are available for current operations.

The Company evaluates its marketable securities with unrealized losses for other-than-temporary impairment. When assessing marketable securities for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates, assumptions and judgments that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates, assumptions and judgments reflected in these condensed consolidated financial statements include, but are not limited to, revenue, the accrual of research and development expenses and the valuation of stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.

Classification and Accretion of Redeemable Convertible Preferred Stock

The Company classified redeemable convertible preferred stock outside of stockholders’ equity (deficit) because the shares contained certain redemption features that were not solely within the control of the Company. The carrying values of the redeemable convertible preferred stock were accreted to their respective redemption values from the date of issuance through the earliest date of redemption.

Collaboration Agreements

The Company follows the accounting guidance for collaboration agreements, which requires that certain transactions between the Company and collaborators be recorded in its consolidated statements of operations and comprehensive loss on either a gross basis or net basis, depending on the characteristics of the collaborative relationship, and requires enhanced disclosure of collaborative relationships. The Company evaluates its collaboration agreements for proper classification in its consolidated statements of operations and comprehensive loss based on the nature of the underlying activity. If payments to and from collaborative partners are not within the scope of other authoritative accounting literature, the consolidated statements of operations classification for the payments is based on a reasonable, rational analogy to authoritative accounting literature that is applied in a consistent manner. When the Company has concluded that it has a customer relationship with one of its collaborators, such as that with Seattle Genetics (see Note 5), the Company follows the guidance in Accounting Standards Codification (“ASC”) Topic 606, Revenue From Contracts With Customers (“ASC 606”).

Revenue Recognition of Collaboration Agreements

On January 1, 2018, the Company adopted the new revenue standard, discussed below under the heading “Recently Adopted Accounting Pronouncements”, which amended revenue recognition principles and provides a single, comprehensive set of criteria for revenue recognition within and across all industries. The new revenue standard provides a five-step framework whereby revenue is recognized when control of promised goods or services is transferred to a customer at an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. To determine revenue recognition for arrangements that the Company determines are within the scope of the new revenue standard, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies a performance obligation. The Company only applies the five-step model to contracts when collectability of the consideration to which the Company is entitled in exchange for the goods or services it transfers to the customer is determined to be probable.

At contract inception, once the contract is determined to be within the scope of the new revenue standard, the Company assesses whether the goods or services promised within each contract are distinct and, therefore, represent a separate performance obligation. Goods and services that are determined not to be distinct are combined with other promised goods and services until a distinct bundle is identified. In determining whether goods or services are distinct, management evaluates certain criteria, including whether (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (capable of being distinct) and (ii) the good or service is separately identifiable from other goods or services in the contract (distinct in the context of the contract).

At the inception of an arrangement that includes options for a customer to purchase additional services or products at agreed upon prices in the future, the Company evaluates whether each option provides a material right. An option that provides a material right will be accounted for as a separate performance obligation.

The Company then determines the transaction price, which is the amount of consideration it expects to be entitled from a customer in exchange for the promised goods or services, for each performance obligation and recognizes the associated revenue as each performance obligation is satisfied. The Company’s estimate of the transaction price for each contract includes all variable consideration to which it expects to be entitled. Variable consideration includes payments in the form of collaboration payments, regulatory milestone payments, commercial milestone payments, and royalty payments. For collaboration, regulatory milestone, and commercial milestone payments the Company evaluates whether it is probable that the consideration associated with each milestone will not be subject to a significant reversal in the cumulative amount of revenue recognized. Amounts that meet this threshold are included in the transaction price using the most likely amount method, whereas amounts that do not meet this threshold are considered constrained and excluded from the transaction price until they meet this threshold. At the end of each subsequent reporting period, the Company re-evaluates the probability of a significant reversal of the cumulative revenue recognized for its milestones, and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis. The Company excludes sales-based royalties until the sale occurs.

The new revenue standard requires the Company to allocate the arrangement consideration on a relative standalone selling price basis for each performance obligation after determining the transaction price of the contract and identifying the performance obligations to which that amount should be allocated. The relative standalone selling price is defined in the new revenue standard as the price at which an entity would sell a promised good or service separately to a customer. If other observable transactions in which the Company has sold the same performance obligation separately are not available, the Company is required to estimate the standalone selling price of each performance obligation. Key assumptions to determine the standalone selling price may include forecasted revenues, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success. A performance obligation is satisfied and revenue is recognized when “control” of the promised good or service is transferred, either over time or at a point in time, to the customer. A customer obtains control of a good or service if it has the ability to (1) direct its use and (2) obtain substantially all of the remaining benefits from it.

If a contract should be accounted for as a combined performance obligation, the Company determines the period over which the performance obligations will be performed and revenue will be recognized. The Company will recognize revenue using the cost-to-cost method, which it believes best depicts the transfer of control to the customer. Under the cost-to-cost method, the extent of progress towards completion is measured based on the ratio of actual costs incurred to the total estimated costs expected upon satisfying the identified performance obligation. Under this method, revenue will be recorded as a percentage of the estimated transaction price based on the extent of progress towards completion. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which the Company is expected to complete its performance obligations under an arrangement. The estimate of the Company’s measure of progress and estimate of variable consideration to be included in the transaction price will be updated at each reporting date as a change in estimate. The amount of transaction price allocated to the satisfied portion of the performance obligation, based on the Company’s measure of progress, will be recognized immediately on a cumulative catch-up basis, resulting in an adjustment to revenue in the period of change. The amount related to the unsatisfied portion will be recognized as that portion is satisfied over time.

Amounts received prior to satisfying the revenue recognition criteria listed above are recorded as deferred revenue in the accompanying consolidated balance sheets. Amounts expected to be recognized as revenue within 12 months of the balance sheet date are classified as current deferred revenue. Amounts not expected to be recognized as revenue within the following 12 months of the balance sheet date are classified as deferred revenue, net of current portion. At June 30, 2018, the Company had current deferred revenue of $20.4 million and deferred revenue, net of current portion of $0.3 million related to its collaboration. The Company recognized revenue of $1.7 million and $3.9 million during the three and six months ended June 30, 2018, respectively, from the deferred revenue balance at January 1, 2018. The Company recognizes deferred revenue by first allocating from the beginning deferred revenue balance to the extent that the beginning deferred revenue balance exceeds the revenue to be recognized. Billings during the period are added to the deferred revenue balance to be recognized in future periods. To the extent that the beginning deferred revenue balance is less than revenue to be recognized during the period, billings during the period are allocated to revenue. In the event that a collaboration agreement was to be terminated and the Company had no further performance obligations, the Company would recognize as revenue any portion of the upfront payment and other payments that had not previously been recorded as revenue and were classified as deferred revenue at the date of such termination.

Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is that the period between payment by the customer and the transfer of the promised goods or services to the customer will be one year or less. The Company expenses incremental costs of obtaining a contract as and when incurred if the expected amortization period of the asset that the Company would have recognized is one year or less or the amount is immaterial. At June 30, 2018, the Company has not capitalized any costs to obtain its contract.

Recently Adopted Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which supersedes existing revenue recognition guidance under GAAP. The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The standard outlines a five-step process to achieve this principle, and requires companies to use more judgment and make more estimates than under the previous guidance. The Company determined that these judgments and estimates include identifying performance obligations in the customer contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU 2014-09 also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, which delayed the effective date of ASU 2014-09 such that the standard became effective for public entities for annual periods beginning after December 15, 2017 and for interim periods within those fiscal years. The FASB subsequently issued amendments to ASU No. 2014-09 that had the same effective dates and transition requirements as ASU 2014-09, all of which collectively are herein referred to as “ASC 606”.

On January 1, 2018, the Company adopted the new revenue standard by applying the modified retrospective method to its collaboration agreement with Seattle Genetics (see Note 5) which was not completed as of January 1, 2018. As a result, while reporting periods beginning on the Company’s adoption of the new revenue standard are presented under the new revenue standard, prior period amounts have not been adjusted and continue to be presented under the revenue standard in effect prior to January 1, 2018.

The following table summarizes the cumulative effect to the Company’s consolidated balance sheet upon the adoption of the new revenue standard on January 1, 2018 (in thousands):


	Balance at December 31, 2017			Adjustments			Balance at January 1, 2018
Deferred revenue, current and net of current portion	$	15,605		$	6,188		$	21,793
Accumulated deficit	$	(51,339	)	$	(6,188	)	$	(57,527	)

The adjustment is the result of the application of the new revenue standard regarding how entities should measure progress in satisfying performance obligations and the contract’s transaction price. Under ASC 606, the Company will recognize revenue using the cost-to-cost method, which it believes best depicts the transfer of control to the customer. In contrast, under the previous revenue standard, the Company recognized revenue on a straight-line basis over the estimated period of performance. In addition, under ASC 606, the estimated transaction price will include variable consideration for payments expected to be earned for preclinical research and clinical development activities through Phase I, which, under the previous standard, the Company was precluded from including in the estimated transaction price until such payments were determinable and due.

The Company will account for the license, research and development services, and steering committee services under ASC 606 as a single performance obligation under the collaboration agreement, just as it accounted for those items as a single unit of accounting under the previous standard. The options held by Seattle Genetics are expected to continue to be accounted for separately as they do not represent material rights based on the criteria of ASC 606. Further, ASC 606 will not have an impact on the Company’s current accounting for milestone or royalty payments.

In accordance with the new revenue standard requirements, the following tables summarize the impact of adoption on the Company’s consolidated balance sheets, consolidated statements of operations and comprehensive loss, and consolidated statements of cash flows (in thousands):

Consolidated Balance Sheet


	At June 30, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Deferred revenue, current portion	$	20,363		$	7,471		$	12,892
Deferred revenue, net of current portion	$	346		$	5,703		$	(5,357	)
Accumulated deficit	$	(73,285	)	$	(65,750	)	$	(7,535	)

Consolidated Statements of Operations and Comprehensive Loss


	Three Months Ended June 30, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Collaboration revenue	$	1,666		$	2,572		$	(906	)
Net loss	$	(9,023	)	$	(8,117	)	$	(906	)
Net loss attributable to common stockholders	$	(9,023	)	$	(8,117	)	$	(906	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.31	)	$	(0.28	)	$	(0.03	)
Comprehensive loss	$	(9,029	)	$	(8,123	)	$	(906	)


	Six Months Ended June 30, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Collaboration revenue	$	3,886		$	5,233		$	(1,347	)
Net loss	$	(15,758	)	$	(14,411	)	$	(1,347	)
Net loss attributable to common stockholders	$	(15,774	)	$	(14,427	)	$	(1,347	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.80	)	$	(0.73	)	$	(0.07	)
Comprehensive loss	$	(15,755	)	$	(14,408	)	$	(1,347	)

Consolidated Statement of Cash Flows


	Six Months Ended June 30, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Net loss	$	(15,758	)	$	(14,411	)	$	(1,347	)
Change in deferred revenue	$	(1,084	)	$	(2,431	)	$	1,347

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“ASU 2016-15”), to address diversity in practice in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. The Company adopted the standard retrospectively for all periods presented on the required effective date of January 1, 2018, and its adoption had no impact on the Company’s financial position, results of operations or cash flows.

In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows, Restricted Cash, requiring restricted cash and restricted cash equivalents to be included with cash and cash equivalents on the statement of cash flows when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The Company adopted this standard during the first quarter of 2018. Restricted cash is now included as a component of cash, cash equivalents, and restricted cash on the Company’s unaudited consolidated statements of cash flows. The inclusion of restricted cash increased the beginning and ending balances of the unaudited consolidated statements of cash flows by $1.3 million for the six months ended June 30, 2018 and 2017.

In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017-09”), which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. The Company adopted the standard prospectively beginning on the required effective date of January 1, 2018, and its adoption did not have a material impact on the Company’s financial position, results of operations or cash flows.

Recently Issued Accounting Pronouncements

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease, respectively. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. The guidance is effective for public entities for annual reporting periods beginning after December 15, 2018 and for interim periods within those fiscal years, and early adoption is permitted. The Company is currently evaluating the impact that the adoption of ASU 2016-02 will have on its consolidated financial statements.

In July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception (“ASU 2017-11”). Part I applies to entities that issue financial instruments such as warrants, convertible debt or convertible preferred stock that contain down-round features. Part II replaces the indefinite deferral for certain mandatorily redeemable noncontrolling interests and mandatorily redeemable financial instruments of nonpublic entities contained within ASC Topic 480 with a scope exception and does not impact the accounting for these mandatorily redeemable instruments. ASU 2017-11 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. The Company is currently evaluating the impact that the adoption of ASU 2017-11 will have on its consolidated financial statements.

In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”). ASU 2018-07 is intended to simplify aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. ASU 2018-07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the impact that the adoption of ASU 2018-07 will have on its consolidated financial statements.

Fair Value of Financial Assets and Liabilities

6 Months Ended

Jun. 30, 2018

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets and Liabilities

3. Fair Value of Financial Assets and Liabilities

Marketable securities by security type consisted of the following (in thousands):


	June 30, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury notes (due within one year)	$	44,776	$	—	$	(12	)	$	44,764
U.S. Treasury bills (due within one year)		2,952		—		(1	)		2,951

	$	47,728	$	—	$	(13	)	$	47,715


	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury notes (due within one year)	$	5,007	$	—	$	(10	)	$	4,997
U.S. government agency bonds (due within one year)		7,700		—		(6	)		7,694

	$	12,707	$	—	$	(16	)	$	12,691

The following tables present information about the Company’s assets that are measured at fair value on a recurring basis (in thousands):


	Fair Value Measurements at June 30, 2018 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	—	$	29,414	$	—	$	29,414
U.S. Treasury notes		4,996		—		—		4,996
Marketable securities:
U.S. Treasury notes		44,764		—		—		44,764
U.S. Treasury bills		2,951		—		—		2,951

	$	52,711	$	29,414	$	—	$	82,125


	Fair Value Measurements at December 31, 2017 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	—	$	24,196	$	—	$	24,196
Marketable securities:
U.S. Treasury notes		4,997		—		—		4,997
U.S. government agency bonds		—		7,694		—		7,694

	$	4,997	$	31,890	$	—	$	36,887

During the three and six months ended June 30, 2018 and 2017, there were no transfers between Level 1, Level 2 and Level 3.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Accrued Expenses and Other Current Liabilities

6 Months Ended

Jun. 30, 2018

Payables and Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

4. Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):


	June 30, 2018		December 31, 2017
Accrued employee compensation and benefits	$	810	$	1,315
Accrued professional fees		198		980
Accrued external research and development expense		874		478
Accrued other		905		180

	$	2,787	$	2,953

Collaboration Agreement	6 Months Ended
Collaboration Agreement	Jun. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaboration Agreement	5. Collaboration Agreement The Company has a collaboration agreement with Seattle Genetics, entered into in 2015, whereby the parties agreed to jointly develop two product candidates incorporating the Company’s ACTR platform and Seattle Genetics’ antibodies. Under the collaboration agreement, the Company conducts preclinical research and clinical development activities related to the two specified product candidates through Phase I clinical development, and Seattle Genetics provides the funding for those activities. Under the collaboration agreement, the Company recognized revenue of $1.7 million and $3.9 million for the three and six months ended June 30, 2018, respectively, related to research and clinical development activities performed. The Company recognized $2.1 million and $3.9 million for the three and six months ended June 30, 2017, respectively, related to research and clinical development activities performed. As of June 30, 2018 and December 31, 2017, deferred revenue of $20.7 million and $15.6 million, respectively, was recorded related to this agreement. As noted in Note 2 above, deferred revenue was increased by $6.2 million as of January 1, 2018 upon adoption of ASC 606. As of June 30, 2018, the aggregate amount of the transaction price allocated to the remaining performance obligation for preclinical research and clinical development activities related to the two specified product candidates through Phase I is estimated to be approximately $74.0 million, which is expected to be recognized as revenue through December 31, 2022.

Loan and Security Agreement	6 Months Ended
Loan and Security Agreement	Jun. 30, 2018
Debt Disclosure [Abstract]
Loan and Security Agreement	6. Loan and Security Agreement In January 2017, the Company entered into a loan and security agreement (the “Loan Agreement”) with Pacific Western Bank (“PWB”), which provides for term loan borrowings of up to $15.0 million through January 19, 2019. Borrowings under the Loan Agreement bear interest at a variable annual rate equal to the greater of (i) the prime rate plus 0.25% or (ii) 3.75%, and are payable over an interest-only period until January 19, 2019, followed by a 24-month period of equal monthly payments of principal and interest. All amounts outstanding as of the maturity date of January 19, 2021 become immediately due and payable. In connection with the Loan Agreement, the Company agreed to enter into warrant agreements with PWB pursuant to which warrants will be issued to purchase a number of shares of the Company’s capital stock equal to 1% of the amount of each term loan borrowing under the Loan Agreement, divided by the applicable exercise price. No amounts have been borrowed as term loans under the Loan Agreement as of June 30, 2018. Borrowings under the Loan Agreement are collateralized by substantially all of the Company’s assets, except for its intellectual property. Under the Loan Agreement, the Company has agreed to affirmative and negative covenants to which it will remain subject until maturity. These covenants include limitations on the Company’s ability to incur additional indebtedness and engage in certain fundamental business transactions, such as mergers or acquisitions of other businesses. There are no financial covenants associated with the Loan Agreement. Events of default under the Loan Agreement include failure to make payments when due, insolvency events, failure to comply with covenants and material adverse effects with respect to the Company.

Redeemable Convertible Preferred Stock

6 Months Ended

Jun. 30, 2018

Temporary Equity Disclosure [Abstract]

Redeemable Convertible Preferred Stock

7. Redeemable Convertible Preferred Stock

The Company issued Series A redeemable convertible preferred stock (the “Series A preferred stock”) and Series B redeemable convertible preferred stock (the “Series B preferred stock”). The Series A preferred stock and the Series B preferred stock are collectively referred to as the “Preferred Stock”. The Preferred Stock converted to shares of common stock on a 1:1.5701314513884 basis upon the closing of the IPO on April 3, 2018.

As of December 31, 2017, the Preferred Stock consisted of the following (in thousands, except share amounts):


	Preferred Stock Authorized		Preferred Stock Issued and Outstanding		Carrying Value		Liquidation Preference		Common Stock Issuable Upon Conversion
Series A preferred stock		12,297,276		12,297,276	$	12,267	$	12,297		7,832,001
Series B preferred stock		8,494,131		8,474,574		64,884		65,000		5,397,361

		20,791,407		20,771,850	$	77,151	$	77,297		13,229,362

Prior to the closing of the IPO, the holders of the Preferred Stock had the following rights and preferences:

Voting

The holders of Preferred Stock were entitled to vote, together with the holders of common stock, on matters submitted to stockholders for a vote. The holders of Preferred Stock were entitled to the number of votes equal to the number of common shares into which each such share of Preferred Stock could convert. In addition, the holders of Series A preferred stock, voting exclusively and as a separate class, were entitled to elect two directors of the Company. The holders of Series B preferred stock, voting exclusively and as a separate class, were entitled to elect one director of the Company.

Conversion

Each share of Preferred Stock was convertible at the option of the holder at any time after the date of issuance. Each share of Preferred Stock would have automatically converted into shares of common stock at the applicable conversion ratio then in effect upon the closing of a firm commitment public offering with at least $50.0 million of gross proceeds to the Company. Shares of Series A preferred stock would have automatically converted into shares of common stock at the applicable conversion ratio then in effect upon written consent of the holders of at least 65% of the then-outstanding shares of Series A preferred stock. Shares of Series B preferred stock would have automatically converted into shares of common stock at the applicable conversion ratio then in effect upon written consent of the holders of at least a majority of the then-outstanding shares of Series B preferred stock.

The conversion ratio of each series of Preferred Stock was determined by dividing the Original Issue Price of each series by the Conversion Price of each series. The Original Issue Price was $1.00 per share for Series A preferred stock and $7.67 per share for Series B preferred stock. The Conversion Price at issuance was $1.570131 per share for Series A preferred stock and $12.042908 per share for Series B preferred stock, subject to appropriate adjustment in the event of any stock split, stock dividend, combination or other similar recapitalization and other adjustments as set forth in the Company’s certificate of incorporation, as amended and restated.

Dividends

The holders of Preferred Stock were entitled to receive noncumulative dividends if and when declared by the Company’s board of directors. The Company could not declare, pay or set aside any dividends on shares of any other series of capital stock of the Company, other than dividends on common stock payable in common stock, unless the holders of the Series A and Series B preferred stock first received, or simultaneously received, a dividend on each outstanding share of Series A and Series B preferred stock in an amount at least equal to the greater of (i) $0.08 per share in the case of Series A preferred stock and $0.61 per share in the case of Series B preferred stock, each subject to appropriate adjustment in the event of any stock split, stock dividend, combination or other similar recapitalization with respect to such shares, and (ii) (A) in the case of a dividend on common stock or any class or series of stock that was convertible into common stock, that dividend per share of Preferred Stock as would equal the product of (1) the dividend payable on each share of such class or series determined, if applicable, as if all shares of such class or series had been converted into common stock and (2) the number of shares of common stock issuable upon conversion of each share of Preferred Stock, or (B) in the case of a dividend on any class or series that was not convertible into common stock, at a rate per share of Preferred Stock determined by (1) dividing the amount of the dividend payable on each share of such class or series of capital stock by the Original Issue Price of such class or series of capital stock (subject to appropriate adjustment in the event of any stock dividend, stock split, combination of or other similar recapitalization affecting such shares) and (2) multiplying such fraction by an amount equal to the Original Issue Price of each series of Preferred Stock. If the Company declared, paid or set aside, on the same date, a dividend on shares of more than one class or series of capital stock of the Company, the dividend payable to the holders of the Preferred Stock would have been calculated based upon the dividend on the class or series of capital stock that would have resulted in the highest Preferred Stock dividend. Stockholders were not entitled to any accruing dividends. No dividends had been declared or paid.

Liquidation

In the event of any voluntary or involuntary liquidation, dissolution or winding-up of the Company or Liquidating Event (as described below), the holders of shares of Preferred Stock would have received, in preference to the common stockholders, an amount equal to the greater of (i) the Original Issue Price per share of the respective share of Preferred Stock, plus all dividends declared but unpaid on such shares, or (ii) the amount the holders would have received if the Preferred Stock were converted into common stock prior to such liquidation event. In the event that the assets available for distribution to the Company’s stockholders had not been sufficient to permit payment to the holders of Preferred Stock in the full amount to which they were entitled, the assets available for distribution would have been distributed on a pro rata basis among the holders of the Series A and Series B preferred stock. After the payment of all preferential amounts to the holders of the Preferred Stock then, to the extent available, the remaining assets available for distribution would have been distributed among the holders of the common stock ratably based on the number of shares of common stock held by each holder.

Unless the holders of at least two-thirds of the then-outstanding shares of Preferred Stock, voting together as a single class on an as-converted basis, elected otherwise, a Liquidating Event would have included a merger or consolidation (other than one in which stockholders of the Company own a majority by voting power of the outstanding shares of the surviving or acquiring corporation) or a sale, lease, transfer, exclusive license or other disposition of all or substantially all of the assets of the Company.

Redemption

At any time on or after June 10, 2020, shares of each of the Series A and Series B preferred stock were subject to mandatory redemption by the Company in three equal annual installments beginning 60 days after receipt of a notice of redemption from the holders of at least two-thirds of the combined voting power of the holders of outstanding shares of Series A and Series B preferred stock, voting together as a single class, in an amount equal to the Original Issue Price per share of each series of Preferred Stock plus any dividends declared but unpaid thereon.

Common Stock	6 Months Ended
Common Stock	Jun. 30, 2018
Equity [Abstract]
Common Stock	8. Common Stock Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are not entitled to receive dividends, unless declared by the board of directors.

Stock-Based Compensation

6 Months Ended

Jun. 30, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock-Based Compensation

9. Stock-Based Compensation

2015 Stock Incentive Plan

The Company’s 2015 Stock Incentive Plan (the “2015 Plan”) provided for the Company to grant incentive stock options or nonqualified stock options, restricted stock, restricted stock units and other equity awards to employees, directors and consultants of the Company. The 2015 Plan was administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The board of directors could also delegate to one or more officers of the Company the power to grant awards to employees and certain officers of the Company. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated. Stock options granted under the 2015 Plan with service-based vesting conditions generally vest over four years and expire after ten years. The total number of shares of common stock that could have been issued under the 2015 Plan was 4,144,876 shares. Upon effectiveness of the Company’s 2018 Stock Option and Incentive Plan, the (“2018 Plan”) in March 2018, the remaining 1,041,700 shares available under the 2015 Plan became available for issuance under the 2018 Plan and no future issuance will be made under the 2015 Plan. Additionally, outstanding options under the 2015 Plan that are expired, terminated, surrendered or canceled without having been fully exercised will be available for future issuance under the 2018 Plan.

The exercise price for stock options granted is not less than the fair value of common shares as determined by the board of directors as of the date of grant. Prior to the Company’s IPO, the Company’s board of directors valued the Company’s common stock, taking into consideration its most recently available valuation of common stock performed by third parties as well as additional factors which may have changed since the date of the most recent contemporaneous valuation through the date of grant.

2018 Stock Option and Incentive Plan

On March 16, 2018, the Company’s stockholders approved the 2018 Plan, which became effective on March 27, 2018. The 2018 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock units, restricted stock awards, unrestricted stock awards, cash-based awards and dividend equivalent rights. The number of shares initially reserved for issuance under the 2018 Plan is 2,800,721 plus the 1,041,700 shares of common stock remaining available for issuance under the 2015 Plan. The number of shares reserved shall be cumulatively increased on January 1, 2019 and each January 1 thereafter by 4% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or a lesser number of shares determined by the Company’s board of directors. The shares of common stock underlying any awards that are forfeited, canceled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, repurchased or are otherwise terminated by the Company under the 2018 Plan or the 2015 Plan will be added back to the shares of common stock available for issuance under the 2018 Plan. As of June 30, 2018, 3,080,796 shares remained available for future issuance under the 2018 Plan.

2018 Employee Stock Purchase Plan

On March 16, 2018, the Company’s stockholders approved the 2018 Employee Stock Purchase Plan (the “ESPP”), which became effective on March 28, 2018. A total of 314,000 shares of common stock were reserved for issuance under this plan. In addition, the number of shares of common stock that may be issued under the ESPP will automatically increase on January 1, 2019, and each January 1 thereafter through January 1, 2027, by the least of (i) 500,000 shares of common stock, (ii) 1% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or (iii) such lesser number of shares as determined by the ESPP administrator.

Stock Option Issuances

During the six months ended June 30, 2018, the Company granted options to employees, consultants and directors for the purchase of 829,546 shares of common stock with a weighted average grant-date fair value of $6.39 per share.

Stock-Based Compensation

The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands):


	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Research and development expenses	$	607	$	246	$	1,008	$	506
General and administrative expenses		240		36		318		76

Total	$	847	$	282	$	1,326	$	582

As of June 30, 2018, total unrecognized compensation cost related to the unvested stock-based awards was $9.1 million, which is expected to be recognized over a weighted average period of 2.9 years.

Income Taxes	6 Months Ended
Income Taxes	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	10. Income Taxes 2017 U.S. Tax Reform On December 22, 2017, the Tax Cuts and Jobs Act (the “TCJA”) was signed into United States law. The TCJA includes a number of changes to existing tax law, including, among other things, a permanent reduction in the federal corporate income tax rate from 34% to 21%, effective as of January 1, 2018, as well as limitation of the deduction for net operating losses to 80% of annual taxable income and elimination of net operating loss carrybacks, in each case, for losses arising in taxable years beginning after December 31, 2017 (though any such net operating losses may be carried forward indefinitely). The tax rate change resulted in (i) a reduction in the gross amount of the Company’s deferred tax assets recorded as of December 31, 2017, without an impact on the net amount of its deferred tax assets, which are recorded with a full valuation allowance, and (ii) no income tax expense or benefit being recognized as of the enactment date of the TCJA. In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”) which allows the Company to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. Since the Act was passed late in the fourth quarter of 2017, and ongoing guidance and accounting interpretation are expected over the next 12 months, the Company considers the accounting for the effect of the TCJA to be provisional in accordance with SAB 118 at June 30, 2018. During the three and six months ended June 30, 2018, the Company did not make any adjustments to the provisional amounts recorded as a result of the TCJA. Income Taxes During the three and six months ended June 30, 2018 and 2017, the Company recorded no income tax benefits for the net operating losses incurred or for the research and development tax credits generated in each year due to its uncertainty of realizing a benefit from those items. The Company has provided a valuation allowance for the full amounts of its net deferred tax assets because, at June 30, 2018 and December 31, 2017, it was more likely than not that any future benefit from deductible temporary differences and net operating loss and tax credit carryforwards would not be realized. As of June 30, 2018 and December 31, 2017, the Company had not recorded any amounts for unrecognized tax benefits. The Company files income tax returns in the U.S. and Massachusetts. The statute of limitations for assessment by the Internal Revenue Service and Massachusetts tax authorities remains open for all years since 2014. No federal or state tax audits are currently in process.

Commitments and Contingencies

6 Months Ended

Jun. 30, 2018

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

11. Commitments and Contingencies

Operating Leases

The Company leases its facility under a non-cancelable operating lease that expires in April 2023. Under the terms of the lease, the Company secured a $1.3 million letter of credit as security for its leased facility. The underlying cash securing this letter of credit has been classified as non-current restricted cash in the accompanying consolidated balance sheets. The lease includes annual rent escalations, which are accrued, such that rent expense is recognized on a straight-line basis over the terms of occupancy.

The following table summarizes the future minimum payments due under the operating leases as of June 30, 2018 (in thousands):


Year Ending December 31,
2018 (six months ending December 31)	$	921
2019		1,878
2020		1,933
2021		1,989
2022		2,046
2023		689

	$	9,456

Rent expense for the three months ended June 30, 2018 and 2017 was $0.4 million and $0.4 million, respectively. Rent expense for the six months ended June 30, 2018 and 2017 was $0.9 million and $0.9 million, respectively.

In January 2017, the Company entered into a 12-month sublease agreement with a tenant for up to 2,500 square feet of general office and laboratory space at its headquarters. In December 2017, the Company entered into a new sublease agreement with a tenant for approximately 5,000 square feet of general office and laboratory space at its headquarters. The sublease ended in June 2018. The Company recognized $0.2 million and $0.1 million received under the subleases as other income in the consolidated statements of operations and comprehensive loss for the three months ended June 30, 2018 and 2017, respectively. The Company recognized $0.3 million and $0.1 million received under the subleases as other income in the consolidated statements of operations and comprehensive loss for the six months ended June 30, 2018 and 2017, respectively.

License Agreement

Under its license agreement with National University of Singapore and St. Jude Children’s Research Hospital, Inc. (collectively the “Licensors”) entered into in 2014, the Company is obligated to pay license maintenance fees on each anniversary of the effective date of the agreement that escalate from less than $0.1 million for each of the first seven years to $0.1 million on the eighth anniversary and each year thereafter. The Company is also obligated to make aggregate milestone payments of up to 5.5 million Singapore dollars (equivalent to approximately $4.0 million as of June 30, 2018) upon the achievement of specified clinical and regulatory milestones and to pay tiered royalties ranging in the low single-digit percentages on annual net sales of licensed products sold by the Company or its sublicensees. The royalties are payable on a product-by-product and country-by-country basis and may be reduced in specified circumstances. Additionally, under certain circumstances, the Company is obligated to pay the Licensors a percentage of amounts received from sublicensees.

The license agreement will expire on a country-by-country basis until the last to expire of the patents and patent applications covering such licensed product or service. The Licensors may terminate the license agreement within 60 days after written notice in the event of a breach of contract. The Licensors may also terminate the agreement upon written notice in the event of the Company’s bankruptcy, liquidation, or insolvency. In addition, the Company has the right to terminate this agreement in its entirety at will upon 90 days’ advance written notice to the Licensors. However, if the Company has commenced the commercialization of licensed products, the Company can only terminate at will if it ceases all development and commercialization of licensed products.

Manufacturing Commitment

As of June 30, 2018, the Company had non-cancelable minimum purchase commitments under contract manufacturing agreements for payments totaling $1.7 million over the following 12 months.

Indemnification Agreements

In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its executive officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of June 30, 2018 or December 31, 2017.

Legal Proceedings

The Company is not currently party to any material legal proceedings. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses as incurred the costs related to such legal proceedings.

Net Loss Per Share

6 Months Ended

Jun. 30, 2018

Earnings Per Share [Abstract]

Net Loss Per Share

12. Net Loss Per Share

Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts):


	Three Months Ended June 30,						Six Months Ended June 30,
	2018			2017			2018			2017
Numerator:
Net loss	$	(9,023	)	$	(5,863	)	$	(15,758	)	$	(11,802	)
Accretion of redeemable convertible preferred stock to redemption value		—			(17	)		(16	)		(33	)

Net loss attributable to common stockholders	$	(9,023	)	$	(5,880	)	$	(15,774	)	$	(11,835	)

Denominator:
Weighted average common shares outstanding, basic and diluted		29,155,790			10,190,228			19,732,542			10,190,228

Net loss per share attributable to common stockholders, basic and diluted	$	(0.31	)	$	(0.58	)	$	(0.80	)	$	(1.16	)

The Company’s potential dilutive securities have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated above because including them would have had an anti-dilutive effect:


	June 30,
	2018		2017
Redeemable convertible preferred shares (as converted to common stock)		—		13,229,362
Stock options to purchase common stock		3,830,271		2,549,808

		3,830,271		15,779,170

Retirement Plan	6 Months Ended
Retirement Plan	Jun. 30, 2018
Retirement Benefits [Abstract]
Retirement Plan	13. Retirement Plan The Company has a defined-contribution plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. As currently established, the Company is not required to make and to date has not made any contributions to the 401(k) Plan. The Company did not make any matching contributions during the three or six months ended June 30, 2018 or 2017.

Summary of Significant Accounting Policies (Policies)

6 Months Ended

Jun. 30, 2018

Accounting Policies [Abstract]

Unaudited Interim Financial Information

Unaudited Interim Financial Information

Concentrations of Credit Risk and of Significant Suppliers

Concentrations of Credit Risk and of Significant Suppliers

Cash Equivalents

Cash Equivalents

The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents.

Fair Value Measurements

Fair Value Measurements

•

Level 1—Quoted prices in active markets for identical assets or liabilities.

•

Marketable Securities

Marketable Securities

Use of Estimates

Use of Estimates

Classification and Accretion of Redeemable Convertible Preferred Stock

Classification and Accretion of Redeemable Convertible Preferred Stock

Collaboration Agreements

Collaboration Agreements

Revenue Recognition of Collaboration Agreements

Revenue Recognition of Collaboration Agreements

Recently Adopted and Issued Accounting Pronouncements

Recently Adopted Accounting Pronouncements

The following table summarizes the cumulative effect to the Company’s consolidated balance sheet upon the adoption of the new revenue standard on January 1, 2018 (in thousands):


	Balance at December 31, 2017			Adjustments			Balance at January 1, 2018
Deferred revenue, current and net of current portion	$	15,605		$	6,188		$	21,793
Accumulated deficit	$	(51,339	)	$	(6,188	)	$	(57,527	)

Consolidated Balance Sheet


	At June 30, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Deferred revenue, current portion	$	20,363		$	7,471		$	12,892
Deferred revenue, net of current portion	$	346		$	5,703		$	(5,357	)
Accumulated deficit	$	(73,285	)	$	(65,750	)	$	(7,535	)

Consolidated Statements of Operations and Comprehensive Loss


	Three Months Ended June 30, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Collaboration revenue	$	1,666		$	2,572		$	(906	)
Net loss	$	(9,023	)	$	(8,117	)	$	(906	)
Net loss attributable to common stockholders	$	(9,023	)	$	(8,117	)	$	(906	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.31	)	$	(0.28	)	$	(0.03	)
Comprehensive loss	$	(9,029	)	$	(8,123	)	$	(906	)


	Six Months Ended June 30, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Collaboration revenue	$	3,886		$	5,233		$	(1,347	)
Net loss	$	(15,758	)	$	(14,411	)	$	(1,347	)
Net loss attributable to common stockholders	$	(15,774	)	$	(14,427	)	$	(1,347	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.80	)	$	(0.73	)	$	(0.07	)
Comprehensive loss	$	(15,755	)	$	(14,408	)	$	(1,347	)

Consolidated Statement of Cash Flows


	Six Months Ended June 30, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Net loss	$	(15,758	)	$	(14,411	)	$	(1,347	)
Change in deferred revenue	$	(1,084	)	$	(2,431	)	$	1,347

Recently Issued Accounting Pronouncements

Summary of Significant Accounting Policies (Tables) - ASU 2014-09 [Member]

6 Months Ended

Jun. 30, 2018

Summary of Cumulative Effect to Consolidated Balance Sheet upon Adoption of New Revenue Standard

The following table summarizes the cumulative effect to the Company’s consolidated balance sheet upon the adoption of the new revenue standard on January 1, 2018 (in thousands):


	Balance at December 31, 2017			Adjustments			Balance at January 1, 2018
Deferred revenue, current and net of current portion	$	15,605		$	6,188		$	21,793
Accumulated deficit	$	(51,339	)	$	(6,188	)	$	(57,527	)

Summary of Impact of Adoption of New Revenue Standard Requirements on Company's Financial Statements

Consolidated Balance Sheet


	At June 30, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Deferred revenue, current portion	$	20,363		$	7,471		$	12,892
Deferred revenue, net of current portion	$	346		$	5,703		$	(5,357	)
Accumulated deficit	$	(73,285	)	$	(65,750	)	$	(7,535	)

Consolidated Statements of Operations and Comprehensive Loss


	Three Months Ended June 30, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Collaboration revenue	$	1,666		$	2,572		$	(906	)
Net loss	$	(9,023	)	$	(8,117	)	$	(906	)
Net loss attributable to common stockholders	$	(9,023	)	$	(8,117	)	$	(906	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.31	)	$	(0.28	)	$	(0.03	)
Comprehensive loss	$	(9,029	)	$	(8,123	)	$	(906	)


	Six Months Ended June 30, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Collaboration revenue	$	3,886		$	5,233		$	(1,347	)
Net loss	$	(15,758	)	$	(14,411	)	$	(1,347	)
Net loss attributable to common stockholders	$	(15,774	)	$	(14,427	)	$	(1,347	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.80	)	$	(0.73	)	$	(0.07	)
Comprehensive loss	$	(15,755	)	$	(14,408	)	$	(1,347	)

Consolidated Statement of Cash Flows


	Six Months Ended June 30, 2018
	Under Topic 606			Under Topic 605			Effect of Change
Net loss	$	(15,758	)	$	(14,411	)	$	(1,347	)
Change in deferred revenue	$	(1,084	)	$	(2,431	)	$	1,347

Fair Value of Financial Assets and Liabilities (Tables)

6 Months Ended

Jun. 30, 2018

Fair Value Disclosures [Abstract]

Schedule of Marketable Securities by Type

Marketable securities by security type consisted of the following (in thousands):


	June 30, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury notes (due within one year)	$	44,776	$	—	$	(12	)	$	44,764
U.S. Treasury bills (due within one year)		2,952		—		(1	)		2,951

	$	47,728	$	—	$	(13	)	$	47,715


	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury notes (due within one year)	$	5,007	$	—	$	(10	)	$	4,997
U.S. government agency bonds (due within one year)		7,700		—		(6	)		7,694

	$	12,707	$	—	$	(16	)	$	12,691

Schedule of Assets Measured at Fair Value on Recurring Basis

The following tables present information about the Company’s assets that are measured at fair value on a recurring basis (in thousands):


	Fair Value Measurements at June 30, 2018 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	—	$	29,414	$	—	$	29,414
U.S. Treasury notes		4,996		—		—		4,996
Marketable securities:
U.S. Treasury notes		44,764		—		—		44,764
U.S. Treasury bills		2,951		—		—		2,951

	$	52,711	$	29,414	$	—	$	82,125


	Fair Value Measurements at December 31, 2017 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	—	$	24,196	$	—	$	24,196
Marketable securities:
U.S. Treasury notes		4,997		—		—		4,997
U.S. government agency bonds		—		7,694		—		7,694

	$	4,997	$	31,890	$	—	$	36,887

Accrued Expenses and Other Current Liabilities (Tables)

6 Months Ended

Jun. 30, 2018

Payables and Accruals [Abstract]

Schedule of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):


	June 30, 2018		December 31, 2017
Accrued employee compensation and benefits	$	810	$	1,315
Accrued professional fees		198		980
Accrued external research and development expense		874		478
Accrued other		905		180

	$	2,787	$	2,953

Redeemable Convertible Preferred Stock (Tables)

6 Months Ended

Jun. 30, 2018

Temporary Equity Disclosure [Abstract]

Summary of Preferred Stock

As of December 31, 2017, the Preferred Stock consisted of the following (in thousands, except share amounts):


	Preferred Stock Authorized		Preferred Stock Issued and Outstanding		Carrying Value		Liquidation Preference		Common Stock Issuable Upon Conversion
Series A preferred stock		12,297,276		12,297,276	$	12,267	$	12,297		7,832,001
Series B preferred stock		8,494,131		8,474,574		64,884		65,000		5,397,361

		20,791,407		20,771,850	$	77,151	$	77,297		13,229,362

Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Stock Based Compensation Expense

The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands):


	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Research and development expenses	$	607	$	246	$	1,008	$	506
General and administrative expenses		240		36		318		76

Total	$	847	$	282	$	1,326	$	582

Commitments and Contingencies (Tables)

6 Months Ended

Jun. 30, 2018

Commitments and Contingencies Disclosure [Abstract]

Summary of Future Minimum Payments Due under Operating Leases

The following table summarizes the future minimum payments due under the operating leases as of June 30, 2018 (in thousands):


Year Ending December 31,
2018 (six months ending December 31)	$	921
2019		1,878
2020		1,933
2021		1,989
2022		2,046
2023		689

	$	9,456

Net Loss Per Share (Tables)

6 Months Ended

Jun. 30, 2018

Earnings Per Share [Abstract]

Schedule of Earnings Per Share Basic and Diluted

Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts):


	Three Months Ended June 30,						Six Months Ended June 30,
	2018			2017			2018			2017
Numerator:
Net loss	$	(9,023	)	$	(5,863	)	$	(15,758	)	$	(11,802	)
Accretion of redeemable convertible preferred stock to redemption value		—			(17	)		(16	)		(33	)

Net loss attributable to common stockholders	$	(9,023	)	$	(5,880	)	$	(15,774	)	$	(11,835	)

Denominator:
Weighted average common shares outstanding, basic and diluted		29,155,790			10,190,228			19,732,542			10,190,228

Net loss per share attributable to common stockholders, basic and diluted	$	(0.31	)	$	(0.58	)	$	(0.80	)	$	(1.16	)

Summary of Potential Dilutive Securities


	June 30,
	2018		2017
Redeemable convertible preferred shares (as converted to common stock)		—		13,229,362
Stock options to purchase common stock		3,830,271		2,549,808

		3,830,271		15,779,170

Nature of the Business and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands				3 Months Ended		6 Months Ended
	Apr. 25, 2018 USD ($) $ / shares shares	Apr. 03, 2018 USD ($) $ / shares shares	Mar. 16, 2018	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Reverse stock split description						One-for-1.5701314513884 reverse stock split
Reverse stock split ratio			0.636889
Net proceeds from initial public offering						$ 66,793
Net loss				$ (9,023)	$ (5,863)	(15,758)	$ (11,802)
Accumulated deficit				$ (73,285)		$ (73,285)		$ (51,339)
Funding term of borrowings for operating expense and capital expenditure requirement						12 months
Common Stock [Member] \| IPO [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares of common stock issued and sold \| shares	215,000	5,770,000
Public offering price, per share \| $ / shares	$ 12.00	$ 12.00
Net proceeds from initial public offering	$ 2,400	$ 61,500
Common Stock [Member] \| Private Placement [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Sale of stock, consideration received on transaction		$ 5,000
Sale of stock, number of shares issued in transaction \| shares		416,666
Period of lock-up agreement		180 days

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Deferred revenue, current portion	$ 20,363,000	$ 20,363,000		$ 6,891,000
Deferred revenue, net of current portion	346,000	346,000		$ 8,714,000
Revenue recognized	1,700,000	$ 3,900,000
New accounting pronouncement revenue recognition timing		The Company recognized revenue of $1.7 million and $3.9 million during the three and six months ended June 30, 2018, respectively, from the deferred revenue balance at January 1, 2018. The Company recognizes deferred revenue by first allocating from the beginning deferred revenue balance to the extent that the beginning deferred revenue balance exceeds the revenue to be recognized. Billings during the period are added to the deferred revenue balance to be recognized in future periods. To the extent that the beginning deferred revenue balance is less than revenue to be recognized during the period, billings during the period are allocated to revenue.
Capitalized contract costs	$ 0	$ 0
Maximum [Member]
Significant Accounting Policies [Line Items]
Expenses incremental costs expiration period		1 year
Period of Transfer of promised goods or services to customer		1 year
ASU 2016-18 [Member]
Significant Accounting Policies [Line Items]
Increase in restricted cash		$ 1,300,000	$ 1,300,000

Summary of Significant Accounting Policies - Summary of Cumulative Effect to Consolidated Balance Sheet upon Adoption of New Revenue Standard (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current and net of current portion			$ 15,605
Accumulated deficit	$ (73,285)		$ (51,339)
ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current and net of current portion		$ 21,793
Accumulated deficit		(57,527)
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] \| ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current and net of current portion		6,188
Accumulated deficit	$ (7,535)	$ (6,188)

Summary of Significant Accounting Policies - Summary of Impact of Adoption of New Revenue Standard Requirements on Company's Financial Statements (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current portion	$ 20,363		$ 20,363			$ 6,891
Deferred revenue, net of current portion	346		346			8,714
Accumulated deficit	(73,285)		(73,285)			$ (51,339)
Collaboration revenue	1,666	$ 2,079	3,886	$ 3,906
Net loss	(9,023)	(5,863)	(15,758)	(11,802)
Net loss attributable to common stockholders	$ (9,023)	$ (5,880)	$ (15,774)	$ (11,835)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.31)	$ (0.58)	$ (0.80)	$ (1.16)
Comprehensive loss	$ (9,029)	$ (5,870)	$ (15,755)	$ (11,821)
Net loss	(9,023)	$ (5,863)	(15,758)	(11,802)
Change in deferred revenue			(1,084)	$ (1,762)
ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit					$ (57,527)
Calculated under Revenue Guidance in Effect before Topic 606 [Member] \| ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current portion	7,471		7,471
Deferred revenue, net of current portion	5,703		5,703
Accumulated deficit	(65,750)		(65,750)
Collaboration revenue	2,572		5,233
Net loss	(8,117)		(14,411)
Net loss attributable to common stockholders	$ (8,117)		$ (14,427)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.28)		$ (0.73)
Comprehensive loss	$ (8,123)		$ (14,408)
Net loss	(8,117)		(14,411)
Change in deferred revenue			(2,431)
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] \| ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current portion	12,892		12,892
Deferred revenue, net of current portion	(5,357)		(5,357)
Accumulated deficit	(7,535)		(7,535)		$ (6,188)
Collaboration revenue	(906)		(1,347)
Net loss	(906)		(1,347)
Net loss attributable to common stockholders	$ (906)		$ (1,347)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.03)		$ (0.07)
Comprehensive loss	$ (906)		$ (1,347)
Net loss	$ (906)		(1,347)
Change in deferred revenue			$ 1,347

Fair Value of Financial Assets and Liabilities - Schedule of Marketable Securities by Type (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 47,728	$ 12,707
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(13)	(16)
Fair Value	47,715	12,691
U.S. Treasury Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	44,776	5,007
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(12)	(10)
Fair Value	44,764	4,997
U.S. Government Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		7,700
Gross Unrealized Gains		0
Gross Unrealized Losses		(6)
Fair Value		$ 7,694
US Treasury Bill Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,952
Gross Unrealized Gains	0
Gross Unrealized Losses	(1)
Fair Value	$ 2,951

Fair Value of Financial Assets and Liabilities - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	$ 82,125	$ 36,887
Money Market Funds [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	29,414	24,196
U.S. Treasury Notes [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	4,996
U.S. Treasury Notes [Member] \| Marketable securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	44,764	4,997
U.S. Government Agency Bonds [Member] \| Marketable securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis		7,694
US Treasury Bill Securities [Member] \| Marketable securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	2,951
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	52,711	4,997
Level 1 [Member] \| U.S. Treasury Notes [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	4,996
Level 1 [Member] \| U.S. Treasury Notes [Member] \| Marketable securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	44,764	4,997
Level 1 [Member] \| US Treasury Bill Securities [Member] \| Marketable securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	2,951
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	29,414	31,890
Level 2 [Member] \| Money Market Funds [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis	$ 29,414	24,196
Level 2 [Member] \| U.S. Government Agency Bonds [Member] \| Marketable securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Measured at Fair Value on Recurring Basis		$ 7,694

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Fair value asset, transfers between Level 1, Level 2 and Level 3, amount	$ 0	$ 0

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued employee compensation and benefits	$ 810	$ 1,315
Accrued professional fees	198	980
Accrued external research and development expense	874	478
Accrued other	905	180
Total	$ 2,787	$ 2,953

Collaboration Agreement - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue related to research and clinical development activities	$ 1,666	$ 2,079	$ 3,886	$ 3,906
Deferred revenue						$ 15,605
Remaining performance obligation related to two product candidates	74,000		74,000
ASU 2014-09 [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue					$ 21,793
ASU 2014-09 [Member] \| Cumulative Effect of Changes in Accounting Principle [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue					$ 6,200
Seattle Genetics [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue related to research and clinical development activities	1,700	$ 2,100	3,900	$ 3,900
Deferred revenue	$ 20,700		$ 20,700			$ 15,600

Loan and Security Agreement - Additional Information (Detail) - Loan and Security Agreement [Member] - Pacific Western Bank [Member]	6 Months Ended
	Jun. 30, 2018 USD ($)
Debt Instrument [Line Items]
Warrant to be issued to purchase number of shares capital stock equal to the amount of each term loan borrowing percentage	1.00%
Term Loan [Member]
Debt Instrument [Line Items]
Maximum term loan borrowings	$ 15,000,000
Interest rate during period	3.75%
Term loan payment terms	In January 2017, the Company entered into a loan and security agreement (the “Loan Agreement”) with Pacific Western Bank (“PWB”), which provides for term loan borrowings of up to $15.0 million through January 19, 2019. Borrowings under the Loan Agreement bear interest at a variable annual rate equal to the greater of (i) the prime rate plus 0.25% or (ii) 3.75%, and are payable over an interest-only period until January 19, 2019, followed by a 24-month period of equal monthly payments of principal and interest. All amounts outstanding as of the maturity date of January 19, 2021 become immediately due and payable.
Outstanding amount maturity date	Jan. 19, 2021
Term loans borrowed	$ 0
Term Loan [Member] \| Prime Rate [Member]
Debt Instrument [Line Items]
Annual variable interest rate	0.25%

Redeemable Convertible Preferred Stock - Additional Information (Detail)			6 Months Ended
	Apr. 03, 2018	Mar. 16, 2018	Jun. 30, 2018 USD ($) $ / shares
Temporary Equity [Line Items]
Reverse stock split description			One-for-1.5701314513884 reverse stock split
Reverse stock split ratio		0.636889
Voting, description			The holders of Preferred Stock were entitled to vote, together with the holders of common stock, on matters submitted to stockholders for a vote. The holders of Preferred Stock were entitled to the number of votes equal to the number of common shares into which each such share of Preferred Stock could convert. In addition, the holders of Series A preferred stock, voting exclusively and as a separate class, were entitled to elect two directors of the Company. The holders of Series B preferred stock, voting exclusively and as a separate class, were entitled to elect one director of the Company.
Conversion, description			Each share of Preferred Stock was convertible at the option of the holder at any time after the date of issuance. Each share of Preferred Stock would have automatically converted into shares of common stock at the applicable conversion ratio then in effect upon the closing of a firm commitment public offering with at least $50.0 million of gross proceeds to the Company. Shares of Series A preferred stock would have automatically converted into shares of common stock at the applicable conversion ratio then in effect upon written consent of the holders of at least 65% of the then-outstanding shares of Series A preferred stock. Shares of Series B preferred stock would have automatically converted into shares of common stock at the applicable conversion ratio then in effect upon written consent of the holders of at least a majority of the then-outstanding shares of Series B preferred stock.
Minimum gross proceeds from Initial Public Offering for automatic conversion \| $			$ 50,000,000
Percentage eligibility of conversion, upon outstanding shares held			65.00%
Preferred dividend description			The Company could not declare, pay or set aside any dividends on shares of any other series of capital stock of the Company, other than dividends on common stock payable in common stock, unless the holders of the Series A and Series B preferred stock first received, or simultaneously received, a dividend on each outstanding share of Series A and Series B preferred stock in an amount at least equal to the greater of (i) $0.08 per share in the case of Series A preferred stock and $0.61 per share in the case of Series B preferred stock, each subject to appropriate adjustment in the event of any stock split, stock dividend, combination or other similar recapitalization with respect to such shares, and (ii) (A) in the case of a dividend on common stock or any class or series of stock that was convertible into common stock, that dividend per share of Preferred Stock as would equal the product of (1) the dividend payable on each share of such class or series determined, if applicable, as if all shares of such class or series had been converted into common stock and (2) the number of shares of common stock issuable upon conversion of each share of Preferred Stock, or (B) in the case of a dividend on any class or series that was not convertible into common stock, at a rate per share of Preferred Stock determined by (1) dividing the amount of the dividend payable on each share of such class or series of capital stock by the Original Issue Price of such class or series of capital stock (subject to appropriate adjustment in the event of any stock dividend, stock split, combination of or other similar recapitalization affecting such shares) and (2) multiplying such fraction by an amount equal to the Original Issue Price of each series of Preferred Stock. If the Company declared, paid or set aside, on the same date, a dividend on shares of more than one class or series of capital stock of the Company, the dividend payable to the holders of the Preferred Stock would have been calculated based upon the dividend on the class or series of capital stock that would have resulted in the highest Preferred Stock dividend. Stockholders were not entitled to any accruing dividends.
Dividend payable \| $			$ 0
Redemption, description			At any time on or after June 10, 2020, shares of each of the Series A and Series B preferred stock were subject to mandatory redemption by the Company in three equal annual installments beginning 60 days after receipt of a notice of redemption from the holders of at least two-thirds of the combined voting power of the holders of outstanding shares of Series A and Series B preferred stock, voting together as a single class, in an amount equal to the Original Issue Price per share of each series of Preferred Stock plus any dividends declared but unpaid thereon.
Preferred Stock [Member]
Temporary Equity [Line Items]
Reverse stock split description	1:1.5701314513884 basis upon the closing of the IPO
Reverse stock split ratio	0.63688935
Series A preferred stock [Member]
Temporary Equity [Line Items]
Original Issue Price, per share			$ 1.00
Conversion Price at issuance			1.570131
Series A preferred stock [Member] \| Minimum [Member]
Temporary Equity [Line Items]
Dividend Rate			0.08
Series B preferred stock [Member]
Temporary Equity [Line Items]
Original Issue Price, per share			7.67
Conversion Price at issuance			12.042908
Series B preferred stock [Member] \| Minimum [Member]
Temporary Equity [Line Items]
Dividend Rate			$ 0.61

Redeemable Convertible Preferred Stock - Summary of Preferred Stock (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Preferred Stock Authorized	0	20,791,407
Preferred Stock Issued	0	20,771,850
Preferred Stock Outstanding	0	20,771,850
Carrying Value		$ 77,151
Liquidation Preference		$ 77,297
Common Stock Issuable Upon Conversion		13,229,362
Series A preferred stock [Member]
Temporary Equity [Line Items]
Preferred Stock Authorized		12,297,276
Preferred Stock Issued		12,297,276
Preferred Stock Outstanding		12,297,276
Carrying Value		$ 12,267
Liquidation Preference		$ 12,297
Common Stock Issuable Upon Conversion		7,832,001
Series B preferred stock [Member]
Temporary Equity [Line Items]
Preferred Stock Authorized		8,494,131
Preferred Stock Issued		8,474,574
Preferred Stock Outstanding		8,474,574
Carrying Value		$ 64,884
Liquidation Preference		$ 65,000
Common Stock Issuable Upon Conversion		5,397,361

Common Stock - Additional Information (Detail)	6 Months Ended
Common Stock - Additional Information (Detail)	Jun. 30, 2018
Equity [Abstract]
Common stock voting right	Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company's stockholders.

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions		6 Months Ended
	Mar. 16, 2018	Jun. 30, 2018	Jan. 01, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option grants		829,546
Weighted average grant date fair value		$ 6.39
Total unrecognized stock-based compensation expense		$ 9.1
Period for recognition of unrecognized expense		2 years 10 months 24 days
2015 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of stock option		4 years
Expiration period of stock option		10 years
Number of increases in authorized shares		4,144,876
2018 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of increases in authorized shares	2,800,721
Shares available for future issuance		3,080,796		1,041,700
Percentage applied to the outstanding shares as annual increase in the number of shares authorized for issuance	4.00%
Two Thousand Eighteen Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance	314,000
Two Thousand Eighteen Employee Stock Purchase Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares of common stock available for issuance	1.00%
Scenario, Forecast [Member] \| Two Thousand Eighteen Employee Stock Purchase Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of increases in authorized shares			500,000

Stock-Based Compensation - Schedule of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share Based Compensation Expense	$ 847	$ 282	$ 1,326	$ 582
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share Based Compensation Expense	607	246	1,008	506
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share Based Compensation Expense	$ 240	$ 36	$ 318	$ 76

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal corporate income tax rate			21.00%		34.00%
Percentage of net operating loss carry forward deductible from current year taxable income			80.00%
Income tax expense or benefit recognized			$ 0
Income tax benefits	$ 0	$ 0	0	$ 0
Unrecognized tax benefits	$ 0		$ 0		$ 0

Commitments and Contingencies - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 SGD ($)	Dec. 31, 2017 ft²	Jan. 31, 2017 ft²
Commitments And Contingencies Disclosure [Line Items]
Lease expiration date			Apr. 30, 2023
Rent expense	$ 400,000	$ 400,000	$ 900,000	$ 900,000
Sublease term						12 months	12 months
Area of subleased property \| ft²						5,000
Sublease rent recognized	200,000	$ 100,000	300,000	$ 100,000
Non-cancelable minimum purchase commitments	1,700,000		1,700,000
License Agreement [Member]
Commitments And Contingencies Disclosure [Line Items]
License maintenance fees on eighth anniversary and each year thereafter	100,000		$ 100,000
License agreement expiration terms			The license agreement will expire on a country-by-country basis until the last to expire of the patents and patent applications covering such licensed product or service. The Licensors may terminate the license agreement within 60 days after written notice in the event of a breach of contract. The Licensors may also terminate the agreement upon written notice in the event of the Company’s bankruptcy, liquidation, or insolvency. In addition, the Company has the right to terminate this agreement in its entirety at will upon 90 days’ advance written notice to the Licensors. However, if the Company has commenced the commercialization of licensed products, the Company can only terminate at will if it ceases all development and commercialization of licensed products.
Maximum [Member]
Commitments And Contingencies Disclosure [Line Items]
Area of subleased property \| ft²							2,500
Maximum [Member] \| License Agreement [Member]
Commitments And Contingencies Disclosure [Line Items]
License maintenance fees for each of first seven years	100,000		$ 100,000
Contingent contractual obligation	4,000,000		4,000,000		$ 5,500,000
Non-current restricted cash [Member] \| Letter of Credit [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease security	$ 1,300,000		$ 1,300,000

Commitments and Contingencies - Summary of Future Minimum Payments Due under Operating Leases (Detail) $ in Thousands	Jun. 30, 2018 USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018 (six months ending December 31)	$ 921
2019	1,878
2020	1,933
2021	1,989
2022	2,046
2023	689
Operating Leases, Future Minimum Payments Due, Total	$ 9,456

Net Loss Per Share - Schedule of Earnings Per Share Basic and Diluted (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net loss	$ (9,023)	$ (5,863)	$ (15,758)	$ (11,802)
Accretion of redeemable convertible preferred stock to redemption value		(17)	(16)	(33)
Net loss attributable to common stockholders	$ (9,023)	$ (5,880)	$ (15,774)	$ (11,835)
Weighted average common shares outstanding, basic and diluted	29,155,790	10,190,228	19,732,542	10,190,228
Net loss per share attributable to common stockholders, basic and diluted	$ (0.31)	$ (0.58)	$ (0.80)	$ (1.16)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Net Loss Per Share - Summary of Potential Dilutive Securities (Detail) - shares	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential dilutive securities excluded from computation of diluted net loss per common share	3,830,271	15,779,170
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential dilutive securities excluded from computation of diluted net loss per common share		13,229,362
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential dilutive securities excluded from computation of diluted net loss per common share	3,830,271	2,549,808

Retirement Plan - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Retirement Benefits [Abstract]
Defined contribution plan, matching amount	$ 0	$ 0	$ 0	$ 0