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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2018	Mar. 07, 2019	Jun. 30, 2018
Document and Entity Information
Entity Registrant Name	Mersana Therapeutics, Inc.
Entity Central Index Key	0001442836
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Public Float			$ 262,605,135
Entity Common Stock, Shares Outstanding		47,684,164

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 59,634	$ 26,591
Short-term marketable securities	10,497	88,143
Accounts receivable	459	784
Prepaid expenses and other current assets	3,715	2,025
Total current assets	74,305	117,543
Property and equipment, net	2,694	2,319
Long-term marketable securities		10,482
Other assets	1,503	371
Total assets	78,502	130,715
Current liabilities:
Accounts payable	10,727	3,070
Accrued expenses	12,375	6,944
Deferred rent	127	232
Deferred revenue	46,196	21,635
Total current liabilities	69,425	31,881
Deferred rent, net of current portion	282	67
Deferred revenue, net of current portion		28,773
Total liabilities	69,707	60,721
Commitments (Note 13)
Stockholders' equity
Preferred stock, $0.0001 par value; 25,000,000 shares authorized; 0 shares issued and outstanding at December 31, 2018 and 2017, respectively
Common stock, $0.0001 par value; 175,000,000 shares authorized; 23,234,472 and 22,765,017 shares issued and outstanding at December 31, 2018 and 2017, respectively	3	3
Additional paid-in capital	172,966	168,018
Accumulated other comprehensive loss	(8)	(149)
Accumulated deficit	(164,166)	(97,878)
Total stockholders’ equity	8,795	69,994
Total liabilities and stockholders’ equity	$ 78,502	$ 130,715

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Condensed Consolidated Balance Sheets
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	25,000,000	25,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	175,000,000	175,000,000
Common stock, shares issued	23,234,472	22,765,017
Common stock, shares outstanding	23,234,472	22,765,017

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Consolidated Statements of Operations and Comprehensive Loss
Collaboration revenue	$ 10,594	$ 17,545	$ 25,171
Operating expenses:
Research and development	59,915	46,700	32,008
General and administrative	16,334	10,462	6,984
Total operating expenses	76,249	57,162	38,992
Other income:
Interest income	1,398	910	121
Total other income	1,398	910	121
Net loss	(64,257)	(38,707)	(13,700)
Other comprehensive loss:
Unrealized gain (loss) on marketable securities	141	(149)
Comprehensive loss	(64,116)	(38,856)	(13,700)
Net loss attributable to common stockholders - basic	$ (64,257)	$ (38,707)	$ (13,700)
Net loss per share attributable to common stockholders — basic and diluted	$ (2.79)	$ (3.22)	$ (10.82)
Weighted-average number of common shares used in net loss per share attributable to common stockholders — basic and diluted	23,032,250	12,022,733	1,266,758
Type of Revenue	us-gaap:CollaborationMember	us-gaap:CollaborationMember	us-gaap:CollaborationMember

Consolidated Statements of Convertible Preferred Stock - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Series A-1 convertible preferred stock
Increase (Decrease) in Temporary Equity
Balance (in shares)	25,085,153	25,085,153
Balance	$ 26,336	$ 26,336
Balance (in shares)		25,085,153
Balance		$ 26,336
Conversion of preferred stock into common stock (in shares)	(25,085,153)
Conversion of preferred stock into common stock	$ (26,336)
Series B-1 convertible preferred stock
Increase (Decrease) in Temporary Equity
Balance (in shares)	32,936,919	9,410,551
Balance	$ 35,232	$ 9,960
Issuance of convertible preferred stock (in shares)		23,526,368
Issuance of convertible preferred stock, net of issuance costs		$ 25,272
Balance (in shares)		32,936,919
Balance		$ 35,232
Conversion of preferred stock into common stock (in shares)	(32,936,919)
Conversion of preferred stock into common stock	$ (35,232)
Series C-1 convertible preferred stock
Increase (Decrease) in Temporary Equity
Balance (in shares)	14,674,062
Balance	$ 32,882
Issuance of convertible preferred stock (in shares)		14,674,062
Issuance of convertible preferred stock, net of issuance costs		$ 32,882
Balance (in shares)		14,674,062
Balance		$ 32,882
Conversion of preferred stock into common stock (in shares)	(14,674,062)
Conversion of preferred stock into common stock	$ (32,882)

Consolidated Statements of Convertible Preferred Stock (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)
Series B-1 convertible preferred stock
Issuance costs	$ 50
Series C-1 convertible preferred stock
Issuance costs	$ 218

Consolidated Statements of Stockholders' (Deficit) Equity - USD ($) $ in Thousands	Common stock IPO	Common stock Over allotment option	Common stock	Additional paid-in capital IPO	Additional paid-in capital Over allotment option	Additional paid-in capital	Accumulated other comprehensive income/(loss)	Accumulated deficit	IPO	Over allotment option	Total
Balance at beginning of period (in shares) at Dec. 31, 2015			1,223,457
Balance at beginning of period at Dec. 31, 2015			$ 1			$ 2,778		$ (45,471)			$ (42,692)
Increase (Decrease) in Stockholders' Equity
Exercise of stock options (in shares)			70,895
Exercise of stock options						105					105
Stock-based compensation expense						668					668
Net loss								(13,700)			(13,700)
Balance at end of period (in shares) at Dec. 31, 2016			1,294,352
Balance at end of period at Dec. 31, 2016			$ 1			3,551		(59,171)			(55,619)
Increase (Decrease) in Stockholders' Equity
Issuance of stock (in shares)	5,000,000	51,977
Issuance of stock				$ 67,420	$ 725				$ 67,420	$ 725
Conversion of preferred shares into common stock (in shares)			16,154,671
Conversion of preferred shares into common stock			$ 2			94,448					94,450
Exercise of stock options and warrants (in shares)			264,017
Exercise of stock options and warrants						452					452
Stock-based compensation expense						1,422					1,422
Other comprehensive loss							$ (149)				(149)
Net loss								(38,707)			(38,707)
Balance at end of period (in shares) at Dec. 31, 2017			22,765,017
Balance at end of period at Dec. 31, 2017			$ 3			168,018	(149)	(97,878)			69,994
Increase (Decrease) in Stockholders' Equity
Cumulative effect adjustment for adoption of ASC 606 \| ASU 2014-09								(2,031)			$ (2,031)
Exercise of stock options (in shares)			427,269								427,269
Exercise of stock options						918					$ 918
Purchase of common stock under ESPP (in shares)			42,186
Purchase of common stock under ESPP						146					146
Stock-based compensation expense						3,884					3,884
Other comprehensive loss							141				141
Net loss								(64,257)			(64,257)
Balance at end of period (in shares) at Dec. 31, 2018			23,234,472
Balance at end of period at Dec. 31, 2018			$ 3			$ 172,966	$ (8)	$ (164,166)			$ 8,795

Consolidated Statements of Stockholders' (Deficit) Equity (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
IPO
Issuance costs	$ 7,580
Over allotment option
Issuance costs	$ 55

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net loss	$ (64,257)	$ (38,707)	$ (13,700)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation	1,257	928	655
Loss on disposal of fixed assets	20
Net amortization of premiums and discounts on investments	(296)	(293)
Stock-based compensation	3,884	1,422	668
Change in deferred rent	110	(159)	102
Changes in operating assets and liabilities:
Accounts receivable	325	267	(411)
Prepaid expenses and other current assets	(1,690)	(1,200)	(245)
Other assets	(1,132)	60	(60)
Accounts payable	7,375	1,335	(325)
Accrued expenses	5,431	3,562	1,726
Deferred revenue	(6,243)	(9,894)	43,178
Net cash provided by (used in) operating activities	(55,216)	(42,679)	31,588
Cash flows from investing activities
Maturities of marketable securities	88,565	47,220
Purchase of property and equipment	(1,370)	(1,143)	(1,084)
Purchase of marketable securities		(145,701)
Net cash provided by (used in) investing activities	87,195	(99,624)	(1,084)
Cash flows from financing activities
Proceeds from exercise of stock options	918	452	105
Proceeds from purchases of common stock under ESPP	146
Net cash provided by financing activities	1,064	68,597	58,259
Increase (decrease) in cash, cash equivalents and restricted cash	33,043	(73,706)	88,763
Cash, cash equivalents and restricted cash, beginning of period	26,962	100,668	11,905
Cash, cash equivalents and restricted cash, end of period	60,005	26,962	100,668
Supplemental disclosures of non-cash activities:
Conversion of preferred stock to common stock upon closing of initial public offering		94,450
Purchases of property and equipment included in accounts payable and accrued expenses	317	35	414
Purchases of property and equipment reimbursed by landlord			356
Series B-1 convertible preferred stock
Cash flows from financing activities
Net proceeds from sale of convertible preferred stock			25,272
Series C-1 convertible preferred stock
Cash flows from financing activities
Net proceeds from sale of convertible preferred stock			$ 32,882
IPO
Cash flows from financing activities
Net proceeds from issuance of stock		67,420
Over allotment option
Cash flows from financing activities
Net proceeds from issuance of stock		$ 725
ASU 2014-09
Supplemental disclosures of non-cash activities:
Adjustment to accumulated deficit and deferred revenue upon adoption of Topic 606	$ 2,031

Nature of business and basis of presentation	12 Months Ended
Nature of business and basis of presentation	Dec. 31, 2018
Nature of business and basis of presentation
Nature of business and basis of presentation	1. Nature of Business and Basis of Presentation Mersana Therapeutics, Inc. is a clinical stage biopharmaceutical company located in Cambridge, Massachusetts. The Company is focused on developing antibody drug conjugates, or ADCs, that offer a clinically meaningful benefit for cancer patients with significant unmet need. The Company leveraged 20 years of industry learning in the ADC field to develop proprietary technologies that enable it to design ADCs to have improved efficacy, safety and tolerability relative to existing ADC therapies. The Company’s most advanced platform, Dolaflexin, has been used to generate a pipeline of proprietary ADC product candidates to address patient populations that are not currently amenable to treatment with traditional ADC‑based therapies. The Company’s lead product candidate, XMT‑1536, is an ADC targeting NaPi2b, an antigen broadly expressed in ovarian cancer and non small cell lung cancer (NSCLC). The first patient was dosed on XMT‑1536 in early 2018 and the study is currently in Phase 1 dose escalation in ovarian cancer, NSCLC and other orphan indications where a majority of patients express NaPi2b including endometrial, papillary renal, papillary thyroid, and salivary duct. In January 2019, the Company announced that the development of XMT-1522 was being discontinued and the partnerships with Takeda were being terminated during the quarter ended March 31, 2019. The Company is subject to risks common to companies in the biotechnology industry, including but not limited to, the need for additional capital, risks of failure of preclinical studies and clinical trials, the need to obtain marketing approval and reimbursement for any drug product candidate that it may identify and develop, the need to successfully commercialize and gain market acceptance of its product candidates, dependence on key personnel, protection of proprietary technology, compliance with government regulations, development of technological innovations by competitors, reliance on third party manufacturers and the ability to transition from pilot-scale production to large-scale manufacturing of products. The Company has incurred net losses since inception. The Company’s net loss was $64,257, $38,707 and $13,700 for the years ended December 31, 2018, 2017 and 2016, respectively. The Company expects to continue to incur operating losses for at least the next several years. As of December 31, 2018, the Company had an accumulated deficit of $164,166. The future success of the Company is dependent on its ability to identify and develop its product candidates, and ultimately upon its ability to attain profitable operations. The Company has devoted substantially all of its financial resources and efforts to research and development and general and administrative expense to support such research and development. The Company’s net losses may fluctuate significantly from quarter to quarter and year to year. Net losses and negative cash flows have had, and will continue to have, an adverse effect on the Company’s stockholders’ equity and working capital. The Company believes that its existing cash, cash equivalents and marketable securities as of December 31, 2018, and the $97,800 in gross proceeds from the offering discussed in Note 15, will enable it to fund its operating plan through at least mid 2021. The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (U.S. GAAP) and the rules and regulations of the Securities and Exchange Commission (SEC). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (ASC) and Accounting Standards Updates (ASU) of the Financial Accounting Standards Board (FASB). All dollar amounts, except per share data in the text and tables herein, are stated in thousands unless otherwise indicated. Effective January 1, 2018, the Company adopted the requirements of Accounting Standards Update (ASU) No. 2014-09, Revenue from Contracts with Customers (Topic 606) using the modified retrospective method as discussed below in Note 2. Summary of Significant Accounting Policies. All 2018 amounts and disclosures set forth in this Annual Report on Form 10-K reflect these changes.

Summary of significant accounting policies

12 Months Ended

Dec. 31, 2018

Summary of significant accounting policies

Summary of significant accounting policies

2. Summary of Significant Accounting Policies

Principles of Consolidation

The accompanying consolidated financial statements include those of the Company and its wholly-owned subsidiary, Mersana Securities Corp. All intercompany balances and transactions have been eliminated.

Use of Estimates

The preparation of the Company's consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, equity, revenue, expenses and related disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenue and expenses during the reporting period. On an ongoing basis, the Company's management evaluates its estimates which include, but are not limited to, management's judgments with respect to the separate units of accounting and best estimate of selling price of those units of accounting within its revenue arrangements, accrued expenses, valuation of stock-based awards and income taxes. Actual results could differ from those estimates.

Segment Information

Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision-maker in deciding how to allocate resources and assess performance. The Company and the Company's chief operating decision-maker, the Company's chief executive officer, view the Company's operations and manage its business as a single operating segment, which is the business of discovering and developing ADCs.

Research and Development

The Company expenses all costs incurred in performing research and development activities. Research and development expenses include salaries and benefits, materials and supplies, preclinical expenses, manufacturing expenses, stock-based compensation expense, depreciation of equipment, contract services and other outside expenses. Costs of certain development activities, such as manufacturing, are recognized based on an evaluation of the progress to completion of specific tasks. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the financial statements as prepaid or accrued research and development costs. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred and capitalized. The capitalized amounts are expensed as the related goods are delivered or the services are performed. Costs associated with collaboration agreements are included in research and development expense.

The Company may also incur costs associated with its collaboration agreements to supply ADC drug substance for which it has an enforceable right to recover such costs at the time the costs are incurred and it is probable the Company will receive some future benefit. Costs may include third party manufacturing and internal labor. The capitalized amounts are expensed when the related goods are delivered.

Revenue Recognition

Effective January 1, 2018, the Company adopted the provisions of Topic 606, using the modified retrospective transition method. Under this method, the Company recorded the cumulative effect of initially applying the new standard to all contracts in process as of the date of adoption. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance, collaboration arrangements and financial instruments.

The adoption of the new revenue recognition guidance resulted in increases of $2,031 in deferred revenue and accumulated deficit as of January 1, 2018. For the year ended December 31, 2018, revenue decreased by $378, net loss increased by $378 and basic and diluted net loss per share increased by $0.02, based on revenue recognition under Topic 606, as compared to the Company’s prior revenue recognition methodology under ASC 605 Revenue Recognition. These changes were primarily caused by the differences in determining and allocating transaction price under Topic 606.

The Company enters into collaboration agreements which are within the scope of Topic 606, under which the Company licenses rights to its technology and certain of the Company’s product candidates and performs research and development services for third parties. The terms of these arrangements typically include payment of one or more of the following: non-refundable, up-front fees; reimbursement of research and development costs; development, regulatory and commercial milestone payments; and royalties on net sales of licensed products.

Under Topic 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of Topic 606, the Company performs the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect consideration it is entitled to in exchange for the goods or services it transfers to the customer.

The promised good or services in the Company’s arrangement typically consist of license rights to the Company’s intellectual property and research and development services. The Company also has optional additional items in contracts, which are considered marketing offers and are accounted for as separate contracts with the customer if such option is elected by the customer, unless the option provides a material right which would not be provided without entering into the contract. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer. Promised goods or services are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources or (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on their own or whether the required expertise is readily available.

The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include both fixed consideration and variable consideration. At the inception of each arrangement that includes variable consideration and at each reporting period, the Company evaluates the amount of potential payment and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected amount method to estimate the amount expected to be received based on which method better predicts the amount expected to be received. If it is probable that a significant revenue reversal would not occur, the variable consideration is included in the transaction price. We assessed each of our revenue generating arrangements in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in any of our arrangements because: (a) the promised consideration approximates the cash selling price of the promised goods and services; and (b) timing of payment approximates the transfer of goods and services and performance is over a relatively short period of time within the context of the entire term of the contract.

The Company’s contracts often include development and regulatory milestone payments. At contract inception and at each reporting period, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are not included in the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect license, collaboration and other revenues and earnings in the period of adjustment.

For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of the Company’s collaboration arrangements.

The Company allocates the transaction price based on the estimated standalone selling price of the underlying performance obligations or in the case of certain variable consideration to one or more performance obligations. The Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company utilizes key assumptions to determine the stand-alone selling price, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs to complete the respective performance obligation. Certain variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated to each performance obligation are consistent with the amounts the Company would expect to receive for each performance obligation.

For performance obligations consisting of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue from non-refundable, up-front fees. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company will recognize revenue from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license.

The Company receives payments from its customers based on billing schedules established in each contract. Such billings generally have 30 day terms. Up-front payments and fees are recorded as deferred revenue upon receipt or when due until the Company performs its obligations under these arrangements. Amounts are recorded as accounts receivable when the right to consideration is unconditional.

Collaborative Arrangements

The Company records the elements of its collaboration agreements that represent joint operating activities in accordance with ASC Topic 808, Collaborative Arrangements (ASC 808). Accordingly, the elements of the collaboration agreements that represent activities in which both parties are active participants and to which both parties are exposed to the significant risks and rewards that are dependent on the commercial success of the activities, are recorded as collaborative arrangements. The Company considers the guidance in ASC Topic 606 in determining the appropriate treatment for the transactions between the Company and its collaborative partners and the transactions between the Company and third parties. Generally, the classification of transactions under the collaborative arrangements is determined based on the nature and contractual terms of the arrangement along with the nature of the operations of the participants. To the extent revenue is generated from a collaboration, the Company will recognize its share of the net sales on a gross basis if it is deemed to be the principal in the transactions with customers, or on a net basis if it is instead deemed to be the agent in the transactions with customers, consistent with the guidance in Topic 606.

Fair Value Measurements

Fair value is defined as the price that would be received upon sale of an asset or paid to transfer a liability between market participants at measurement dates. ASC Topic 820 Fair Value Measurement (ASC 820), establishes a three-level valuation hierarchy for instruments measured at fair value. The hierarchy is based on the transparency of inputs to the valuation of an asset or liability as of the measurement date. The three levels are defined as follows:

Level 1—Inputs to the valuation methodology are quoted prices (unadjusted) for identical assets or liabilities in active markets.

Level 2—Inputs to the valuation methodology include quoted prices for similar assets and liabilities in active markets, and inputs that are observable for the asset or liability, either directly or indirectly, for substantially the full term of the financial instrument.

Level 3—Inputs to the valuation methodology are unobservable and significant to the fair value measurement.

Cash and Cash Equivalents

The Company considers all highly liquid investments with an original maturity, or a remaining maturity at the time of purchase, of three months or less to be cash equivalents. The Company invests excess cash primarily in money market funds, commercial paper and government agency securities, which are highly liquid and have strong credit ratings. These investments are subject to minimal credit and market risks. Cash and cash equivalents are stated at cost, which approximates market value.


	Year ended				Year ended
	December 31, 2018				December 31, 2017
	Beginning		End		Beginning		End
	of period		of period		of period		of period
Cash and cash equivalents	$	26,591	$	59,634	$	100,297	$	26,591
Restricted cash included in other assets		371		371		371		371
Total cash, cash equivalents and restricted cash per statement of cash flows	$	26,962	$	60,005	$	100,668	$	26,962

Marketable Securities

Short-term marketable securities consist of investments in debt securities with maturities greater than three months and less than one year from the balance sheet date. Long-term marketable securities consist of investments with maturities greater than one year that are not expected to be used to fund current operations. The Company classifies all of its marketable securities as available-for-sale. Accordingly, these investments are recorded at fair value. Amortization and accretion of discounts and premiums are recorded as interest income within other income. Unrealized gains and losses on available-for-sale securities are included in other comprehensive loss as a component of stockholders’ equity (deficit) until realized.

Other Assets

The Company recorded other assets of $1,503 and $371 assets as of December 31, 2018 and 2017, respectively. The December 31, 2018 amount is comprised of restricted cash of $371 held as security deposits for a standby letter of credit related to a facility lease and a corporate credit card program and $1,132 held by a service provider. The December 31, 2017 amount is comprised of restricted cash.

Accounting for Stock-based Compensation

The Company accounts for its stock-based compensation in accordance with ASC Topic 718 Compensation—Stock Compensation (ASC 718). ASC 718 requires all stock-based payments to employees and directors to be recognized as expense in the statements of operations based on their grant date fair values. Expense related to stock awards to non-employees is required to be recognized in the statement of operations based on the awards' vesting date fair values. The Company estimates the fair value of options granted using the Black-Scholes option pricing model.

The Black-Scholes option pricing model requires inputs based on certain subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk-free interest rate and (d) expected dividends. Due to the lack of a public market for the Company's common stock prior to completion of the IPO and a lack of company-specific historical and implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The computation of expected volatility is based on the historical volatility of a representative group of companies with similar characteristics to the Company, including stage of product development and life science industry focus. The Company uses the simplified method as prescribed by the SEC Staff Accounting Bulletin No. 107, Share-Based Payment, to calculate the expected term for options granted to employees as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. For options granted to non-employees, the Company utilizes the contractual term of the arrangement as the basis for the expected term assumption. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected term of the stock options. The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to do so.

Through December 31, 2016, the Company was required to estimate forfeitures at the time of grant, and revise those estimates in subsequent periods if actual forfeitures differ from its estimates. The Company used historical data to estimate post-vesting forfeitures and recorded stock-based compensation expense only for those awards that were expected to vest. To the extent that actual forfeitures differ from estimates, the difference was recorded as a cumulative adjustment in the period the estimates were revised. Stock-based compensation expense recognized in the financial statements is based on awards that were ultimately expected to vest. The fair value of stock-based payments was recognized as expense, net of estimated forfeitures, over the requisite service period which is generally the vesting period.

In the first quarter of 2017, the Company made an accounting policy election to recognize forfeitures as they occur upon adoption of guidance per ASU No. 2016-09. The adoption of this ASU did not have a material impact on the Company's financial statements.

Net Loss per Share

Basic net loss per common share is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of common shares outstanding during the period, without consideration for potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common shares and potentially dilutive securities outstanding for the period determined using the treasury stock and if-converted methods.

For purposes of the diluted net loss per share calculation, convertible preferred stock, warrants to purchase common stock and options to purchase common stock are considered to be potentially dilutive securities, but are excluded from the calculation of diluted net loss per share because their effect would be anti-dilutive and therefore, basic and diluted net loss per share were the same for all periods presented.

The following table sets forth the outstanding potentially dilutive securities that have been excluded from the calculation of diluted net loss per share because to include them would be anti-dilutive (in common stock equivalent shares):


	Year ended
	December 31,
	2018	2017	2016
Series A-1 Convertible Preferred Stock	—	—	5,574,467
Series B-1 Convertible Preferred Stock	—	—	7,319,307
Series C-1 Convertible Preferred Stock	—	—	3,260,897
Warrants	110,365	110,365	129,491
Stock options	3,746,567	3,205,485	2,901,985
	3,856,932	3,315,850	19,186,147

Property and Equipment

Property and equipment is stated at cost, less accumulated depreciation. Depreciation is computed using the straight-line method over the estimated useful life of each asset as follows:


Computer equipment, office equipment and software		3 years
Laboratory equipment		5 years
Leasehold improvements		Shorter of useful life or life of lease

Upon retirement or sale, the cost of the assets disposed of and the related accumulated depreciation are eliminated from the balance sheet and related gains or losses are reflected in the statement of operations. There were no material retirements or sales of assets during the years ended December 31, 2018, 2017 and 2016.

The Company reviews its property and equipment for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. If an impairment review is performed to evaluate an asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the asset to its carrying value. If the carrying amount of the asset exceeds its estimated undiscounted future net cash flows, an impairment charge is recognized in the amount by which the carrying amount of the asset exceeds the fair value of the asset. The Company did not recognize impairment charges during the years ended December 31, 2018, 2017 and 2016.

Repairs and maintenance costs are expensed as incurred.

Patent Costs

The Company expenses patent application and related legal costs as incurred and classifies such costs as general and administrative expenses in the accompanying consolidated statements of operations.

Income Taxes

The Company accounts for income taxes using the liability method. The difference between the financial statement and tax basis of the assets and liabilities is determined annually. Deferred income tax assets and liabilities are computed using the tax laws and rates that are expected to apply for periods in which such differences reverse. Valuation allowances are established, if necessary, to reduce the deferred tax asset to the amount that will more likely than not be realized.

The Company recognizes the effect of income tax positions only if those positions are more likely than not to be sustained. Recognized income tax positions are measured at the largest amount that is greater than 50% likely of being realized. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs.

Comprehensive Income (Loss)

Comprehensive income (loss) is comprised of net loss and other comprehensive loss. For the years ended December 31, 2018 and 2017, other comprehensive loss consisted of unrealized income and loss on marketable securities. For the year ended December 31, 2016 comprehensive loss equaled net loss.

Concentration of Credit Risk and Off-balance Sheet Risk

The Company has no financial instruments with off-balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Financial instruments that potentially subject the Company to concentrations of credit risk primarily consist of cash equivalents and marketable securities. Under its investment policy, the Company limits amounts invested in such securities by credit rating, maturity, industry group, investment type and issuer, except for securities issued by the U.S. government. The Company is not exposed to any significant concentrations of credit risk from these financial instruments.

Recently Issued Accounting Pronouncements

In January 2016, the FASB issued ASU No. 2016-01 Financial Instruments (ASU No. 2016-01) related to the recording of financial assets and financial liabilities. Under the amended guidance, equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of the investee) are to be measured at fair value with changes in fair value recognized in net income (loss). However, an entity has the option to measure equity investments without readily determinable fair values either (i) at fair value or (ii) at cost, adjusted for changes in observable prices minus impairment. Changes in measurement under either alternative will be recognized in net income (loss). The amended guidance became effective January 1, 2018. The Company adopted the new standard effective January 1, 2018. Based on the Company’s current investment holdings, the adoption of this new standard did not have a material impact on its consolidated financial statements.

In February 2016, the FASB issued ASU No. 2016-02, Leases (ASU No. 2016-02), which will replace the existing guidance in ASC 840, Leases. The updated standard aims to increase transparency and comparability among organizations by requiring lessees to recognize lease assets and lease liabilities on the balance sheet and requiring disclosure of key information about leasing arrangements. This standard is effective for the Company in the fiscal year beginning after December 15, 2018, but early adoption is permissible. The Company plans to adopt the standard effective January 1, 2019. The Company plans to use the modified retrospective method of adoption and to elect the available practical expedients. The standard is expected to have a material impact on the Consolidated Balance Sheets, but not have an impact on the Consolidated Statements of Operations. The most significant impact will be the recognition of right-of-use assets and lease liabilities for operating leases.

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments. The new standard clarifies certain aspects of the statement of cash flows, including the classification of contingent consideration payments made after a business combination and several other clarifications not currently applicable to the Company. The new standard also clarifies that an entity should determine each separately identifiable source or use within cash receipts and cash payments on the basis of the nature of the underlying cash flows. In situations in which cash receipts and payments have aspects of more than one class of cash flows and cannot be separated by source or use, the appropriate classification should depend on the activity that is likely to be the predominant source or use of cash flows for the item. The Company adopted the new standard effective January 1, 2018. The adoption of this standard did not have a material impact on the Company’s consolidated financial statements.

In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows: Restricted Cash (ASU No. 2016-18). The amendments in this update require that amounts generally described as restricted cash and restricted cash equivalents be included within cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The Company adopted the new standard effective January 1, 2018. The adoption of this standard did not have a material impact on the Company’s consolidated financial statements.

In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting. This guidance is intended to provide clarity and reduce diversity in practice as to when changes to the terms or conditions of share-based payments are accounted for as modifications. Under this new guidance, entities will apply modification accounting if the fair value, vesting conditions or classification of the award changes. This guidance will be effective for annual reporting periods beginning after December 15, 2017, including interim periods within those annual reporting periods, and early adoption is permitted. The guidance per ASU 2017-09 is to be adopted prospectively to an award modified on or after the adoption date. The Company adopted the new standard effective January 1, 2018. The adoption of this standard did not have a material impact on the Company’s consolidated financial statements.

In June 2018, the FASB issued ASU No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting. This guidance simplifies the accounting for share-based payments to non-employees by aligning it with the accounting for share-based payments to employees, with certain exceptions. This guidance will be effective for annual reporting periods beginning after December 15, 2018, including interim periods within those annual reporting periods, and early adoption is permitted. The guidance per ASU 2018-07 is to be adopted by using a modified retrospective approach with the cumulative effect of initially applying the new standard recognized at the date of initial application. The Company does not anticipate a material impact to the consolidated financial statements as a result of the adoption of this guidance.

In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808):Clarifying the Interaction between Topic 808 and Topic 606. The main provisions of ASU 2018-18 include: (i) clarifying that certain transactions between collaborative arrangement participants should be accounted for as revenue when the collaborative arrangement participant is a customer in the context of a unit of account and (ii) precluding the presentation of transactions with collaborative arrangement participants that are not directly related to sales to third parties together with revenue. This guidance will be effective for annual reporting periods beginning after December 15, 2019, including interim periods within those annual reporting periods, and early adoption is permitted. The guidance per ASU 2018-18 is to be adopted retrospectively to the date of initial application of Topic 606. The Company is currently evaluating the potential impact that ASU No. 2018-18 may have on its financial position and results of operations.

Collaboration agreements

12 Months Ended

Dec. 31, 2018

Collaboration Agreements

Collaboration agreements

3. Collaboration Agreements

Merck KGaA

In June 2014, the Company entered into a Collaboration and Commercial License Agreement with Merck KGaA (the Merck KGaA Agreement). Upon the execution of the agreement, Merck KGaA paid the Company a nonrefundable technology access fee of $12,000 for the right to develop ADCs directed to six exclusive targets over a specified period of time. No additional fees are due when a target is designated and the commercial license to the target is granted. Merck KGaA will be responsible for the product development and marketing of any products resulting from this collaboration. All six targets were designated prior to 2018.

Under the terms of the agreement, the Company and Merck KGaA develop research plans to evaluate Merck KGaA's antibodies as ADCs incorporating the Company's technology. The Company receives reimbursement for its efforts under the research plans. The goal of the research plans is to provide Merck KGaA with sufficient information to formally nominate a development candidate and begin IND-enabling studies or cease development on the designated target.

In addition to the payments received for research and development activities performed on behalf of Merck KGaA, the Company is also eligible to receive up to a total of $780,000 in future milestones related to all targets under the agreement, plus low to mid single digit royalties on the commercial sales of any resulting products during the applicable royalty term. The total milestones are categorized as follows: development milestones—$84,000; regulatory milestones—$264,000; and sales milestones—$432,000. There are six individual development milestones per target, payable upon the completion of various activities from the delivery of ADCs meeting defined specifications, through the dosing in a Phase 3 clinical trial. There are five regulatory milestones, which are payable upon regulatory approvals for a first indication in each of the U.S., European Union and Japanese markets and regulatory approvals for both a second and a third indication in the United States. There are three individual commercial milestones, which are payable upon the attainment of certain defined thresholds for annual net sales.

Prior to 2018, the Company had received $3,000 related to development milestones under the agreement. There have been no additional milestone payments in the year ended December 31, 2018. The next potential milestone payment the Company will be eligible to receive will be a development milestone of $500 on Merck KGaA's designation of a preclinical development candidate for any target. Revenue will be recognized upon achievement of the milestone.

Unless earlier terminated, the agreement will expire upon the expiration of the last royalty term for a product under the agreement, after which time, Merck KGaA will have a perpetual, royalty-free license, or if Merck KGaA does not designate any ADC product candidates produced by the Company under the agreement as preclinical development candidates, upon the expiration of the last to expire research program. Merck KGaA may terminate the agreement in its entirety or with respect to any target for convenience upon 60 days' prior written notice. Each party may terminate the Merck KGaA Agreement in its entirety upon bankruptcy or similar proceedings of the other party or upon an uncured material breach of the agreement by the other party. However, if such breach only relates to one target, the agreement may only be terminated with respect to such target.

In May 2018, the Company entered into a Supply Agreement with Merck KGaA (the Merck KGaA Supply Agreement). Under the terms of the agreement, the Company will provide Merck KGaA preclinical non-GMP ADC Drug Substance and clinical GMP Drug Substance for use in clinical trials associated with one of the antibodies designated under the Merck KGaA Agreement. The Company receives fees for its efforts under the Merck KGaA Supply Agreement and reimbursement equal to the supply cost. The Company may also enter into future supply agreements to provide clinical supply material should Merck KGaA pursue clinical development of any other candidates nominated under the agreement.

Accounting Analysis

For periods prior to January 1, 2018, the Company applied the provisions of ASC 605 in accounting for this arrangement.

By applying the contract modification practical expedient, the Company has aggregated the effect of all modifications through the initial date of application of Topic 606 for the purposes of (i) identifying the satisfied and unsatisfied performance obligations, (ii) determining the transaction price and (iii) allocating the transaction price to the satisfied and unsatisfied performance obligations.

The Company identified the following performance obligations under the agreement: (i) exclusive license and research services for six designated targets, (ii) rights to future technological improvements and (iii) participation of project team leaders and providing joint research committee services.

The Company has concluded that each license for a designated target is not distinct from the research services performed related to the designated target as Merck KGaA cannot obtain the benefit of the license without the related research services. Each license for a designated target and the related services performance obligation is considered distinct from every other license for a designated target and related services performance obligation as each research plan is pursued independent of every other research plans for other designated targets.

As of the date of initial application of Topic 606, the total transaction price for the Merck KGaA Agreement was $22,875, which included approximately $7,875 fees for research and development activities which have been or were expected to be received and $3,000 of milestone payments previously earned. The Company utilizes the expected value approach to estimate the amount of consideration related to the payment of fees associated with development and research services. The Company utilizes the most likely amount approach to estimate any development and regulatory milestone payments to be received. As of the date of initial application of Topic 606, there were no milestones payments that had not already been received, included in the estimated transaction price. The Company considered the stage of development and the remaining risks associated with the remaining development required to achieve the milestone, as well as whether the achievement of the milestone is outside the control of the Company or Merck KGaA. The milestone payment amounts were fully constrained, as a result of the uncertainty whether any of the associated milestones would be achieved. The Company has determined that any commercial milestones and sales based royalties will be recognized when the related sales occur as they were determined to relate predominantly to the license granted and therefore have also been excluded from the transaction price. The Company will re-evaluate the transaction price in each reporting period and as uncertain events are resolved or other changes in circumstances occur. In the third quarter of 2018, the Company revised its estimate for fees associated with research and development activities under the Merck KGaA Agreement to $7,070, a decrease of $805. The revised total transaction price for the Merck KGaA Agreement is $22,070.

The transaction price was allocated to the performance obligations based on the relative estimated standalone selling prices of each performance obligation or in the case of certain variable consideration to one or more performance obligations. The estimated standalone selling prices for performance obligations, that include a license and research services, were developed using the estimated selling price of the license and an estimate of the overall effort to perform the research service and an estimated market rate for research services. The estimated standalone selling price of the licenses was established based on comparable transactions. The estimated standalone selling price for the rights to future technological improvements was developed based on the estimated selling prices of a license or rights received, as well as considering the probability that additional technology would be made available or the probability the counterpart would utilize the technology. The estimated standalone selling price for the joint research committee services was developed using an estimate of the time and costs incurred to participate in the committees.

The transaction price as of the date of initial application of Topic 606 of $22,875 was allocated to the performance obligations as follows: approximately $4,226 for each of the license and corresponding research and development services units of account for the first and second designated targets; $3,439 for each of the license and corresponding research and development services units of account for the third, fourth, fifth and sixth designated target; $425 for rights to future technological improvements; and $242 for joint research committee services.

The Company is recognizing revenue related to the exclusive license and research and development services performance obligation over the estimated period of the research and development services using a proportional performance model. The Company measures proportional performance based on the costs incurred relative to the total costs expected to be incurred. To the extent that the Company receives fees for the research services as they are preformed, these amounts are recorded as deferred revenue. Revenue related to future technological improvements and joint research committee services will be recognized ratably over the performance period (which in the case of the joint research committee services approximate the time and cost incurred each period), which are 10 and 5 years, respectively. The Company is continuing to reassess the estimated remaining term at each subsequent reporting period.

During the years ended December 31, 2018, 2017 and 2016, the Company recorded revenue of $2,444, $3,636 and $3,644, respectively, related to its efforts under the collaboration agreement. Included in accounts receivable as of December 31, 2018 and 2017 was $450 and $330, respectively, related to the Merck KGaA Agreement.

As of December 31, 2018 and 2017, the Company had recorded $5,462 and $6,634, respectively, in deferred revenue related to the Merck KGaA Agreement that will be recognized over the remaining performance period.

Takeda XMT-1522 strategic partnership

In January 2016, the Company entered into a Development Collaboration and Commercial License Agreement with Takeda’s wholly owned subsidiary, Millennium Pharmaceuticals, Inc. for the development and commercialization of XMT-1522 (the XMT-1522 Agreement). Under the XMT-1522 Agreement, Takeda was granted the exclusive right to commercialize XMT-1522 outside of the United States and Canada. Under the XMT-1522 Agreement, the Company was responsible for conducting certain Phase 1 development activities for XMT-1522, including the ongoing Phase 1 clinical study, at its own expense. Takeda had the option to conduct Phase 1 development activities at its own expense within its territory. The parties agreed to collaborate on the further development of XMT-1522 in accordance with a global development plan (Post-Phase 1 Development). The parties agreed to share equally all clinical stage manufacturing costs and any Post-Phase 1 Development costs incurred in the performance of activities for the purpose of obtaining regulatory approval in either the United States or Canada and in certain other major markets in the rest of the world. Each party was responsible for all Post-Phase 1 Development costs incurred in the performance of activities solely for the purpose of obtaining regulatory approval in such party's territory. Each party may conduct independent development of XMT-1522, subject to certain restrictions.

The Company received an upfront payment of $26,500 upon execution of the XMT-1522 Agreement. In addition, the Company was entitled to a milestone payment of $20,000 upon achievement of the IND Clearance Date (as defined therein). The Company achieved the IND Clearance Date in October 2016.

In addition to the milestone payment upon achievement of the IND Clearance Date, the Company is entitled to receive future development, regulatory and commercial milestones of up to $288,000, consisting of $87,000 of development milestones, $128,000 of regulatory milestones and $73,000 of commercial milestones, as well as royalties in the low to mid teens on net sales of XMT-1522 in Takeda’s territory during the applicable royalty term. There are development milestones payable upon the achievement of nine separate events: the initiation of Phase 2 clinical trials and Phase 3 clinical trials for four separate specified patient populations and the initiation of a Phase 3 clinical trial for one additional unspecified patient population. There are 14 regulatory milestones, which are payable upon regulatory submissions, regulatory approvals and pricing approvals, as applicable, for the U.S., European Union and Japanese markets for up to four separate patient populations and multiple label indications. In addition, a regulatory milestone is payable upon the receipt of regulatory and pricing approval in two specified markets other than the United States, the European Union or Japan. There are three individual commercial milestones, which are payable upon the attainment of certain thresholds for annual net sales. The next potential milestone the Company will be eligible to receive is a development milestone of $12,000 related to the initiation of a Phase 2 clinical trial.

The XMT-1522 Agreement expires upon the expiration of the royalty term for XMT-1522, after which time, Takeda will have a perpetual, royalty-free license. However, Takeda may terminate the XMT-1522 Agreement in its entirety for convenience upon 30 days' prior written notice at any time up to the initiation of the first Phase 2 clinical study of XMT-1522 or upon 90 days' prior written notice following the initiation of the first Phase 2 clinical study of XMT-1522. Each party may terminate the XMT-1522 Agreement in its entirety upon bankruptcy or similar proceedings of the other party and in its entirety or on a country-by-country basis upon an uncured material breach of the agreement by the other party. Following termination, XMT-1522 will revert to the Company for further development and commercialization.

Takeda strategic research and development partnership

In March 2014, the Company entered into a Research Collaboration and Commercial License Agreement with Takeda’s wholly owned subsidiary, Millennium Pharmaceuticals, Inc. (the 2014 Agreement). The 2014 Agreement was amended in January 2015 and amended and restated in January 2016 (the 2016 Restated Agreement). The agreements initially provided Takeda with the right to develop ADCs directed to a total of seven exclusive targets, designated by Takeda, over a specified period of time. Takeda was responsible for the product development and marketing of any products resulting from this collaboration. To date, the Company has received $24,800 in non-refundable upfront fees, technology access fees or option exercise fees.

For the two targets initially under the 2014 Agreement, the Company initially granted a research license upon designation of a target. To receive a development and commercialization license for these designated targets, Takeda was required to pay an additional option exercise fee of $1,300. For the remaining five targets, the Company grants a research, development and commercialization license upon the designation of a target.

For each designated target, the Company and Takeda develop research plans to evaluate Takeda's antibodies as ADCs incorporating the Company's technology. The Company receives service fees for its efforts under the research plans. The goal of the research plans is to provide Takeda with sufficient information to formally nominate a development candidate and begin Investigational New Drug Application (IND), enabling studies or cease development on the designated target.

As of December 31, 2018, Takeda had designated four targets and received development and commercialization licenses for the first, third and fourth designated targets and a research license for the second designated target. Takeda has limited replacement rights for two designated targets, subject to certain contractual restrictions. Takeda is required to pay $500 to utilize the second limited replacement right.

As of December 31, 2018, if products are successfully developed and commercialized, the Company would be entitled to receive aggregate milestones of up to $474,750 for all eligible designated targets consisting of $50,250 in development milestones, $153,000 in regulatory milestones, and $271,500 in commercial milestones. The total milestones payable on each of the remaining eligible three targets are $158,250. There are four individual development milestones per target, which are payable upon the filing of an IND application and the initiation of Phase 1 through Phase 3 clinical trials. There are eight individual regulatory milestones per target. These are payable upon regulatory submissions, regulatory approvals and pricing approvals, as applicable, for the U.S., European Union and Japanese markets and regulatory approvals for both a second and third indication. There are six individual commercial milestones, which are payable upon the first commercial sale in each of the U.S., European Union and Japanese markets and upon the attainment of three separate defined thresholds for annual net sales. The next potential milestone payment the Company will be eligible to receive is a development milestone of $750 related to the filing of an IND. The Company is also entitled to receive royalties on product sales, if any, during the applicable royalty term. Royalties payable on the remaining designated targets are in the mid to high single digits.

The Company may elect to exercise an option to co-develop and co-commercialize one product incorporating either Takeda's fifth, sixth or seventh target in the United States for a payment of $15,000. If the Company elects to exercise the option to co-develop and co-commercialize a product, the Company will share in 50% of the profits related to the United States. The Company will be responsible for 50% of costs incurred specifically for the United States and 30% of global development costs. Any costs incurred specifically for a foreign country will be borne 100% by Takeda. If the Company elects to co-develop and co-commercialize a product, certain regulatory milestones and royalties related to the United States for that target would not be paid by Takeda.

Unless earlier terminated, the 2016 Restated Agreement will expire upon the expiration of the last royalty term for a product under the agreement, after which time, Takeda will have a perpetual, royalty-free license. Except with respect to the target antigen of a product for which the Company exercised its option to co-develop and co-commercialize in the United States, Takeda may terminate the 2016 Restated Agreement in its entirety or with respect to any target for convenience upon 45 days' prior written notice. Each party may terminate the 2016 Restated Agreement in its entirety upon bankruptcy or similar proceedings of the other party or upon an uncured material breach of the agreement by the other party. However, if such breach only relates to one target, the agreement may only be terminated with respect to such target.

Accounting Analysis

For periods prior to January 1, 2018, the Company applied the provisions of ASC 605 in accounting for these arrangements.

Under ASC 605 and Topic 606, the Company has concluded that the 2016 Restated Agreement and the XMT-1522 Agreement should be accounted for as one arrangement due in part because the agreements are with the same party and were negotiated and executed contemporaneously. Further, in applying the contract modification practical expedient, the Company has aggregated the effect of all modifications through the initial date of application of Topic 606 for the purposes of (i) identifying the satisfied and unsatisfied performance obligations, (ii) determining the transaction price and (iii) allocating the transaction price to the satisfied and unsatisfied performance obligations.

The performance obligations and the allocated transaction price as of the date of initial application of Topic 606 are as follows:


	Allocated
Performance obligations	Transaction Price
XMT-1522 license and research services	$	49,828
Joint research committee services for XMT-1522		449
Exclusive license to the first designated target and research services		6,611
Research license to the second designated target and research services		1,017
Material right related to the exclusive license to the second designated target		526
Exclusive license to the third designated target and research services		4,974
Exclusive license to the fourth designated target and research services		3,678
Material right related to the license to the fifth designated target and research services		3,506
Material right related to the license to the sixth designated target and research services		3,506
Material right to license to the seventh designated target and research services		3,506
Material right related to first replacement right for a designated target		3,506
Material right related to the second replacement right to a designated target		3,116
Rights to future technological improvements		1,750
Joint research committee services		150
	$	86,123

The Company has concluded the license related to each of the designated targets is not distinct from the research services performed related to each of the designated targets as Takeda cannot obtain the benefit of the license without the related research services. Each license to a designated target and the related services performance obligation is considered distinct from every other license to a designated target and related services performance obligation as each research plan is pursued independent of the any other research plans for other designated targets. Further, the material rights provided in the agreement provide Takeda incremental rights for either no additional consideration or for additional fees that contain a significant discount. The material rights are distinct from the other performance obligations in the arrangement as they are options in the contract and are not required for Takeda to obtain the benefit of the other promised goods or services in the arrangement.

Similarly, the Company concluded that the XMT-1522 license and the related research and development services, including the Phase 1 development and the transfer certain materials and know how related to the Company's manufacturing processes are one performance obligation. The license to the Company's intellectual property is not distinct from the research and related development services that the Company is obligated to perform. Takeda would not have the ability to realize the value of the license without the Company performing the related services.

The Company has concluded that the Post-Phase 1 Development activities under the XMT-1522 Agreement represent joint operating activities in which both parties are active participants and of which both parties are exposed to significant risks and rewards that are dependent on the commercial success of the activities. Accordingly, the Company is accounting for the Post-Phase 1 Development activities in accordance with ASC 808 and they are not considered revenue elements under Topic 606. For the years ended December 31, 2018, 2017 and 2016, the Company was billed approximately $8,046, $3,408 and $340, respectively, from Takeda representing the Company's share of Post-Phase 1 Development costs incurred by Takeda. These amounts have been reflected as research and development costs in the consolidated statement of operations. The Company did not perform any Post-Phase 1 Development activities or incur any associated costs prior to January 1, 2018. During the years ended December 31, 2018, 2017 and 2016, the Company billed Takeda $3,746, $0 and $0, respectively, related to ASC 808 costs.

As of the date of initial application of Topic 606, the total transaction price for the 2016 Restated Agreement and the XMT-1522 Agreement was $86,123, which included approximately $14,023 of fees associated with research and development activities which had been or were expected to be provided. The Company utilizes the expected value approach to estimate the amount of consideration related to the fees associated with development and research services. The Company utilizes the most likely amount approach to estimate any development and regulatory milestone payments to be received. As of the date of initial application of Topic 606, there were no milestone payments, which had not been received, included in the estimated transaction price. The Company considered the stage of development and the remaining risks associated with the remaining development required to achieve the milestone, as well as whether the achievement of the milestone is outside the control of the Company or Takeda. The milestone payment amounts were fully constrained, as a result of the uncertainty whether any of the associated milestones would be achieved. The Company has determined that any commercial milestones and sales based royalties will be recognized when the related sales occur as they were determined to relate predominantly to the license granted and therefore have also been excluded from the transaction price. The Company will re-evaluate the transaction price in each reporting period and as uncertain events are resolved or other changes in circumstances occur. In the third quarter of 2018, the Company revised its estimate for fees associated with research and development activities under the XMT-1522 Agreement to $9,615. The revised total transaction price for the 2016 Restated Agreement and the XMT-1522 Agreement is $86,289.

The transaction price was allocated to the performance obligations based on the relative estimated standalone selling prices of each performance obligation or, in the case of certain variable consideration, to one or more performance obligations. The estimated standalone selling prices for performance obligations which include a license and research services, was developed using the estimated selling price of the license and an estimate of the overall effort to perform the research service and an estimated market rate for research services. The estimated standalone selling price of the licenses was established based on comparable transactions. The estimated standalone selling price for the material rights and rights to future technological improvements were developed based on the estimated selling prices of a license or rights received and any fees payable upon exercise of the associated option, as well as considering the probability that additional technology would be made available or the probability the counterpart would utilize the technology or exercise the option. The estimated standalone selling price for the joint research committee services was developed using an estimate of the time and costs incurred to participate in the committees.

The Company will recognize revenue related to the performance obligations, which include research licenses or an exclusive development and commercialization license and the related research services, over the estimated period of the research and development services using a proportional performance model. The Company measures proportional performance based on the costs incurred relative to the total estimated costs of the research. To the extent that the Company receives fees for the research services as they are performed, these amounts are recorded as deferred revenue. Revenue related to material rights will be recognized when the option is exercised, unless there are additional research services that the Company is required to perform related to the designated target (in which case revenue will be recognized based on the proportional performance model) or at the time the option right lapses. To the extent that the Company receives a fee upon exercise of the option, such amounts are recorded as deferred revenue. Revenue related to the material rights related to replacement rights will be recognized over the research term of the replacement target once the replacement right is exercised or at the time the right lapses unused. To the extent that the Company receives a fee upon exercise of the replacement right, such amounts are recorded as deferred revenue. Revenue related to future technological improvements and joint research committee services will be recognized ratably over the performance period (which in the case of the joint research committee services approximates the time and cost incurred), which is expected to be ten years and six years, respectively. The Company will reassess the estimated remaining term at each subsequent reporting period.

For the years ended December 31, 2018, 2017 and 2016, the Company recorded total revenue of $5,868, $13,784 and $21,401, respectively, related to its efforts under the 2016 Restated Agreement and the XMT-1522 Agreement. Included in accounts receivable as of December 31, 2018 and 2017 was $9 and $454, respectively, related to the Takeda agreements. Included in accounts payable as of December 31, 2018 was $2,749 related to the Takeda agreements. During the quarter ended September 30, 2018, the Company revised its estimate of the total costs to complete research services under the 2016 Restated Agreement and the XMT-1522 Agreement, which changed the total consideration to be received under the agreements and the amount of revenue recognized during the year ended December 31, 2018. The Company recognized approximately $1,535 less for the year ended December 31, 2018 as a result of the Company’s change in estimate. The change in estimate increased the net loss by $1,535, or $0.07 per common share, for the year ended December 31, 2018.

As of December 31, 2018 and 2017, the Company had $39,960 and $43,579, respectively, of deferred revenue related to the Takeda agreements that will be recognized over the remaining performance periods for the applicable obligations.

On January 2, 2019, the Company received notices from Takeda stating that Takeda was exercising its right to: (a) terminate the 2016 Restated Agreement upon 45 days’ prior written notice and (b) terminate the XMT-1522 Agreement upon 30 days’ prior written notice. The Company and Takeda are working to wind down activities under the 2016 Restated Agreement and the XMT-1522 Agreement over the course of the applicable notice periods. The respective notice periods expired in February 2019 and the agreements were terminated. Under Topic 606, the Company has concluded that the performance obligations were not modified during the year ended December 31, 2018, and that the Company will account for the termination notice as an event in the first quarter of the fiscal year ended December 31, 2019.

Summary of Contract Assets and Liabilities

The following table presents changes in the balances of our contract assets and liabilities during the year ended December 31, 2018:


	Balance at
	Beginning						Balance at
	of Period		Additions		Deductions		End of Period
Year ended December 31, 2018
Contract assets	$	—	$	—	$	—	$	—
Contract liabilities:
Deferred revenue	$	52,439	$	2,851	$	9,094	$	46,196

The impact of the adoption of the new revenue recognition guidance is reflected within the balance as of the beginning of period.

During the year ended December 31, 2018, the Company recognized the following revenues as a result of changes in the contract asset and the contract liability balances in the respective periods:


	Year ended
	December 31, 2018
Revenue recognized in the period from:
Amounts included in the contract liability at the beginning of the period	$	8,704
Performance obligations satisfied in previous periods	$	—

Other Revenue

The Company has provided limited services for a collaboration partner, Asana BioSciences. For the years ended December 31, 2018, 2017 and 2016, the Company recorded revenue of $782, $125 and $125, respectively, related to these services. In addition, during the year ended December 31, 2018, the Company recognized revenue of $1,500 related to a milestone achieved upon the completion of a GLP toxicology study by Asana BioSciences. The next potential milestone the Company is eligible to receive is $2,500 upon dosing the fifth patient in a Phase 1 clinical study by Asana BioSciences.

Fair value measurements

12 Months Ended

Dec. 31, 2018

Fair value measurements

Fair value measurements

4. Fair Value Measurements

The following table presents information about the Company’s assets and liabilities regularly measured and carried at a fair value and indicates the level within fair value hierarchy of the valuation techniques utilized to determine such value as of December 31, 2018 and 2017:


					Significant
			Quoted Prices		Other		Significant
			in Active		Observable		Unobservable
	Fair		Markets		Inputs		Inputs
	Value		(Level 1)		(Level 2)		(Level 3)
December 31, 2018
Marketable securities:
U.S. Treasuries	$	10,497	$	10,497	$	—	$	—
	$	10,497	$	10,497	$	—	$	—


					Significant
			Quoted Prices		Other		Significant
			in Active		Observable		Unobservable
	Fair		Markets		Inputs		Inputs
	Value		(Level 1)		(Level 2)		(Level 3)
December 31, 2017
Marketable securities:
U.S. Treasuries	$	62,640	$	62,640	$	—	$	—
Commercial paper		24,931		—		24,931		—
Corporate bonds		11,054		—		11,054		—
	$	98,625	$	62,640	$	35,985	$	—

There were no changes in valuation techniques or transfers between fair value measurement levels during the years ended December 31, 2018, 2017 and 2016. As of December 31, 2018 and 2017, cash and cash equivalents were comprised of cash and money market funds.

Marketable securities

12 Months Ended

Dec. 31, 2018

Marketable securities

Marketable securities

5. Marketable Securities

The following table summarizes marketable securities held at December 31, 2018 and 2017.


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
December 31, 2018
U.S. Treasuries	$	10,505	$	—	$	(8)	$	10,497
	$	10,505	$	—	$	(8)	$	10,497


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
December 31, 2017
U.S. Treasuries	$	62,777	$	—	$	(137)	$	62,640
Commercial paper		24,931		—		—		24,931
Corporate bonds		11,066		—		(12)		11,054
	$	98,774	$	—	$	(149)	$	98,625

As of December 31, 2018, the Company held three securities that were in an unrealized loss position. The aggregate fair value of securities held by the Company in an unrealized loss position at December 31, 2018 was $10,497. These securities were held by the Company in an unrealized loss position for more than 12 months. As of December 31, 2018, the Company did not intend to sell, and would not be more likely than not required to sell, the securities in an unrealized loss position before recovery of their amortized cost basis. Furthermore, the Company has determined that there was no material change in the credit risk of these securities. As a result, the Company determined it did not hold any securities with any other-than-temporary impairment as of December 31, 2018.

There were no realized gains or losses on available-for-sale securities during the years ended December 31, 2018, 2017 and 2016.

Property and Equipment

12 Months Ended

Dec. 31, 2018

Property and Equipment

Property and Equipment

6. Property and Equipment

Property and equipment consists of the following as of December 31, 2018 and 2017:


	December 31,		December 31,
	2018		2017
Laboratory equipment	$	6,134	$	5,237
Computer equipment, office equipment and software		1,035		718
Leasehold improvements		1,886		1,504
Total property and equipment at cost		9,055		7,459
Less: Accumulated depreciation		(6,361)		(5,140)
	$	2,694	$	2,319

Depreciation expense for the years ended December 31, 2018, 2017 and 2016 was $1,257, $928 and $655, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2018

Accrued expenses

Accrued expenses

7. Accrued Expenses

Accrued expenses consist of the following as of December 31, 2018 and 2017:


	December 31,		December 31,
	2018		2017
Accrued payroll and related expenses	$	3,042	$	3,041
Accrued preclinical, manufacturing and clinical expenses		8,314		3,183
Accrued professional fees		567		492
Accrued other		452		228
	$	12,375	$	6,944

Preferred Stock	12 Months Ended
Preferred Stock	Dec. 31, 2018
Preferred Stock
Preferred Stock	8. Preferred Stock As of December 31, 2018, the Company has 25,000,000 shares of authorized preferred stock. No shares of preferred stock have been issued.

Stockholders' equity (deficit)

12 Months Ended

Dec. 31, 2018

Stockholders' equity (deficit)

Stockholders’ Equity (Deficit)

9. Stockholders’ Equity (Deficit)

Common Stock

The holders of the common stock are entitled to one vote for each share held. Common stockholders are not entitled to receive dividends, unless declared by the Board of Directors (the Board).

As of December 31, 2018 and 2017 there were 3,856,932 and 3,315,850, respectively, shares of common stock reserved for the exercise of outstanding stock options and warrants.


	December 31,	December 31,
	2018	2017
Warrants	110,365	110,365
Stock options	3,746,567	3,205,485
	3,856,932	3,315,850

Warrants

In connection with a 2013 Series A‑1 Preferred Stock issuance, the Company granted to certain investors warrants to purchase 129,491 shares of common stock. The warrants have a $0.05 per share exercise price and a contractual life of 10 years. The fair value of these warrants was recorded as a component of equity at the time of issuance. During the year ended December 31, 2017, the Company issued 19,071 shares of common stock upon the exercise of a warrant.

Stock options

12 Months Ended

Dec. 31, 2018

Stock options

Stock options

10. Stock Options

Stock Option Plan

As of June 30, 2017, there were 3,141,625 options outstanding under the Company’s 2007 Stock Incentive Plan. The 2007 Plan expired in June 2017. Any cancellations under the 2007 Stock Incentive Plan will increase the options available under the 2017 Stock Incentive Plan as described below.

In June 2017 the Company’s shareholders approved the 2017 Stock Incentive Plan (the 2017 Plan or the Plan). Under the 2017 Plan initially, up to 2,255,000 shares of common stock may be granted to the Company's employees, officers, directors, consultants and advisors in the form of options, restricted stock awards or other stock-based awards. The number of shares of common stock issuable under the Plan will be cumulatively increased annually by 4% of the outstanding shares or such lesser amount specified by the Board. The terms of the awards are determined by the Board, subject to the provisions of the Plan. As of December 31, 2018 there were 2,085,048 shares available for future issuance under the Plan. In January 2019, the number of shares of common stock that might be issued under the Plan was increased by 929,378 shares.

With respect to incentive stock options, the exercise price per share will equal the fair market value of the common stock on the date of grant, as determined by the Board, and the vesting period is generally four years. Nonqualified stock options will be granted at an exercise price established by the Board at its sole discretion (which has not been less than fair market value on the date of grant) and the vesting periods may vary. Options granted under the Plan expire no later than 10 years from the date of grant. The Board may accelerate vesting or extend the expiration of granted options in the case of a merger, consolidation, dissolution, or liquidation of the Company.

A summary of the activity under the Plan is as follows:


		Weighted-		Remaining
	Number	Average		Contractual Life	Aggregate
	of Shares	Exercise Price		(in years)	Intrinsic Value

Options outstanding at January 1, 2018	3,205,485	$	3.44	7.8	$	41,709
Granted	1,273,115		13.33
Exercised	(427,269)		2.15
Cancelled	(304,764)		7.99
Options outstanding at December 31, 2018	3,746,567	$	6.58	7.6	$	3,897

Options exercisable at December 31, 2018	1,998,527	$	3.57	6.6	$	3,460

The weighted-average grant date fair value of options granted during the years ended December 31, 2018, 2017 and 2016, was $8.78, $5.53 and $2.34 per share, respectively.

Cash received from the exercise of stock options was $918, $452 and $105 for the years ended December 31, 2018, 2017 and 2016, respectively.

Stock-Based Compensation

The Company uses the provisions of ASC 718, Stock Compensation, to account for all stock-based awards to employees and nonemployees.

The measurement date for employee awards is generally the date of grant. Stock-based compensation expense is recognized over the requisite service period, which is generally the vesting period, using the straight-line method.

For the years ended December 31, 2018, 2017 and 2016, the Company recorded stock-based compensation expense of $3,828, $1,366 and $664, respectively, related to employee grants. The Company has an aggregate of $10,289 of unrecognized stock compensation cost as of December 31, 2018 remaining to be amortized over the weighted-average period of 2.6 years. The fair value of each option award is estimated on the date of grant using the Black–Scholes option pricing model with the following weighted average assumptions:


	December 31,
	2018		2017		2016

Risk-free interest rate	2.7	%	2.2	%	1.5	%
Expected dividend yield	—	%	—	%	—	%
Expected term (years)	6.07		6.21		6.25
Expected stock price volatility	73	%	67	%	69	%

Expected volatility for the Company’s common stock was determined based on the historical volatility of comparable publicly traded companies. The risk-free interest rate is based on the yield of U.S. Treasury securities consistent with the expected term of the option. No dividend yield was assumed as the Company has not historically and does not expect to pay dividends on its common stock. The expected term of the options granted is based on the use of the simplified method, in which the expected term is presumed to be the mid-point between the vesting date and the end of the contractual term.

Prior to the Company’s initial public offering in June 2017, the fair value of the common stock had been determined by the Board at each date of grant based on the variety of factors, including the Company’s financial position and historical financial performance, the status of developments within the Company’s research and development activities, the composition and ability of the current research and management team, an evaluation of the Company’s competition, the current climate in the marketplace, the illiquid nature of the common stock, the effect of the rights and preferences of the preferred shareholders, and the prospects of the liquidity event, among others.

The Company granted stock option awards to non-employees. Total expense recorded during the years ended December 31, 2018, 2017 and 2016 related to these awards was $56, $56 and $4, respectively.

Employee Stock Purchase Plan

During the year ended December 31, 2017, the Board adopted and the Company's stockholders approved the 2017 employee stock purchase plan (the 2017 ESPP). The Company initially reserved 225,000 shares of common stock for issuance under the 2017 ESPP. During the year ended December 31, 2018 the Company issued 42,186 shares under the 2017 ESPP, with 182,814 available for issuance as of December 31, 2018. In January 2019, the number of shares of common stock for issuance under the 2017 ESPP was increased by 232,344 shares.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Taxes

Income Taxes

11. Income Taxes

For the years ended December 31, 2018, 2017 and 2016, the Company recorded no income tax benefit for the net operating losses incurred in each year, due to its uncertainty of realizing a benefit from those items.

A reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations as of December 31, 2018, 2017 and 2016 are as follows:


	2018		2017		2016
Income tax computed at federal statutory tax rate	21.0	%	34.0	%	34.0	%
State taxes, net of federal benefit	6.5	%	5.1	%	5.1	%
Permanent differences	0.6	%	(0.8)	%	(1.3)	%
Research and development expenditures	—	%	(2.3)	%	—	%
General business credits	4.2	%	8.2	%	12.9	%
Impact of tax reform	—	%	(27.3)	%	—	%
Other	—	%	—	%	(0.1)	%
Change in valuation allowance	(32.3)	%	(16.9)	%	(50.6)	%
	—	%	—	%	—	%

In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (SAB No. 118), which allowed the Company to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. The Company’s provisional estimate associated with the reduction in the U.S. federal corporate tax rate from 35% to 21% impacted the changes in the valuation allowance and change in tax rate component of the Company’s effective tax rate reconciliation as well as its ending deferred tax assets and valuation allowance in the deferred tax footnote disclosure. In the fourth quarter of 2018, we completed our analysis to determine the effect of the Tax Act and recorded no adjustments as of December 31, 2018.

Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Significant components of the Company’s net deferred tax assets as of December 31, 2018 and 2017 are as follows:


	2018		2017
Deferred tax assets:
Net operating losses	$	27,916	$	9,324
Tax credit carryforwards		10,178		7,516
Deferred revenue		12,410		13,718
Licensed technology		1,160		1,266
Depreciation		405		268
Accrued expenses		1,585		53
Deferred expenses		112		313
Unrealized loss		43		41
Other state credits		131		103
Total deferred tax assets		53,940		32,602
Valuation allowance		(53,940)		(32,602)
Net deferred tax assets	$	—	$	—

The Company has incurred net operating losses (NOL) since inception. At December 31, 2018, the Company had Federal and State net operating loss carryforwards of approximately $102,112 and $102,405, respectively. Of the $102,112 of Federal net operating loss carryforwards, $34,115 expire at various dates through 2037. The remaining $67,997 of Federal net operating loss carryforwards do not expire. The State net operating loss carryforwards expire at various dates through 2038. At December 31, 2018, the Company had Federal and State research and development tax credit carryforwards of approximately $7,708 and $3,294, respectively, which expire at various dates through 2038.

As required by ASC 740, management of the Company has evaluated the evidence bearing upon the reliability of its deferred tax assets. Based on the weight of available evidence, both positive and negative, management has determined that it is more likely than not that the Company will not realize the benefits of these assets. Accordingly, the Company recorded a valuation allowance of $53,940 and $32,602 at December 31, 2018 and December 31, 2017, respectively. The valuation allowance increased by $21,338 and $6,611 during the years ended December 31, 2018 and 2017, respectively, primarily as a result of net operating losses generated during the periods.

Utilization of the NOLs and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 due to ownership change limitations that have occurred previously or that could occur in the future in accordance with Section 382, as well as similar state provisions. These ownership changes may limit the amount of NOLs and research and development tax credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. If a change in control as defined by Section 382 has occurred at any time since the Company’s formation, utilization of its NOLs or research and development tax credit carryforwards would be subject to an annual limitation under Section 382, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term tax-exempt rate, which could then be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the NOLs or research and development tax carryforwards before their utilization. The Company has determined that ownership changes have occurred through December 31, 2015 and that certain NOLs and research and development tax credit carryforwards will be subject to limitation. The amounts presented do not include NOLs or research and development tax credit carryforwards that will expire unused due to ownership changes.

The Company applies the accounting guidance in ASC 740 related to accounting for uncertainty in income taxes. The Company’s reserves related to taxes are based on a determination of whether, and how much of, a tax benefit taken by the Company in its tax filings or positions is more likely than not to be realized following resolution of any potential contingencies present related to the tax benefit. As of December 31, 2018 and 2017, the Company had no unrecognized tax benefits.

The Company has not conducted a study of its research and development credit carryforwards. This study may result in an adjustment to research and development credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company’s research and development credits and, if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. Thus, there would be no impact to the balance sheets or statements of operations if an adjustment were required.

Interest and penalties related to uncertain tax positions would be classified as income tax expense in the accompanying statements of operations. As of December 31, 2018 and 2017, the Company had no accrued interest or penalties related to uncertain tax positions.

The Company files income tax returns in the United States federal tax jurisdiction and one state jurisdiction. The Company did not have any foreign operations during the years ended December 31, 2018, 2017 and 2016. The statute of limitations for assessment by the Internal Revenue Service and state tax authorities is closed for tax years prior to 2015, although carryforward attributes that were generated prior to tax year 2015 may still be adjusted upon examination to the extent utilized in a future period. There are no federal or state audits currently in progress.

Employee Benefit Plan	12 Months Ended
Employee Benefit Plan	Dec. 31, 2018
Employee Benefit Plan
Employee Benefit Plan	12. Employee Benefit Plan The Company has a defined contribution plan established under Section 401(k) of the Internal Revenue Code (401(k) Plan), which covers substantially all employees. Employees who have attained the age of 21 are eligible to participate in the 401(k) Plan with no service requirement. Employees may contribute up to 75% of eligible pay on a pre–tax basis up to the federal annual limits. The Company matches the employees’ contributions at 50% on the first 6% up to $6. For the years ended December 31, 2018, 2017 and 2016, the Company recorded expense of $332, $273 and $136, respectively, related to its contribution to its 401(k) Plan.

Commitments

12 Months Ended

Dec. 31, 2018

Commitments

Commitments

13. Commitments

Operating Leases

The Company leases office space in Cambridge, MA under an operating lease, which is effective through March 2021. The Company has an option to extend the lease term for an additional five years. The lease also provided the Company with a tenant improvement allowance of up to $356. The Company fully utilized the allowance and recorded the assets acquired with the allowance as leasehold improvements. The Company recorded the tenant improvement allowance incurred as a deferred lease incentive and has amortized the deferred lease incentive through a reduction of rent expense ratably over the lease term.

In connection with the office lease, the Company has a letter of credit agreement for the benefit of its landlord in the amount of $321 as of each December 31, 2018 and 2017, respectively, collateralized by a money market account.

In addition, the Company leases certain equipment under leases that expire through February 2024. Future minimum lease payments under leases, as amended, were as follows:


2019	$	2,333
2020		2,468
2021		760
2022		83
2023 and thereafter		86
	$	5,730

Rent expense was approximately $1,994, $1,834 and $1,572 for the years ended December 31, 2018, 2017 and 2016, respectively.

The Company is recording rent expense on a straight-line basis over the term of the lease and has recorded deferred rent in the consolidated balance sheets accordingly.

License Agreements

Through December 31, 2018 the Company has licensed intellectual property from two biotechnology companies. The consideration included upfront payments and a commitment to pay annual license fees, milestone payments, and, upon product commercialization, royalties on revenue generated from the sale of products covered by the licenses. During the year ended December 31, 2017, the Company recorded expense related to milestone payments of $2,750 related to these agreements.

Related party transactions	12 Months Ended
Related party transactions	Dec. 31, 2018
Related party transactions
Related party transactions	14. Related Party Transactions Included in Series C‑1 financing and the Company’s initial public offering were investments of $10,000 and $10,000, respectively, by Takeda.

Subsequent events	12 Months Ended
Subsequent events	Dec. 31, 2018
Subsequent events
Subsequent events	15. Subsequent Events The Company considered the events or transactions occurring after the balance sheet date, but prior to the issuance of the consolidated financial statements, for potential recognition or disclosure in its consolidated financial statements. All significant subsequent events have been properly disclosed in the consolidated financial statements. On January 2, 2019, the Company received notices from Takeda stating that Takeda was exercising its right to: (a) terminate the 2016 Restated Agreement upon 45 days’ prior written notice and (b) terminate the XMT-1522 Agreement upon 30 days’ prior written notice. On March 5, 2019, the Company completed a secondary public offering, in which the Company issued and sold an aggregate of 24,437,500 shares of its common stock at the public offering price of $4.00 per share, which included the exercise in full of the underwriters’ option to purchase additional shares of common stock. The gross proceeds from the offering were approximately $97,800.

Selected quarterly financial data (Unaudited)

12 Months Ended

Dec. 31, 2018

Selected quarterly financial data (unaudited)

Selected quarterly financial data (unaudited)

16. Selected Quarterly Financial Data (unaudited)

The following table contains selected quarterly financial information for 2018 and 2017. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.


	Three months ended

	March 31, 2018		June 30, 2018		September 30, 2018		December 31, 2018
Collaboration revenue	$	3,064	$	4,191	$	2,151	$	1,188
Operating expenses:
Research and development		12,256		12,663		15,180		19,816
General and administrative		3,571		4,231		4,380		4,152
Total operating expenses		15,827		16,894		19,560		23,968
Other income:
Interest income		360		349		340		349
Total other income		360		349		340		349
Net loss	$	(12,403)	$	(12,354)	$	(17,069)	$	(22,431)
Net loss per share attributable to common stockholders — basic and diluted	$	(0.54)	$	(0.54)	$	(0.74)	$	(0.97)
Weighted-average number of common shares used in net loss per share attributable to common stockholders — basic and diluted		22,816,521		22,966,314		23,152,019		23,184,459


	Three months ended

	March 31, 2017		June 30, 2017		September 30, 2017		December 31, 2017
Collaboration revenue	$	4,290	$	3,727	$	6,267	$	3,261
Operating expenses:
Research and development		10,106		10,627		11,412		14,555
General and administrative		2,296		2,204		2,905		3,057
Total operating expenses		12,402		12,831		14,317		17,612
Other income:
Interest income		51		158		318		383
Total other income		51		158		318		383
Net loss	$	(8,061)	$	(8,946)	$	(7,732)	$	(13,968)
Net loss per share attributable to common stockholders — basic and diluted	$	(6.02)	$	(6.33)	$	(0.35)	$	(0.61)
Weighted-average number of common shares used in net loss per share attributable to common stockholders — basic and diluted		1,338,475		1,412,308		22,242,129		22,750,425

Summary of significant accounting policies (Policies)

12 Months Ended

Dec. 31, 2018

Summary of significant accounting policies

Principles of Consolidation

Use of Estimates

Segment Information

Research and Development

Revenue Recognition

Collaborative Arrangements

Fair Value Measurements

Level 1—Inputs to the valuation methodology are quoted prices (unadjusted) for identical assets or liabilities in active markets.

Level 3—Inputs to the valuation methodology are unobservable and significant to the fair value measurement.

Cash and Cash Equivalents


	Year ended				Year ended
	December 31, 2018				December 31, 2017
	Beginning		End		Beginning		End
	of period		of period		of period		of period
Cash and cash equivalents	$	26,591	$	59,634	$	100,297	$	26,591
Restricted cash included in other assets		371		371		371		371
Total cash, cash equivalents and restricted cash per statement of cash flows	$	26,962	$	60,005	$	100,668	$	26,962

Marketable Securities

Other Assets

Accounting for Stock-based Compensation

Net Loss per Share


	Year ended
	December 31,
	2018	2017	2016
Series A-1 Convertible Preferred Stock	—	—	5,574,467
Series B-1 Convertible Preferred Stock	—	—	7,319,307
Series C-1 Convertible Preferred Stock	—	—	3,260,897
Warrants	110,365	110,365	129,491
Stock options	3,746,567	3,205,485	2,901,985
	3,856,932	3,315,850	19,186,147

Property and Equipment

Property and equipment is stated at cost, less accumulated depreciation. Depreciation is computed using the straight-line method over the estimated useful life of each asset as follows:


Computer equipment, office equipment and software		3 years
Laboratory equipment		5 years
Leasehold improvements		Shorter of useful life or life of lease

Repairs and maintenance costs are expensed as incurred.

Patent Costs

The Company expenses patent application and related legal costs as incurred and classifies such costs as general and administrative expenses in the accompanying consolidated statements of operations.

Income Taxes

Comprehensive Income (Loss)

Concentration of Credit Risk and Off-balance Sheet Risk

Recently Issued Accounting Pronouncements

Summary of significant accounting policies (Tables)

12 Months Ended

Dec. 31, 2018

Summary of significant accounting policies

Schedule of cash, cash equivalents and restricted cash


	Year ended				Year ended
	December 31, 2018				December 31, 2017
	Beginning		End		Beginning		End
	of period		of period		of period		of period
Cash and cash equivalents	$	26,591	$	59,634	$	100,297	$	26,591
Restricted cash included in other assets		371		371		371		371
Total cash, cash equivalents and restricted cash per statement of cash flows	$	26,962	$	60,005	$	100,668	$	26,962

Schedule of outstanding potentially dilutive securities excluded from calculation of diluted net loss per share


	Year ended
	December 31,
	2018	2017	2016
Series A-1 Convertible Preferred Stock	—	—	5,574,467
Series B-1 Convertible Preferred Stock	—	—	7,319,307
Series C-1 Convertible Preferred Stock	—	—	3,260,897
Warrants	110,365	110,365	129,491
Stock options	3,746,567	3,205,485	2,901,985
	3,856,932	3,315,850	19,186,147

Schedule of estimated useful life of property and equipment


Computer equipment, office equipment and software		3 years
Laboratory equipment		5 years
Leasehold improvements		Shorter of useful life or life of lease

Collaboration Agreements (Tables)

12 Months Ended

Dec. 31, 2018

Collaboration Agreements

Schedule of allocated transaction price of performance obligations


	Allocated
Performance obligations	Transaction Price
XMT-1522 license and research services	$	49,828
Joint research committee services for XMT-1522		449
Exclusive license to the first designated target and research services		6,611
Research license to the second designated target and research services		1,017
Material right related to the exclusive license to the second designated target		526
Exclusive license to the third designated target and research services		4,974
Exclusive license to the fourth designated target and research services		3,678
Material right related to the license to the fifth designated target and research services		3,506
Material right related to the license to the sixth designated target and research services		3,506
Material right to license to the seventh designated target and research services		3,506
Material right related to first replacement right for a designated target		3,506
Material right related to the second replacement right to a designated target		3,116
Rights to future technological improvements		1,750
Joint research committee services		150
	$	86,123

Schedule of changes in the balances of our contract assets and liabilities and recognized revenue

The following table presents changes in the balances of our contract assets and liabilities during the year ended December 31, 2018:


	Balance at
	Beginning						Balance at
	of Period		Additions		Deductions		End of Period
Year ended December 31, 2018
Contract assets	$	—	$	—	$	—	$	—
Contract liabilities:
Deferred revenue	$	52,439	$	2,851	$	9,094	$	46,196

The impact of the adoption of the new revenue recognition guidance is reflected within the balance as of the beginning of period.

During the year ended December 31, 2018, the Company recognized the following revenues as a result of changes in the contract asset and the contract liability balances in the respective periods:


	Year ended
	December 31, 2018
Revenue recognized in the period from:
Amounts included in the contract liability at the beginning of the period	$	8,704
Performance obligations satisfied in previous periods	$	—

Fair value measurements (Tables)

12 Months Ended

Dec. 31, 2018

Fair value measurements

Schedule of assets and liabilities measured and carried at fair value


					Significant
			Quoted Prices		Other		Significant
			in Active		Observable		Unobservable
	Fair		Markets		Inputs		Inputs
	Value		(Level 1)		(Level 2)		(Level 3)
December 31, 2018
Marketable securities:
U.S. Treasuries	$	10,497	$	10,497	$	—	$	—
	$	10,497	$	10,497	$	—	$	—


					Significant
			Quoted Prices		Other		Significant
			in Active		Observable		Unobservable
	Fair		Markets		Inputs		Inputs
	Value		(Level 1)		(Level 2)		(Level 3)
December 31, 2017
Marketable securities:
U.S. Treasuries	$	62,640	$	62,640	$	—	$	—
Commercial paper		24,931		—		24,931		—
Corporate bonds		11,054		—		11,054		—
	$	98,625	$	62,640	$	35,985	$	—

- Definition

Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances.

+ References + Details

Marketable securities (Tables)

12 Months Ended

Dec. 31, 2018

Marketable securities

Schedule of reconciliation of marketable securities from cost basis to fair value


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
December 31, 2018
U.S. Treasuries	$	10,505	$	—	$	(8)	$	10,497
	$	10,505	$	—	$	(8)	$	10,497


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
December 31, 2017
U.S. Treasuries	$	62,777	$	—	$	(137)	$	62,640
Commercial paper		24,931		—		—		24,931
Corporate bonds		11,066		—		(12)		11,054
	$	98,774	$	—	$	(149)	$	98,625

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2018

Property and Equipment

Schedule of property and equipment


	December 31,		December 31,
	2018		2017
Laboratory equipment	$	6,134	$	5,237
Computer equipment, office equipment and software		1,035		718
Leasehold improvements		1,886		1,504
Total property and equipment at cost		9,055		7,459
Less: Accumulated depreciation		(6,361)		(5,140)
	$	2,694	$	2,319

Accrued expenses (Tables)

12 Months Ended

Dec. 31, 2018

Accrued expenses

Schedule of accrued expenses


	December 31,		December 31,
	2018		2017
Accrued payroll and related expenses	$	3,042	$	3,041
Accrued preclinical, manufacturing and clinical expenses		8,314		3,183
Accrued professional fees		567		492
Accrued other		452		228
	$	12,375	$	6,944

Stockholders' equity (deficit) (Tables)

12 Months Ended

Dec. 31, 2018

Stockholders' equity (deficit)

Schedule for number of common stock reserved for conversion of outstanding Preferred Stock and for the exercise of outstanding stock options and warrants


	December 31,	December 31,
	2018	2017
Warrants	110,365	110,365
Stock options	3,746,567	3,205,485
	3,856,932	3,315,850

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

+ References

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+ Details

Stock options (Tables)

12 Months Ended

Dec. 31, 2018

Stock options

Schedule of stock option activity under Plan


		Weighted-		Remaining
	Number	Average		Contractual Life	Aggregate
	of Shares	Exercise Price		(in years)	Intrinsic Value

Options outstanding at January 1, 2018	3,205,485	$	3.44	7.8	$	41,709
Granted	1,273,115		13.33
Exercised	(427,269)		2.15
Cancelled	(304,764)		7.99
Options outstanding at December 31, 2018	3,746,567	$	6.58	7.6	$	3,897

Options exercisable at December 31, 2018	1,998,527	$	3.57	6.6	$	3,460

Schedule of weighted average assumptions for estimating fair value of option awards


	December 31,
	2018		2017		2016

Risk-free interest rate	2.7	%	2.2	%	1.5	%
Expected dividend yield	—	%	—	%	—	%
Expected term (years)	6.07		6.21		6.25
Expected stock price volatility	73	%	67	%	69	%

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Taxes

Schedule of reconciliation of income tax rates


	2018		2017		2016
Income tax computed at federal statutory tax rate	21.0	%	34.0	%	34.0	%
State taxes, net of federal benefit	6.5	%	5.1	%	5.1	%
Permanent differences	0.6	%	(0.8)	%	(1.3)	%
Research and development expenditures	—	%	(2.3)	%	—	%
General business credits	4.2	%	8.2	%	12.9	%
Impact of tax reform	—	%	(27.3)	%	—	%
Other	—	%	—	%	(0.1)	%
Change in valuation allowance	(32.3)	%	(16.9)	%	(50.6)	%
	—	%	—	%	—	%

Schedule of significant components of net deferred tax assets


	2018		2017
Deferred tax assets:
Net operating losses	$	27,916	$	9,324
Tax credit carryforwards		10,178		7,516
Deferred revenue		12,410		13,718
Licensed technology		1,160		1,266
Depreciation		405		268
Accrued expenses		1,585		53
Deferred expenses		112		313
Unrealized loss		43		41
Other state credits		131		103
Total deferred tax assets		53,940		32,602
Valuation allowance		(53,940)		(32,602)
Net deferred tax assets	$	—	$	—

Commitments (Tables)

12 Months Ended

Dec. 31, 2018

Commitments

Schedule of future minimum lease payments under operating leases


2019	$	2,333
2020		2,468
2021		760
2022		83
2023 and thereafter		86
	$	5,730

Selected quarterly financial data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Selected quarterly financial data (unaudited)

Selected quarterly financial data (unaudited)


	Three months ended

	March 31, 2018		June 30, 2018		September 30, 2018		December 31, 2018
Collaboration revenue	$	3,064	$	4,191	$	2,151	$	1,188
Operating expenses:
Research and development		12,256		12,663		15,180		19,816
General and administrative		3,571		4,231		4,380		4,152
Total operating expenses		15,827		16,894		19,560		23,968
Other income:
Interest income		360		349		340		349
Total other income		360		349		340		349
Net loss	$	(12,403)	$	(12,354)	$	(17,069)	$	(22,431)
Net loss per share attributable to common stockholders — basic and diluted	$	(0.54)	$	(0.54)	$	(0.74)	$	(0.97)
Weighted-average number of common shares used in net loss per share attributable to common stockholders — basic and diluted		22,816,521		22,966,314		23,152,019		23,184,459


	Three months ended

	March 31, 2017		June 30, 2017		September 30, 2017		December 31, 2017
Collaboration revenue	$	4,290	$	3,727	$	6,267	$	3,261
Operating expenses:
Research and development		10,106		10,627		11,412		14,555
General and administrative		2,296		2,204		2,905		3,057
Total operating expenses		12,402		12,831		14,317		17,612
Other income:
Interest income		51		158		318		383
Total other income		51		158		318		383
Net loss	$	(8,061)	$	(8,946)	$	(7,732)	$	(13,968)
Net loss per share attributable to common stockholders — basic and diluted	$	(6.02)	$	(6.33)	$	(0.35)	$	(0.61)
Weighted-average number of common shares used in net loss per share attributable to common stockholders — basic and diluted		1,338,475		1,412,308		22,242,129		22,750,425

Nature of business and basis of presentation - Liquidity (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Mar. 05, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net loss		$ (22,431)	$ (17,069)	$ (12,354)	$ (12,403)	$ (13,968)	$ (7,732)	$ (8,946)	$ (8,061)	$ (64,257)	$ (38,707)	$ (13,700)
Accumulated deficit		$ (164,166)				$ (97,878)				$ (164,166)	$ (97,878)
Subsequent event \| Secondary public offering
Gross proceeds from the public offering	$ 97,800

- Definition

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

+ References + Details

Summary of significant accounting policies - Revenue Recognition (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Collaboration agreements
Deferred revenue	$ 46,196				$ 52,439				$ 46,196	$ 52,439
Accumulated deficit	(164,166)				(97,878)				(164,166)	(97,878)
Collaboration revenue	$ 1,188	$ 2,151	$ 4,191	$ 3,064	$ 3,261	$ 6,267	$ 3,727	$ 4,290	10,594	17,545	$ 25,171
Net income (loss)									$ (64,257)	$ (38,707)	$ (13,700)
Basic and diluted net income (loss) per share (in dollars per share)	$ (0.97)	$ (0.74)	$ (0.54)	$ (0.54)	$ 0.61	$ 0.35	$ 6.33	$ 6.02	$ (2.79)	$ (3.22)	$ (10.82)
Billing term									30 days
Difference between Revenue Guidance in Effect before and after Topic 606 \| ASU 2014-09
Collaboration agreements
Deferred revenue												$ 2,031
Accumulated deficit												$ (2,031)
Collaboration revenue									$ (378)
Net income (loss)									$ (378)
Basic and diluted net income (loss) per share (in dollars per share)									$ (0.02)

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

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-Name Accounting Standards Codification
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+ Details

Summary of significant accounting policies - Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash, cash equivalents and restricted cash
Cash and cash equivalents	$ 59,634	$ 26,591	$ 100,297
Restricted cash included in other assets	371	371	371
Total cash, cash equivalents and restricted cash per statement of cash flows	$ 60,005	$ 26,962	$ 100,668	$ 11,905
Restricted cash, Balance Sheet Location	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent

Summary of significant accounting policies - Other Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Assets
Other assets	$ 1,503	$ 371
Restricted cash included in other assets	$ 371	$ 371	$ 371
Restricted cash, Balance Sheet Location	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent
Held by service provider	$ 1,132

Summary of significant accounting policies - Antidilutive securities (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Anti-dilutive securities
Anti-dilutive securities	3,856,932	3,315,850	19,186,147
Series A-1 convertible preferred stock
Anti-dilutive securities
Anti-dilutive securities			5,574,467
Series B-1 convertible preferred stock
Anti-dilutive securities
Anti-dilutive securities			7,319,307
Series C-1 convertible preferred stock
Anti-dilutive securities
Anti-dilutive securities			3,260,897
Warrants
Anti-dilutive securities
Anti-dilutive securities	110,365	110,365	129,491
Stock options.
Anti-dilutive securities
Anti-dilutive securities	3,746,567	3,205,485	2,901,985

Summary of Significant Accounting Policies - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2018
Computer equipment, office equipment and software
Property, equipment and software, net
Estimated useful life	3 years
Laboratory equipment
Property, equipment and software, net
Estimated useful life	5 years

Summary of significant accounting policies - Risks (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Concentration of Credit Risk and Off-balance Sheet Risk
Financial instruments with off-balance sheet risk	$ 0

Collaboration agreements - Merck KGaA Agreement (Details) - Merck KGaA $ in Thousands	1 Months Ended	12 Months Ended	36 Months Ended
	Jun. 30, 2014 USD ($) item	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($) item
Collaboration agreements
Upfront payment received	$ 12,000
Number of targets \| item	6
Amount of additional fees receivable when target is designated and commercial license to target is granted	$ 0
Number of targets designated \| item		6	6
Aggregate milestones	780,000
Development milestones	84,000
Regulatory milestones	264,000
Sales milestones	$ 432,000
Number of individual development milestones per target \| item	6
Number of individual regulatory milestones per target \| item	5
Number of individual commercial milestones \| item	3
Amount of development milestone payment received		$ 0	$ 3,000
Next potential development milestone payment eligible to receive		$ 500
Prior written notice for termination period		60 days

Collaboration agreements - Merck KGaA Agreement - Accounting (Details) $ in Thousands	3 Months Ended								12 Months Ended			36 Months Ended
	Dec. 31, 2018 USD ($) item	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) item	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) item	Jan. 01, 2018 USD ($)
Collaboration agreements
Revenue, Practical Expedient, Initial Application and Transition, Nonrestatement of Modified Contract									true
Collaboration revenue	$ 1,188	$ 2,151	$ 4,191	$ 3,064	$ 3,261	$ 6,267	$ 3,727	$ 4,290	$ 10,594	$ 17,545	$ 25,171
Accounts receivable	459				784				459	784		$ 784
Deferred revenue	46,196				52,439				46,196	52,439		52,439
Deferred revenue, current	$ 46,196				$ 21,635				$ 46,196	$ 21,635		$ 21,635
Merck KGaA
Collaboration agreements
Number of targets designated \| item	6				6				6	6		6
Allocated Transaction Price													$ 22,875
Estimated amount of research and development fees received or expected to be received included in transaction price		7,070											7,875
Amount of milestone payment earned												$ 3,000
Amount of milestone payments, which had not been received, included in the estimated transaction price													0
Adjusted transaction price		22,070
Collaboration revenue									$ 2,444	$ 3,636	$ 3,644
Accounts receivable	$ 450				$ 330				450	330		330
Deferred revenue	$ 5,462				$ 6,634				$ 5,462	$ 6,634		$ 6,634
Merck KGaA \| Revised estimate of costs to complete research services
Collaboration agreements
Increase (decrease) in transaction price		$ (805)
Merck KGaA \| License and corresponding research and development services, First and second targets
Collaboration agreements
Allocated Transaction Price													4,226
Merck KGaA \| License and corresponding research and development services, Third target
Collaboration agreements
Allocated Transaction Price													3,439
Merck KGaA \| License and corresponding research and development services, Fourth target
Collaboration agreements
Allocated Transaction Price													3,439
Merck KGaA \| License and corresponding research and development services, Fifth target
Collaboration agreements
Allocated Transaction Price													3,439
Merck KGaA \| License and corresponding research and development services, Sixth target
Collaboration agreements
Allocated Transaction Price													3,439
Merck KGaA \| Rights to future technological improvements
Collaboration agreements
Allocated Transaction Price													425
Merck KGaA \| Joint research committee services
Collaboration agreements
Allocated Transaction Price													$ 242
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-01-01 \| Merck KGaA \| Rights to future technological improvements
Collaboration agreements
Expected recognition period	10 years								10 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-01-01 \| Merck KGaA \| Joint research committee services
Collaboration agreements
Expected recognition period	5 years								5 years

Collaboration agreements - XMT-1522 Agreement (Details) - XMT-1522 Agreement $ in Thousands	1 Months Ended
	Jan. 31, 2016 USD ($) item	Dec. 31, 2018 USD ($)
Collaboration agreements
Upfront payment received \| $	$ 26,500
Milestone payment upon achievement of IND Clearance Date \| $	20,000
Aggregate milestones, excluding milestone payment upon achievement of IND Clearance Date \| $	288,000
Development milestones \| $	87,000
Regulatory milestones \| $	128,000
Commercial milestones \| $	$ 73,000
Number of separate events to be achieved \| item	9
Number of separate specified patient populations \| item	4
Number of additional unspecified patient populations \| item	1
Number of individual commercial milestones \| item	3
Next potential development milestone payment eligible to receive \| $		$ 12,000
Prior written notice for termination, before initiation of clinical study	30 days
Prior written notice for termination, after initiation of clinical study	90 days
United States, European Union and Japan
Collaboration agreements
Number of regulatory milestones \| item	14
Locations other than United States, European Union and Japan
Collaboration agreements
Number of specified markets in which regulatory milestone is payable \| item	2
Maximum \| United States, European Union and Japan
Collaboration agreements
Number of separate patient populations, regulatory milestone \| item	4

Collaboration agreements - Takeda partnership (Details) $ in Thousands	1 Months Ended	58 Months Ended
	Jan. 31, 2016 USD ($) item	Dec. 31, 2018 USD ($) item	Mar. 31, 2014 USD ($) item
2014, 2015 and 2016 Takeda Agreements
Collaboration agreements
Number of targets	7
Fee payments received \| $		$ 24,800
Number of targets designated		4
Number of targets for which Tokeda has limited replacement rights		2
Amount to be received for second limited replacement right \| $		$ 500
Aggregate milestones \| $		474,750
Development milestones \| $		50,250
Regulatory milestones \| $		153,000
Commercial milestones \| $		$ 271,500
Number of remaining eligible targets		3
Milestone payable on each of the remaining eligible targets \| $		$ 158,250
Number of individual development milestones per target		4
Number of individual regulatory milestones per target		8
Number of individual commercial milestones		6
Number of thresholds for annual net sales		3
Next potential development milestone payment eligible to receive \| $		$ 750
2015 and 2016 Takeda Agreements
Collaboration agreements
Number of targets	5
2014 Tokeda Agreement
Collaboration agreements
Number of targets			2
Amount of option exercise fee for designated targets \| $			$ 1,300
2016 Restated Tokeda Agreement
Collaboration agreements
Number of products entity may elect to co-develop and co-commercialize	1
Payment for option to co-develop and co-commercialize product \| $	$ 15,000
Share of profit related to United States (as a percent)	50.00%
Costs incurred in United States (as a percent)	50.00%
Global development costs (as a percent)	30.00%
Costs incurred specifically for a foreign country borne by Takeda (as a percent)	100.00%
Prior written notice for termination period	45 days
Number of targets with respect to breach	1

Collaboration agreements - 2016 Takeda and XMT-1522 Agreements - Accounting (Details)	12 Months Ended
	Dec. 31, 2018 item
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Revenue, Practical Expedient, Initial Application and Transition, Nonrestatement of Modified Contract	true
2016 Restated Tokeda Agreement and XMT-1522 Agreement
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Number of arrangements	1

Collaboration agreements - 2016 Takeda and XMT-1522 Agreements - Performance obligations (Details) - 2016 Restated Tokeda Agreement and XMT-1522 Agreement $ in Thousands	Jan. 01, 2018 USD ($)
Performance obligations
Allocated Transaction Price	$ 86,123
XMT-1522 license and research services
Performance obligations
Allocated Transaction Price	49,828
Joint research committee services for XMT-1522
Performance obligations
Allocated Transaction Price	449
Exclusive license to the first designated target and research services
Performance obligations
Allocated Transaction Price	6,611
Research license to the second designated target and research services
Performance obligations
Allocated Transaction Price	1,017
Material right related to the exclusive license to the second designated target
Performance obligations
Allocated Transaction Price	526
Exclusive license to the third designated target and research services
Performance obligations
Allocated Transaction Price	4,974
Exclusive license to the fourth designated target and research services
Performance obligations
Allocated Transaction Price	3,678
Material right related to the license to the fifth designated target and research services
Performance obligations
Allocated Transaction Price	3,506
Material right related to the license to the sixth designated target and research services
Performance obligations
Allocated Transaction Price	3,506
Material right to license to the seventh designated target and research services
Performance obligations
Allocated Transaction Price	3,506
Material right related to first replacement right for a designated target
Performance obligations
Allocated Transaction Price	3,506
Material right related to the second replacement right to a designated target
Performance obligations
Allocated Transaction Price	3,116
Rights to future technological improvements
Performance obligations
Allocated Transaction Price	1,750
Joint research committee services
Performance obligations
Allocated Transaction Price	$ 150

Collaboration agreements - 2016 Takeda and XMT-1522 Agreements - Additional Accounting (Details) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
	Jan. 02, 2019	Dec. 31, 2018 USD ($) item $ / shares	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares	Jun. 30, 2017 USD ($) $ / shares	Mar. 31, 2017 USD ($) $ / shares	Dec. 31, 2018 USD ($) item $ / shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	Jan. 01, 2018 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Research and development		$ 19,816	$ 15,180	$ 12,663	$ 12,256	$ 14,555	$ 11,412	$ 10,627	$ 10,106	$ 59,915	$ 46,700	$ 32,008
Collaboration revenue		1,188	$ 2,151	$ 4,191	$ 3,064	3,261	$ 6,267	$ 3,727	$ 4,290	10,594	17,545	25,171
Accounts receivable		459				784				459	784
Accounts payable		$ 10,727				$ 3,070				10,727	3,070
Net loss										$ (64,257)	$ (38,707)	$ (13,700)
Basic and diluted net income (loss) per share (in dollars per share) \| $ / shares		$ (0.97)	$ (0.74)	$ (0.54)	$ (0.54)	$ 0.61	$ 0.35	$ 6.33	$ 6.02	$ (2.79)	$ (3.22)	$ (10.82)
Deferred revenue		$ 46,196				$ 52,439				$ 46,196	$ 52,439
License and related research and development services, including Phase 1 development and transfer certain materials and know how
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Number of performance obligations \| item		1								1
2016 Restated Tokeda Agreement and XMT-1522 Agreement
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Allocated Transaction Price													$ 86,123
Estimated amount of research and development fees received or expected to be received included in transaction price													14,023
Amount of milestone payments, which had not been received, included in the estimated transaction price													0
Adjusted transaction price		$ 86,289								$ 86,289
Collaboration revenue										5,868	13,784	$ 21,401
2016 Restated Tokeda Agreement and XMT-1522 Agreement \| Revised estimate of costs to complete research services
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Collaboration revenue										(1,535)
Net loss										$ (1,535)
Basic and diluted net income (loss) per share (in dollars per share) \| $ / shares										$ (0.07)
2016 Restated Tokeda Agreement and XMT-1522 Agreement \| XMT-1522 license and research services
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Allocated Transaction Price													49,828
2016 Restated Tokeda Agreement and XMT-1522 Agreement \| Joint research committee services for XMT-1522
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Allocated Transaction Price													449
2016 Restated Tokeda Agreement and XMT-1522 Agreement \| Joint research committee services for XMT-1522 \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Expected recognition period		6 years								6 years
2016 Restated Tokeda Agreement and XMT-1522 Agreement \| Rights to future technological improvements
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Allocated Transaction Price													$ 1,750
2016 Restated Tokeda Agreement and XMT-1522 Agreement \| Rights to future technological improvements \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Expected recognition period		10 years								10 years
2014, 2015 and 2016 Takeda Agreements
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Accounts receivable		$ 9				454				$ 9	454
Accounts payable		2,749								2,749
Deferred revenue		$ 39,960				$ 43,579				39,960	43,579
2016 Restated Tokeda Agreement \| Subsequent event
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Period of time until agreement will be terminated	45 days
XMT-1522 Agreement
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Research and development										8,046	3,408	340
ASC 808 costs billed to Takeda										$ 3,746	$ 0	$ 0
Estimated amount of research and development fees received or expected to be received included in transaction price			$ 9,615
XMT-1522 Agreement \| Subsequent event
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Period of time until agreement will be terminated	30 days

Collaboration Agreements - Contract Assets and Liabilities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Contract liabilities
Deferred revenue, Balance at Beginning of Period	$ 52,439
Deferred revenue, Additions	2,851
Deferred revenue, Deductions	9,094
Deferred revenue, Balance at End of Period	46,196
Revenue recognized in the period from:
Amounts included in the contract liability at the beginning of the period	$ 8,704

Collaboration Agreements - Other Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaboration agreements
Collaboration revenue	$ 1,188	$ 2,151	$ 4,191	$ 3,064	$ 3,261	$ 6,267	$ 3,727	$ 4,290	$ 10,594	$ 17,545	$ 25,171
Asana BioSciences
Collaboration agreements
Next potential milestone payment eligible to receive	$ 2,500								2,500
Asana BioSciences \| Limited services
Collaboration agreements
Collaboration revenue									782	$ 125	$ 125
Asana BioSciences \| Milestone
Collaboration agreements
Collaboration revenue									$ 1,500

Fair value measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair value measurements
Marketable securities	$ 10,497	$ 98,625
Transfers between fair value measurement level 1 to level 2	0	0	$ 0
Transfers between fair value measurement level 2 to level 1	0	0	$ 0
Recurring basis
Fair value measurements
Total assets	10,497	98,625
U.S. Treasuries \| Recurring basis
Fair value measurements
Marketable securities	10,497	62,640
Commercial paper \| Recurring basis
Fair value measurements
Marketable securities		24,931
Corporate bonds \| Recurring basis
Fair value measurements
Marketable securities		11,054
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Recurring basis
Fair value measurements
Total assets	10,497	62,640
Quoted Prices in Active Markets for Identical Assets (Level 1) \| U.S. Treasuries \| Recurring basis
Fair value measurements
Marketable securities	$ 10,497	62,640
Significant Other Observable Inputs (Level 2) \| Recurring basis
Fair value measurements
Total assets		35,985
Significant Other Observable Inputs (Level 2) \| Commercial paper \| Recurring basis
Fair value measurements
Marketable securities		24,931
Significant Other Observable Inputs (Level 2) \| Corporate bonds \| Recurring basis
Fair value measurements
Marketable securities		$ 11,054

Marketable securities (Details) $ in Thousands	12 Months Ended
Marketable securities (Details) $ in Thousands	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Marketable securities
Amortized Cost	$ 10,505	$ 98,774
Gross Unrealized Losses	(8)	(149)
Marketable securities	$ 10,497	98,625
Number of securities in an unrealized loss position \| security	3
Aggregate fair value of securities in an unrealized loss position for more than 12 months	$ 10,497
Realized gains on available-for-sale securities	0	0	$ 0
Realized losses on available-for-sale securities	0	0	$ 0
U.S. Treasuries
Marketable securities
Amortized Cost	10,505	62,777
Gross Unrealized Losses	(8)	(137)
Marketable securities	$ 10,497	62,640
Commercial paper
Marketable securities
Amortized Cost		24,931
Marketable securities		24,931
Corporate bonds
Marketable securities
Amortized Cost		11,066
Gross Unrealized Losses		(12)
Marketable securities		$ 11,054

Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, equipment and software, net
Total property and equipment at cost	$ 9,055	$ 7,459
Less: Accumulated depreciation	(6,361)	(5,140)
Property and equipment	2,694	2,319
Depreciation expense	1,257	928	$ 655
Laboratory equipment
Property, equipment and software, net
Total property and equipment at cost	6,134	5,237
Computer equipment, office equipment and software
Property, equipment and software, net
Total property and equipment at cost	1,035	718
Leasehold improvement
Property, equipment and software, net
Total property and equipment at cost	$ 1,886	$ 1,504

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued expenses
Accrued payroll and related expenses	$ 3,042	$ 3,041
Accrued preclinical, manufacturing and clinical expenses	8,314	3,183
Accrued professional fees	567	492
Accrued other	452	228
Total	$ 12,375	$ 6,944

Preferred Stock (Details) - shares	Dec. 31, 2018	Dec. 31, 2017
Preferred Stock
Preferred stock, shares authorized	25,000,000	25,000,000
Preferred stock, shares issued	0	0

Stockholders' Equity - Stock and warrants (Details)	12 Months Ended
Stockholders' Equity - Stock and warrants (Details)	Dec. 31, 2018 Vote shares	Dec. 31, 2017 shares	Dec. 31, 2013 $ / shares shares
Common stock
Number of votes for each shares held \| Vote	1
Warrants
Number of shares of common stock into which warrants may be converted			129,491
Warrant exercise price per share (in dollars per share) \| $ / shares			$ 0.05
Contractual life of warrants			10 years
Common stock issued upon exercise of warrant (in shares)		19,071
Stock options.
Common stock
Number of shares reserved for future issuance	3,746,567	3,205,485
Warrants
Common stock
Number of shares reserved for future issuance	110,365	110,365
Warrants and options
Common stock
Number of shares reserved for future issuance	3,856,932	3,315,850

Stock options - Plans (Details) - shares	1 Months Ended		12 Months Ended
Stock options - Plans (Details) - shares	Jan. 31, 2019	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017
Stock options
Number of shares outstanding			3,746,567	3,205,485
2007 Plan
Stock options
Number of shares outstanding		3,141,625
2017 Plan
Stock options
Number of shares authorized		2,255,000
Cumulative increase in number of shares issuable (as a percent)		4.00%
Number of shares available for future issuance			2,085,048
Vesting period (in years)			4 years
2017 Plan \| Subsequent event
Stock options
Additional shares authorized	929,378
2017 Plan \| Maximum
Stock options
Expiration period			10 years

Stock options - Activity under stock option plan (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Shares
Outstanding at beginning of period (in shares)	3,205,485
Granted (in shares)	1,273,115
Exercised (in shares)	(427,269)
Cancelled (in shares)	(304,764)
Outstanding at end of period (in shares)	3,746,567	3,205,485
Options exercisable, at end of period (in shares)	1,998,527
Weighted- Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 3.44
Granted (in dollars per share)	13.33
Exercised (in dollars per share)	2.15
Cancelled (in dollars per share)	7.99
Outstanding at end of period (in dollars per share)	6.58	$ 3.44
Options exercisable, at end of period (in dollars per share)	$ 3.57
Additional Disclosures
Remaining Contractual Life, Options outstanding	7 years 7 months 6 days	7 years 9 months 18 days
Remaining Contractual Life, Options exercisable	6 years 7 months 6 days
Aggregate Intrinsic Value, Options outstanding (in dollars)	$ 3,897	$ 41,709
Aggregate Intrinsic Value, Options exercisable (in dollars)	$ 3,460
Weighted-average grant date fair value of options granted (in dollars per share)	$ 8.78	$ 5.53	$ 2.34
Cash received from the exercise of stock options (in dollars)	$ 918	$ 452	$ 105

Stock options - Stock based compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock options
Share-based compensation charge	$ 3,828	$ 1,366	$ 664
Unrecognized stock compensation cost	$ 10,289
Weighted-average amortization period of unrecognized stock compensation cost	2 years 7 months 6 days

Stock options - Fair value of each option award (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Weighted average fair value assumptions
Risk-free interest rate (as a percent)	2.70%	2.20%	1.50%
Expected term	6 years 26 days	6 years 2 months 16 days	6 years 3 months
Expected stock price volatility (as a percent)	73.00%	67.00%	69.00%

Stock options - Non-employees (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock options
Total expense for stock options granted to non-employees	$ 56	$ 56	$ 4

Stock options - Employee Stock Purchase Plan (Details) - shares	1 Months Ended
	Jan. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee Stock Purchase Plan
Number of shares issued		23,234,472	22,765,017
Employee Stock Purchase Plan
Employee Stock Purchase Plan
Number of shares reserved for future issuance		225,000
Number of shares issued		42,186
Number of shares available for future issuance		182,814
Employee Stock Purchase Plan \| Subsequent event
Employee Stock Purchase Plan
Additional shares authorized	232,344

Income Taxes - Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of income taxes
Income tax benefit	$ 0	$ 0	$ 0
Income tax computed at federal statutory tax rate	21.00%	34.00%	34.00%
State taxes, net of federal benefit	6.50%	5.10%	5.10%
Permanent differences	0.60%	(0.80%)	(1.30%)
Research and development expenditures		(2.30%)
General business credits	4.20%	8.20%	12.90%
Impact of tax reform		(27.30%)
Other			(0.10%)
Change in valuation allowance	(32.30%)	(16.90%)	(50.60%)
Maximum
Reconciliation of income taxes
Income tax computed at federal statutory tax rate		35.00%

Income Taxes - Components of deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating losses	$ 27,916	$ 9,324
Tax credit carryforwards	10,178	7,516
Deferred revenue	12,410	13,718
Licensed technology	1,160	1,266
Depreciation	405	268
Accrued expenses	1,585	53
Deferred expenses	112	313
Unrealized loss	43	41
Other state credits	131	103
Total deferred tax assets	53,940	32,602
Valuation allowance	$ (53,940)	$ (32,602)

Income Taxes - Net operating loss carryforwards (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Federal
Operating loss carry-forwards
Net operating loss carry forwards	$ 102,112
Net operating loss carryforwards subject to expiration	34,115
Net operating loss carryforwards not subject to expiration	67,997
State
Operating loss carry-forwards
Net operating loss carry forwards	$ 102,405

Income Taxes - Tax credit carryforward (Details) - Research and Development $ in Thousands	Dec. 31, 2018 USD ($)
Federal
Tax credit carryforwards
Tax credit carryforwards	$ 7,708
State
Tax credit carryforwards
Tax credit carryforwards	$ 3,294

Income Taxes - Valuation allowance (Details) $ in Thousands	12 Months Ended
Income Taxes - Valuation allowance (Details) $ in Thousands	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)
Valuation allowance
Valuation allowance	$ 53,940	$ 32,602
Increase (decrease) in valuation allowance	21,338	6,611
Unrecognized tax benefits	0	0
Accrued interest or penalties related to uncertain tax position	$ 0	$ 0
Number of state jurisdictions in which entity files income tax returns \| item	1
Number of federal or state audits in progress \| item	0

Employee Benefit Plan (Details) - USD ($) $ in Thousands	12 Months Ended
Employee Benefit Plan (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Benefit Plan
Eligible age to participate	21 years
Maximum contribution by employee (as a percent)	75.00%
Percentage employer matches of the employee's percentage contribution matched	50.00%
Employer matching contribution (as a percent)	6.00%
Maximum annual amount employer contributes per employee	$ 6
Employee benefit expenses	$ 332	$ 273	$ 136

Commitments - Operating leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating leases
Option to extend lease term (years)	5 years
Letter of credit for security deposit	$ 321	$ 321
Future minimum lease payments under operating leases
2019	2,333
2020	2,468
2021	760
2022	83
2023 and thereafter	86
Total	5,730
Rent expense	1,994	$ 1,834	$ 1,572
Maximum
Operating leases
Tenant improvement allowance	$ 356

Commitments - License agreements (Details) - License agreements $ in Thousands	12 Months Ended
	Dec. 31, 2018 item	Dec. 31, 2017 USD ($)
Commitments
Number of biotechnology companies where Company has licensed intellectual property \| item	2
Milestone payments \| $		$ 2,750

Related party transactions (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2016
Takeda
Related party transactions
Proceeds from IPO		$ 10,000
Series C-1 convertible preferred stock
Related party transactions
Amount invested in Company			$ 32,882
Series C-1 convertible preferred stock \| Takeda
Related party transactions
Amount invested in Company	$ 10,000

Subsequent events (Details) - Subsequent event - USD ($) $ / shares in Units, $ in Thousands	Mar. 05, 2019	Jan. 02, 2019
Secondary public offering
Subsequent events
Issuance of stock (in shares)	24,437,500
Share price (in dollars per share)	$ 4.00
Gross proceeds from the public offering	$ 97,800
2016 Restated Tokeda Agreement
Subsequent events
Period of time until agreement will be terminated		45 days
XMT-1522 Agreement
Subsequent events
Period of time until agreement will be terminated		30 days

Selected quarterly financial data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Data
Collaboration revenue	$ 1,188	$ 2,151	$ 4,191	$ 3,064	$ 3,261	$ 6,267	$ 3,727	$ 4,290	$ 10,594	$ 17,545	$ 25,171
Operating expenses:
Research and development	19,816	15,180	12,663	12,256	14,555	11,412	10,627	10,106	59,915	46,700	32,008
General and administrative	4,152	4,380	4,231	3,571	3,057	2,905	2,204	2,296	16,334	10,462	6,984
Total operating expenses	23,968	19,560	16,894	15,827	17,612	14,317	12,831	12,402	76,249	57,162	38,992
Other income:
Interest income	349	340	349	360	383	318	158	51	1,398	910	121
Total other income	349	340	349	360	383	318	158	51	1,398	910	121
Net loss	$ (22,431)	$ (17,069)	$ (12,354)	$ (12,403)	$ (13,968)	$ (7,732)	$ (8,946)	$ (8,061)	$ (64,257)	$ (38,707)	$ (13,700)
Net loss per share attributable to common stockholders — basic and diluted	$ (0.97)	$ (0.74)	$ (0.54)	$ (0.54)	$ 0.61	$ 0.35	$ 6.33	$ 6.02	$ (2.79)	$ (3.22)	$ (10.82)
Weighted-average number of common shares used in net loss per share attributable to common stockholders — basic and diluted	23,184,459	23,152,019	22,966,314	22,816,521	22,750,425	22,242,129	1,412,308	1,338,475	23,032,250	12,022,733	1,266,758