Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2017	Mar. 26, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Trading Symbol	ALRN
Entity Registrant Name	AILERON THERAPEUTICS INC
Entity Central Index Key	0001420565
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		14,734,383
Entity Public Float			$ 61,589,199

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 11,863	$ 20,715
Investments	38,889	0
Prepaid expenses and other current assets	1,000	333
Restricted cash	88	25
Total current assets	51,840	21,073
Property and equipment, net	154	107
Restricted cash		63
Other assets	694	778
Total assets	52,688	22,021
Current liabilities:
Accounts payable	1,600	1,971
Accrued expenses and other current liabilities	3,291	2,100
Total current liabilities	4,891	4,071
Deferred rent		11
Total liabilities	4,891	4,082
Commitments and contingencies (Note 12)
Redeemable convertible preferred stock (Series A, A-1, B, C-1, C-2, D, D-1, E, E-1, E-2, E-3 and F), $0.01 par value; no shares and 151,557,293 shares authorized at December 31, 2017 and 2016, respectively; no shares and 105,631,019 shares issued and outstanding at December 31, 2017 and 2016, respectively	0	129,745
Stockholders’ equity (deficit):
Preferred stock, $0.001 par value; 5,000,000 shares and no shares authorized at December 31, 2017 and 2016, respectively; no shares issued and outstanding at December 31, 2017 and 2016
Common stock, $0.001 par value; 150,000,000 shares and 143,500,000 shares authorized at December 31, 2017 and 2016, respectively; 14,723,818 and 432,413 shares issued and outstanding at December 31, 2017 and 2016, respectively	15
Additional paid-in capital	184,761	2,536
Accumulated other comprehensive loss	(33)
Accumulated deficit	(136,946)	(114,342)
Total stockholders’ equity (deficit)	47,797	(111,806)
Total liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)	$ 52,688	$ 22,021

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Redeemable convertible preferred stock, authorized (in shares)	0	151,557,293
Redeemable convertible preferred stock, issued (in shares)	0	105,631,019
Redeemable convertible preferred stock, outstanding (in shares)	0	105,631,019
Redeemable convertible preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	143,500,000
Common stock, shares issued	14,723,818	432,413
Common stock, shares outstanding	14,723,818	432,413

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenue									$ 0	$ 0	$ 0
Operating expenses:
Research and development	$ 4,311	$ 3,825	$ 3,161	$ 2,942	$ 3,092	$ 2,605	$ 2,086	$ 2,493	14,239	10,276	7,832
General and administrative	2,730	2,601	1,791	1,647	1,356	1,627	3,464	1,446	8,769	7,893	5,059
Total operating expenses	7,041	6,426	4,952	4,589	4,448	4,232	5,550	3,939	23,008	18,169	12,891
Loss from operations	(7,041)	(6,426)	(4,952)	(4,589)	(4,448)	(4,232)	(5,550)	(3,939)	(23,008)	(18,169)	(12,891)
Interest income	176	167	29	32	6	10	16	14	404	46	13
Net loss	(6,865)	(6,259)	(4,923)	(4,557)	(4,442)	(4,222)	(5,534)	(3,925)	(22,604)	(18,123)	(12,878)
Accretion of redeemable convertible preferred stock to redemption value			(21)	(20)	(19)	(19)	(19)	(18)	(41)	(75)	(71)
Net loss attributable to common stockholders	$ (6,865)	$ (6,259)	$ (4,944)	$ (4,577)	$ (4,461)	$ (4,241)	$ (5,553)	$ (3,943)	$ (22,645)	$ (18,198)	$ (12,949)
Net loss per share attributable to common stockholders—basic and diluted	$ (0.47)	$ (0.45)	$ (10.98)	$ (10.58)	$ (10.31)	$ (9.84)	$ (12.95)	$ (9.25)	$ (3.04)	$ (42.35)	$ (32.31)
Weighted average common shares outstanding—basic and diluted	14,720,734	13,939,950	450,495	432,728	432,413	430,929	428,870	426,487	7,443,078	429,686	400,713
Comprehensive loss:
Net loss	$ (6,865)	$ (6,259)	$ (4,923)	$ (4,557)	$ (4,442)	$ (4,222)	$ (5,534)	$ (3,925)	$ (22,604)	$ (18,123)	$ (12,878)
Other comprehensive loss:
Unrealized loss on investments, net of tax of $0	(27)	(6)			1	(3)	(5)	7	(33)		(4)
Total other comprehensive loss	(27)	(6)			1	(3)	(5)	7	(33)		(4)
Total comprehensive loss	$ (6,892)	$ (6,265)	$ (4,923)	$ (4,557)	$ (4,441)	$ (4,225)	$ (5,539)	$ (3,918)	$ (22,637)	$ (18,123)	$ (12,882)

Statement of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]	Series E-1 Redeemable Convertible Preferred Stock [Member]	Series F Redeemable Convertible Preferred Stock [Member]
Redeemable convertible preferred stock, beginning balance at Dec. 31, 2014	$ 97,610
Redeemable convertible preferred stock, beginning balance, shares at Dec. 31, 2014	81,975,780
Accretion of redeemable convertible preferred stock to redemption value	$ 71
Redeemable convertible preferred stock, ending balance at Dec. 31, 2015	$ 97,681
Redeemable convertible preferred stock, ending balance, shares at Dec. 31, 2015	81,975,780
Beginning balance at Dec. 31, 2014	$ (82,044)		$ 1,293	$ 4	$ (83,341)
Beginning balance, shares at Dec. 31, 2014		386,335
Exercise of stock options	52		52
Exercise of stock options, shares		39,865
Stock-based compensation expense	626		626
Accretion of redeemable convertible preferred stock to redemption value	(71)		(71)
Unrealized loss on investments, net of tax of $0	(4)			(4)
Net loss	(12,878)				(12,878)
Ending balance at Dec. 31, 2015	(94,319)		1,900		(96,219)
Ending balance, shares at Dec. 31, 2015		426,200
Issuance of redeemable convertible preferred stock, net of issuance costs						$ 13,183	$ 18,806
Issuance of redeemable convertible preferred stock, net of issuance costs, Shares						9,705,882	13,949,357
Accretion of redeemable convertible preferred stock to redemption value	75
Redeemable convertible preferred stock, ending balance at Dec. 31, 2016	$ 129,745
Redeemable convertible preferred stock, ending balance, shares at Dec. 31, 2016	105,631,019
Exercise of stock options	$ 23		23
Exercise of stock options, shares		6,213
Stock-based compensation expense	688		688
Accretion of redeemable convertible preferred stock to redemption value	(75)		(75)
Net loss	(18,123)				(18,123)
Ending balance at Dec. 31, 2016	$ (111,806)		2,536		(114,342)
Ending balance, shares at Dec. 31, 2016	432,413	432,413
Issuance of redeemable convertible preferred stock, net of issuance costs							$ 626
Issuance of redeemable convertible preferred stock, net of issuance costs, Shares							483,501
Accretion of redeemable convertible preferred stock to redemption value	$ 41
Conversion of redeemable convertible preferred stock to common stock	$ (130,412)
Conversion of redeemable convertible preferred stock to common stock, Shares	(106,114,520)
Redeemable convertible preferred stock, ending balance at Dec. 31, 2017	$ 0
Redeemable convertible preferred stock, ending balance, shares at Dec. 31, 2017	0
Exercise of stock options	$ 111		111
Exercise of stock options, shares	31,631	31,631
Stock-based compensation expense	$ 1,749		1,749
Accretion of redeemable convertible preferred stock to redemption value	(41)		(41)
Conversion of redeemable convertible preferred stock to common stock	130,412	$ 11	130,401
Conversion of redeemable convertible preferred stock to common stock, Shares		10,509,774
Issuance of common stock upon completion of initial public offering, net of commissions, underwriting discounts and offering costs	50,009	$ 4	50,005
Issuance of common stock upon completion of initial public offering, net of commissions, underwriting discounts and offering costs, shares		3,750,000
Unrealized loss on investments, net of tax of $0	(33)			(33)
Net loss	(22,604)				(22,604)
Ending balance at Dec. 31, 2017	$ 47,797	$ 15	$ 184,761	$ (33)	$ (136,946)
Ending balance, shares at Dec. 31, 2017	14,723,818	14,723,818

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (22,604)	$ (18,123)	$ (12,878)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	129	232	325
Net amortization of premiums and discounts on investments	(152)
Stock-based compensation expense	1,749	688	626
Change in deferred rent	(11)	(18)	(21)
Loss on disposal of property and equipment		12
Write-off of deferred offering costs		1,500
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(764)	(75)	(109)
Other assets	147	(778)
Accounts payable	(241)	1,215	(237)
Accrued expenses and other current liabilities	1,216	333	584
Net cash used in operating activities	(20,531)	(15,014)	(11,710)
Cash flows from investing activities:
Purchases of property and equipment	(142)		(94)
Purchases of investments	(71,056)	(12,634)	(2,368)
Proceeds from sales or maturities of investments	32,286	12,634	12,378
Changes in restricted cash		(25)	713
Net cash provided by (used in) investing activities	(38,912)	(25)	10,629
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	471	32,143	(91)
Proceeds from exercise of stock options	111	23	52
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts	52,313
Payments of initial public offering costs	(2,304)	(180)	(1,320)
Net cash provided by (used in) financing activities	50,591	31,986	(1,359)
Net increase (decrease) in cash and cash equivalents	(8,852)	16,947	(2,440)
Cash and cash equivalents at beginning of period	20,715	3,768	6,208
Cash and cash equivalents at end of period	11,863	20,715	3,768
Supplemental disclosure of non-cash financing activities:
Accretion of redeemable convertible preferred stock to redemption value	$ 41	75	71
Issuance costs for redeemable convertible preferred stock included in accounts payable and accrued expenses		$ 154
Deferred offering costs included in accounts payable and accrued expenses			$ 180

Nature of the Business and Basis of Presentation	12 Months Ended
Nature of the Business and Basis of Presentation	Dec. 31, 2017
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Aileron Therapeutics, Inc. (“Aileron” or the “Company”) is a clinical-stage biopharmaceutical company that is focused on developing and commercializing a novel class of therapeutics called stapled peptides. The Company’s lead product candidate, ALRN-6924, targets the tumor suppressor p53 for the treatment of a wide variety of cancers. ALRN-6924 reactivates p53-mediated tumor suppression by targeting the two primary p53 suppressor proteins, MDMX and MDM2. ALRN-6924 was in multiple clinical trials as of December 31, 2017. The Company is subject to risks common to companies in the biotechnology industry, including but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure, and extensive compliance-reporting capabilities. The Company’s product candidates are in development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary governmental regulatory approval or that any approved products will be commercially viable. Even if the Company’s drug development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its key employees and consultants. The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Reverse Stock Split On June 16, 2017, in connection with its initial public offering (“IPO”), the Company effected a one-for-9.937 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s redeemable convertible preferred stock (see Note 7). Accordingly, all common share and per share amounts for all periods presented in the accompanying financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and the associated adjustment of the preferred stock conversion ratios. Initial Public Offering On June 28, 2017, the Company’s registration statement on Form S-1 relating to its IPO was declared effective by the Securities and Exchange Commission (“SEC”). In the IPO, which closed on July 5, 2017, the Company issued and sold 3,750,000 shares of common stock at a public offering price of $15.00 per share for net proceeds of $50,009 after deducting underwriting discounts and commissions of $3,937 and offering expenses of $2,304. Upon the closing of the IPO, all 106,114,520 shares of redeemable convertible preferred stock then outstanding converted into an aggregate of 10,509,774 common shares. Liquidity In accordance with Accounting Standards Update (“ASU”) No. 2014-15, Disclosures of Uncertainties about an Entity’s Ability to Continue as a Going Concern (Subtopic 205-40), management must evaluate whether there are conditions or events, considered in the aggregate, that raise substantial doubt about the company’s ability to continue as a going concern within one year after the date that the financial statements are issued. This evaluation initially does not take into consideration the potential mitigating effect of management’s plans that have not been fully implemented as of the date the financial statements are issued. When substantial doubt exists under this methodology, management evaluates whether the mitigating effect of its plans sufficiently alleviates substantial doubt about the company’s ability to continue as a going concern. The mitigating effect of management’s plans, however, is only considered if both (1) it is probable that the plans will be effectively implemented within one year after the date that the financial statements are issued, and (2) it is probable that the plans, when implemented, will mitigate the relevant conditions or events that raise substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the financial statements are issued. Generally, to be considered probable of being effectively implemented, the plans must have been approved before the date that the financial statements are issued. The Company’s financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities in the ordinary course of business. Through December 31, 2017, the Company has funded its operations with net proceeds of $50,009 from its IPO, $131,211 from sales of preferred stock and $34,910 from a collaboration agreement. As of December 31, 2017, the Company had cash, cash equivalents and investments of $50,752. The Company has incurred losses and negative cash flows from operations and had an accumulated deficit of $136,946 as of December 31, 2017. The Company expects to continue to generate losses for the foreseeable future. As of April 2, 2018, the date of issuance of these financial statements, the Company expects that its cash, cash equivalents and investments of $50,752 as of December 31, 2017 will be sufficient to fund its operating expenses and capital expenditure requirements through at least the next twelve months. The future viability of the Company is dependent on its ability to raise additional capital to finance its operations. Although the Company has been successful in raising capital in the past, there is no assurance that it will be successful in obtaining such additional financing on terms acceptable to the Company, if at all. To execute its business plans, the Company will need substantial funding to support its continuing operations and pursue its growth strategy. Until such time as the Company can generate significant revenue from product sales, if ever, it expects to finance its operations through the sale of common stock in public offering and/or private placements, debt financings or other capital sources, including collaborations with other companies or other strategic transactions. The Company may not be able to obtain financing on acceptable terms or at all. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion plans or commercialization efforts, which could adversely affect its business prospects.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Use of Estimates

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

Cash Equivalents

The Company considers all short-term, highly liquid investments with original maturities of 90 days or less at acquisition date to be cash equivalents. Cash equivalents, which consist of money market accounts, are stated at fair value.

Restricted Cash

As of December 31, 2017, current restricted cash of $88 consisted of $25 of cash deposited in a separate restricted bank account as a security deposit for the Company’s corporate credit cards and $63 of cash deposited in a separate restricted bank account as a security deposit for the lease of the Company’s facilities. As of December 31, 2016, current restricted cash consisted of $25 of cash deposited in a separate restricted bank account as a security deposit for the Company’s corporate credit cards and non-current restricted cash consisted of $63 of cash deposited in a separate restricted bank account as a security deposit for the lease of the Company’s facilities.

Investments

The Company classifies its available-for-sale investments as current assets on the balance sheet if they mature within one year from the balance sheet date.

The Company classifies all of its investments as available-for-sale securities. The Company’s investments are measured and reported at fair value using quoted prices in active markets for similar securities or using other inputs that are observable or can be corroborated by observable market data. Unrealized gains and losses on available-for-sale securities are reported as accumulated other comprehensive income (loss), which is a separate component of stockholders’ equity (deficit). The cost of securities sold is determined on a specific identification basis, and realized gains and losses are included in other income (expense) within the statements of operations and comprehensive loss.

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

Concentration of Credit Risk and of Significant Suppliers

Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and investments. From time to time, the Company has maintained all of its cash, cash equivalents and investment balances at three accredited financial institutions, in amounts that exceed federally insured limits. The Company generally invests its excess cash in money market funds, commercial paper and corporate notes that are subject to minimal credit and market risk. Management has established guidelines relative to credit ratings and maturities intended to safeguard principal balances and maintain liquidity. The investment portfolio is maintained in accordance with the Company’s investment policy, which defines allowable investments, specifies credit quality standards and limits the credit exposure of any single issuer.

The Company is dependent on third-party manufacturers to supply products for research and development activities of its programs, including preclinical and clinical testing. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs.

Fair Value Measurements

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

•

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

•

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

•

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

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

Deferred Offering Costs

The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the offering. Should the planned equity financing be abandoned, the deferred offering costs would be expensed immediately as a charge to operating expenses in the statement of operations and comprehensive loss.

The Company’s IPO was completed in July 2017 and previously deferred offering costs of $2,304 were recorded as a reduction to stockholder's equity. As of December 31, 2017 and 2016, the Company did not have any deferred offering costs recorded. As of December 31, 2015, the Company had recorded $1,500 of deferred offering costs in contemplation of a probable 2016 equity financing. The Company determined in June 2016 that the equity financing was no longer probable of being consummated and, at that time, recorded general and administrative expense of $1,500 in the statement of operations and comprehensive loss to write off the deferred offering costs that had been capitalized.

Property and Equipment

Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the following estimated useful lives:


Laboratory equipment	5 years
Computer equipment and software	3 to 5 years
Furniture and fixtures	7 years
Leasehold improvements	Shorter of 7 years or term of lease

Expenditures for repairs and maintenance of assets are charged to expense as incurred. Upon retirement or sale, the cost and related accumulated depreciation and amortization of assets disposed of are removed from the accounts and any resulting gain or loss is included in the statements of operations and comprehensive loss.

Impairment of Long-Lived Assets

Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any impairment losses on long-lived assets.

Research and Development Costs

Research and development expenditures are expensed as incurred. Research and development expenses are comprised of salaries, stock-based compensation and benefits of employees, third-party license fees and other operational costs related to the Company’s research and development activities, including allocated facility-related expenses and external costs of outside vendors engaged to conduct both preclinical studies and clinical trials.

Research Contract Costs and Accruals

The Company has entered into various research and development contracts with research institutions and other companies. These agreements are cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. This process involves reviewing open contracts and purchase orders, communicating with personnel to identify services that have been performed and estimating level of service performed and the associated costs incurred for the services for which the Company has not yet been invoiced. Significant judgment and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.

Patent Costs

All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.

Accounting for Stock-Based Compensation

The Company measures all stock options and other stock-based awards granted to employees and directors based on the fair value on the date of the grant and recognizes compensation expense of those awards, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award. The Company applies the straight-line method of expense recognition to all awards with only service-based vesting conditions and applies the graded vesting method to all awards with performance-based vesting conditions or both service-based and performance-based vesting conditions.

For stock-based awards granted to non-employees, compensation expense is recognized over the period during which services are rendered by such non-employees until completed. At the end of each financial reporting period prior to the completion of the service, the fair value of these awards is remeasured using the then-current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model. The Company classifies stock-based compensation expense in its statement of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified.

The Company recognizes compensation expense for only the portion of awards that are expected to vest. In developing a forfeiture rate estimate, the Company has considered its historical experience to estimate pre-vesting forfeitures for awards with service-based vesting conditions. The impact of a forfeiture rate adjustment will be recognized in full in the period of adjustment, and if the actual forfeiture rate is materially different from the Company’s estimate, the Company may be required to record adjustments to stock-based compensation expense in future periods.

The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. For options with service-based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

Income Taxes

The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies.

The Company accounts for uncertainty in income taxes recognized in the financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.

Segment Data

The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on developing a novel class of therapeutics for the treatment of cancer and other diseases. All of the Company’s tangible assets are held in the United States.

Comprehensive Loss

Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. The Company’s only element of other comprehensive loss in all periods presented was unrealized gains (losses) on available-for-sale investments.

Net Income (Loss) per Share

The Company follows the two-class method when computing net income (loss) per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed.

Basic net income (loss) per share attributable to common stockholders is computed by dividing the net income (loss) attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) attributable to common stockholders is computed by adjusting income (loss) per share attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period, including potential dilutive common shares. For purpose of this calculation, outstanding options to purchase common stock and shares of redeemable convertible preferred stock are considered potential dilutive common shares.

The Company’s redeemable convertible preferred stock contractually entitled the holders of such shares to participate in dividends but contractually did not require the holders of such stock to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive.

Recently Issued Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (the “FASB”) issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. The FASB has continued to issue accounting standards updates to clarify and provide implementation guidance related to Revenue from Contracts with Customers, including ASU 2016-08, Revenue from Contract with Customers: Principal versus Agent Considerations, ASU 2016-10, Revenue from Contracts with Customers: Identifying Performance Obligations and Licensing, and ASU 2016-12, Revenue from Contracts with Customers: Narrow-Scope Improvements and Practical Expedients. These amendments address a number of areas, including the entity’s identification of its performance obligations in a contract, collectability, non-cash consideration, presentation of sales tax and an entity’s evaluation of the nature of its promise to grant a license of intellectual property and whether or not that revenue is recognized over time or at a point in time. These new standards will be effective for the Company beginning January 1, 2018. The adoption of these standards is not expected to have an impact on the Company’s financial position, results of operations or cash flows as the Company does not currently have any revenue-generating arrangements.

In February 2016, the FASB issued ASU No. 2016-02, Leases, (“ASU 2016-02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e. lessees or lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease, respectively. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months, regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. ASU 2016-02 (Accounting Standards Codification (“ASC”) Topic 842) supersedes the previous leases standard, ASC 840, Leases. The standard is effective for public entities for annual periods beginning after December 15, 2018 and for interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the potential impact that the adoption of ASU 2016-02 will have on its financial statements.

In March 2016, the FASB issued ASU No. 2016-09, Compensation—Stock Compensation (“ASU 2016-09”), which amends ASC Topic 718, Compensation—Stock Compensation (“ASC Topic 718”). The new standard identifies areas for simplification involving several aspects of accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, an option to recognize gross stock compensation expense with actual forfeitures recognized as they occur, as well as certain classifications on the statements of cash flows. The standard is effective for annual periods beginning after December 15, 2016 and for interim periods within those fiscal years. The Company adopted ASU 2016-09 on the required effective date of January 1, 2017. The Company elected to maintain its existing policy to estimate forfeitures when determining periodic stock-based compensation expense. The adoption of the other provisions of ASU 2016-09 had no impact on the Company’s financial position, results of operations or cash flows.

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“ASU 2016-15”). This guidance addresses the presentation and classification of certain cash receipts and cash payments in the statement of cash flows. The standard is effective for annual periods beginning after December 15, 2017 and for interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the potential impact that the adoption of ASU 2016-15 will have on its financial statements.

In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash (“ASU 2016-18”). The new standard requires restricted cash and restricted cash equivalents be included with cash and cash equivalents when reconciling the total beginning and ending amounts for the periods shown on the statement of cash flows. The new standard is effective for annual periods beginning after December 15, 2017 and for interim periods within those fiscal years. Early adoption is permitted. The adoption of this standard is not expected to have a material impact on the Company’s financial position, results of operations or cash flows.

In May 2017, the FASB issued ASU No. 2017-09, Compensation – Stock Compensation (“ASU 2017-09”), Scope of Modification Accounting which amends ASC Topic 718. This new standard clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. The standard is effective for all companies for annual periods beginning on or after December 15, 2017 and for interim periods within those fiscal years. The Company is currently evaluating the potential impact that the adoption of ASU 2017-09 will have on its financial statements.

Other accounting standards that have been issued or proposed by the FASB or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on the Company’s financial statements upon adoption.

Fair Value of Financial Assets

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets

3. Fair Value of Financial Assets

The following tables present information about the Company’s assets that are measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values:

	Fair Value Measurements as of December 31, 2017 using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	10,509	$	—	$	—	$	10,509
Investments:
Corporate notes		—		25,710		—		25,710
Commercial paper		—		13,179		—		13,179
	$	10,509	$	38,889	$	—	$	49,398

	Fair Value Measurements as of December 31, 2016 using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	20,016	$	—	$	—	$	20,016
	$	20,016	$	—	$	—	$	20,016

As of December 31, 2017, the Company’s cash equivalents and investments were invested in money market funds, corporate notes and commercial paper and were valued based on Level 1 and Level 2 inputs. As of December 31, 2016, the Company’s cash equivalents were invested in money market funds and were valued based on Level 1 inputs. In determining the fair value of its corporate notes and commercial paper at each date presented above, the Company relied on quoted prices for similar securities in active markets or using other inputs that are observable or can be corroborated by observable market data. The Company’s cash equivalents have original maturities of less than 90 days from the date of purchase. All available-for-sale investments have contractual maturities of less than one year. During the years ended December 31, 2017 and 2016, there were no transfers between Level 1, Level 2 and Level 3.

- Definition

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

+ References + Details

Investments

12 Months Ended

Dec. 31, 2017

Investments Debt And Equity Securities [Abstract]

Investments

4. Investments

As of December 31, 2016, the Company had no available-for-sale investments. As of December 31, 2017, the fair value of available-for-sale investments by type of security was as follows:

	December 31, 2017
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value
Investments:
Corporate notes	$	25,733	$	—	$	(23	)	$	25,710
Commercial paper		13,189		—		(10	)		13,179
	$	38,922	$	—	$	(33	)	$	38,889

Property and Equipment, Net

12 Months Ended

Dec. 31, 2017

Property Plant And Equipment [Abstract]

Property and Equipment, Net

5. Property and Equipment, Net

Property and equipment, net consisted of the following:

	December 31,
	2017			2016
Laboratory equipment	$	1,308		$	1,240
Leasehold improvements		559			559
Computer equipment and software		195			331
Furniture and fixtures		71			71
		2,133			2,201
Less: Accumulated depreciation and amortization		(1,979	)		(2,094	)
	$	154		$	107

Depreciation and amortization expense for the years ended December 31, 2017, 2016 and 2015 was $95, $232 and $325, respectively. During the year ended December 31, 2017, fully depreciated assets with a cost of $210 were disposed of for no proceeds, resulting in neither a gain nor a loss and during the year ended December 31, 2016, assets with a cost of $12 were disposed of for no proceeds, resulting in a loss of $12. No assets were disposed of during the year ended December 31, 2015.

Accrued Expenses and Other Current Liabilities

12 Months Ended

Dec. 31, 2017

Payables And Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

6. Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:

	December 31, 2017		December 31, 2016
Payroll and payroll-related costs	$	1,120	$	899
External research and development services		1,284		723
Professional fees		536		322
Other		351		156
	$	3,291	$	2,100

Redeemable Convertible Preferred Stock

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Redeemable Convertible Preferred Stock

7. Redeemable Convertible Preferred Stock

As of December 31, 2017 and 31, 2016, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue no shares and 151,557,293 shares of $0.01 par value redeemable convertible preferred stock, respectively.

The Company previously had issued Series A, Series A-1, Series B, Series C-1 and Series C-2 redeemable convertible preferred stock (collectively, the “Junior Preferred Stock”) and Series D, Series D-1, Series E, Series E-1, Series E-2, Series E-3 and Series F redeemable convertible preferred stock (collectively, the “Senior Preferred Stock”), together the “Redeemable Preferred Stock”. The Redeemable Preferred Stock is classified outside of stockholders’ equity (deficit) because the shares contain redemption features that are not solely within the control of the Company.

In January 2016, the Company issued 9,705,882 shares of Series E-1 preferred stock at a price of $1.36 per share, resulting in proceeds of $13,183, net of issuance costs of $17.

In December 2016, the Company issued 13,949,357 shares of Series F redeemable convertible preferred stock (the “Series F preferred stock”) at a price of $1.36 per share, resulting in proceeds of $18,806, net of issuance costs of $165. As part of the Series F preferred stock purchase agreement, the investors agreed to purchase an additional 8,192,477 shares of Series F preferred stock at a price of $1.36 per share upon the Company achieving specified clinical milestones (the “second tranche closing”) for an aggregate purchase price of $11,142. In February 2017, the Company amended the Series F preferred stock purchase agreement to permit the sale of up to 758,458 additional shares of Series F preferred stock. In February 2017, pursuant to the amended Series F preferred stock purchase agreement, the Company issued 483,501 shares of Series F preferred stock at a price of $1.36 per share, resulting in proceeds of $626, net of issuance costs of $32. The purchasers of Series F preferred stock in this February 2017 closing agreed to purchase an additional 274,957 shares of Series F preferred stock at a price of $1.36 per share in the second tranche closing, which increased the aggregate number of shares of Series F preferred stock to be purchased in the second tranche closing to 8,467,434 shares for an aggregate purchase price of $11,516. The Company determined that the future tranche obligations of the Series F preferred stock purchase agreement, as amended, did not meet the definition of a freestanding financial instrument because, while separately exercisable, they were not legally detachable. Further, the Company determined that the embedded future tranche obligations did not require bifurcation for accounting purposes as they are clearly and closely related to the economic characteristics and risks of the initial preferred shares and would not qualify as a derivative on a standalone basis.

In December 2016, pursuant to the Series F preferred stock purchase agreement, holders of 8,927,582 shares of Series E redeemable convertible preferred stock (the “Series E preferred stock”) that participated in the Series F preferred stock financing elected to convert their shares of Series E preferred stock into 8,927,582 shares of Series E-2 preferred stock, and holders of 16,567,108 shares of Series E-1 preferred stock that participated in the Series F preferred stock financing elected to convert their shares of Series E-1 preferred stock into 16,567,108 shares of Series E-3 redeemable convertible preferred stock (the “Series E-3 preferred stock”). Holders of Series E preferred stock and Series E-1 preferred stock that did not participate in the Series F preferred stock financing were not entitled to convert their shares into Series E-2 preferred stock and Series E-3 preferred stock, respectively. In February 2017, pursuant to the amended Series F preferred stock purchase agreement, holders of 4,411,765 shares of Series E-1 preferred stock that participated in the February 2017 closing elected to convert their shares of Series E-1 preferred stock into 4,411,765 shares of Series E-3 preferred stock.

In February 2017, the Company issued 483,501 shares of Series F preferred stock at a price of $1.36 per share, resulting in proceeds of $626, net of issuance costs of $32.

The Company determined that the conversion of shares of preferred stock that occurred in December 2016 and February 2017 represented modifications of these securities for accounting purposes; however, the modifications did not result in the recognition of a deemed dividend for accounting purposes because the modifications did not result in a transfer of value from common stockholders to preferred stockholders.

Pursuant to the terms of the amended Series F preferred stock purchase agreement, if the second tranche closing did not occur prior to the closing of the Company’s initial public offering of common stock, then, immediately prior to such closing, the purchasers of the Series F preferred stock would be required to purchase a number of shares of the Company’s common stock equal to $11,516 divided by the price per share paid by the public in the initial public offering in a concurrent private offering. This requirement to purchase shares immediately prior to the closing of the Company’s initial public offering could be waived in whole or in part by the Company’s board of directors. As of December 31, 2016, the specified clinical milestones had not been achieved and the second tranche closing had not occurred.

On June 15, 2017, the Company’s board of directors waived in whole, effective immediately prior to the closing of the Company’s IPO, the requirement of the purchasers of Series F preferred stock to purchase shares of the Company’s common stock in a concurrent private offering in connection with the Company’s initial public offering.

Upon the completion of the Company’s IPO on July 5, 2017, all shares of the Redeemable Preferred Stock converted into an aggregate of 10,509,774 shares of common stock. As of December 31, 2017, there were no shares of Redeemable Preferred Stock authorized, issued or outstanding.

Redeemable Preferred Stock consisted of the following as of:

	December 31, 2016
	Preferred Shares Authorized		Preferred Shares Issued and Outstanding		Carrying Value		Liquidation Preference		Common Stock Issuable Upon Conversion
Series A preferred stock		1,250,000		1,250,000	$	1,250	$	1,250		12,579
Series A-1 preferred stock		615,384		615,384		800		800		6,192
Series B preferred stock		3,706,056		3,706,056		1,506		1,506		372,955
Series C-1 preferred stock		5,934,050		5,934,050		6,997		7,000		597,167
Series C-2 preferred stock		8,689,144		8,689,144		10,248		10,250		874,423
Series D preferred stock		34,142,865		34,142,865		40,263		40,276		3,435,932
Series D-1 preferred stock		363,636		363,636		2,000		2,000		36,594
Series E preferred stock		12,715,822		3,788,240		4,453		4,469		381,225
Series E-1 preferred stock		24,264,705		7,697,597		10,446		10,469		774,639
Series E-2 preferred stock		9,226,082		8,927,582		10,493		10,531		898,418
Series E-3 preferred stock		21,237,785		16,567,108		22,483		22,531		1,667,214
Series F preferred stock		29,411,764		13,949,357		18,806		18,971		1,403,779
		151,557,293		105,631,019	$	129,745	$	130,053		10,461,117

The holders of the Redeemable Preferred Stock had the following rights and preferences prior to conversion:

Voting Rights

The holders of the Redeemable Preferred Stock were entitled to vote, together with the holders of common stock, on all matters submitted to stockholders for a vote and had the right to vote the number of shares equal to the number of whole shares of common stock into which such holders of Redeemable Preferred Stock could convert on the record date for determination of stockholders entitled to vote. In addition, holders of the Senior Preferred Stock, voting as a single class, were entitled to elect three directors of the Company. The holders of the Junior Preferred Stock, voting as a single class, were entitled to elect two directors of the Company.

Dividends

The holders of the Redeemable Preferred Stock, in order of preference, were entitled to receive noncumulative dividends when and if declared by the Company’s board of directors. The Company could not declare, pay or set aside any dividends on shares of any other class or series of capital stock of the Company unless the holders of the Redeemable Preferred Stock then outstanding first received, or simultaneously received, a dividend on each outstanding share of Redeemable Preferred Stock in an amount at least equal to the greater of (i) $0.08 per share for Series A redeemable convertible preferred stock (“Series A preferred stock”), $0.104 per share for Series A-1 redeemable convertible preferred stock (“Series A-1 preferred stock”), $0.03251 per share for Series B redeemable convertible preferred stock (“Series B preferred stock”), $0.09437 per share for Series C-1 redeemable convertible preferred stock (“Series C-1 preferred stock”), $0.09437 per share for Series C-2 redeemable convertible preferred stock (“Series C-2 preferred stock”), $0.09437 per share for Series D redeemable convertible preferred stock (“Series D preferred stock”), Series E preferred stock and Series E-2 preferred stock, $0.40 per share for Series D-1 redeemable convertible preferred stock (“Series D-1 preferred stock”), and $0.1088 per share for Series E-1 preferred stock, Series E-3 preferred stock and Series F preferred stock and (ii) (A) in the case of a dividend on common stock or any class or series of stock that is convertible into common stock, that dividend per share of Redeemable Preferred Stock as would equal the product of (1) the dividend payable on each share of such class or series determined, if applicable, as if all shares of such class or series had been converted into common stock and (2) the number of shares of common stock issuable upon conversion of each share of Redeemable Preferred Stock, or (B) in the case of a dividend on any class or series that is not convertible into common stock, at a rate per share of Redeemable Preferred Stock determined by (1) dividing the amount of the dividend payable on each share of such class or series of capital stock by the Original Issue Price (as defined below) of such class or series of capital stock (subject to appropriate adjustment in the event of any stock dividend, stock split, combination of or other similar recapitalization affecting such shares) and (2) multiplying such fraction by an amount equal to the Original Issue Price of each series of Redeemable Preferred Stock. If the Company declared, paid or set aside, on the same date, a dividend on shares of more than one class or series of capital stock of the Company, the dividend payable to the holders of the Redeemable Preferred Stock would be calculated based upon the dividend on the class or series of capital stock that would result in the highest Redeemable Preferred Stock dividend. Stockholders were not entitled to any accruing dividends. No dividends have been declared or paid during the year ended December 31, 2017 or 2016.

The Original Issue Price per share was $1.00 for Series A, $1.30 for Series A-1, $0.4064 for Series B, $1.179633 for Series C-1, $1.179633 for Series C-2, $1.179633 for Series D, $5.50 for Series D-1, $1.179633 for Series E, $1.36 for Series E-1, $1.179633 for Series E-2, $1.36 for Series E-3 and $1.36 for Series F preferred stock, subject to appropriate adjustment in the event of any stock dividend, stock split, combination or other similar recapitalization with respect to the Redeemable Preferred Stock.

Liquidation Preference

In the event of any liquidation event, voluntary or involuntary, dissolution or winding up of the Company or Deemed Liquidation Event (as defined below), the holders of the then outstanding Series F preferred stock were entitled to receive, prior and in preference to any distributions to the holders of the common stock and other preferred stock, $1.36 per share, plus any dividends declared but unpaid on the Series F preferred stock.

After the payment of all preferential amounts to the holders of Series F preferred stock, then, to the extent available, the holders of the Series E-2 and Series E-3 preferred stock would have been paid $1.179633 per share and $1.36 per share, respectively, plus any dividends declared but unpaid on the Series E-2 and Series E-3 preferred stock, prior and in preference to any distributions to the holders of common stock, Series E, Series E-1, Series D and Series D-1 preferred stock and Junior Preferred Stock.

After the payment of all preferential amounts to the holders of Series E-2 and Series E-3 preferred stock, then, to the extent available, the holders of the Series E preferred stock and the Series E-1 preferred stock would have been entitled to receive, prior and in preference to any distributions to the holders of the common stock and other preferred stock, $1.179633 per share and $1.36 per share, respectively, plus any dividends declared but unpaid on the Series E and Series E-1 preferred stock, prior and in preference to any distributions to the holders of common stock, Series D and Series D-1 preferred stock and Junior Preferred Stock.

After the payment of all preferential amounts to the holders of Series E and Series E-1 preferred stock, then, to the extent available, the holders of the Series D and Series D-1 preferred stock would have been paid $1.179633 per share and $5.50 per share, respectively, plus any dividends declared but unpaid on the Series D and Series D-1 preferred stock, prior and in preference to any distributions to the holders of common stock and Junior Preferred Stock.

After the payment of all preferential amounts to the holders of the Senior Preferred Stock, then, to the extent available, the holders of Series C-1 and Series C-2 preferred stock would have been paid $1.179633 per share plus any dividends declared but unpaid on the Series C-1 and Series C-2 preferred stock, prior and in preference to any distributions to the holders of common stock and Series B, Series A and Series A-1 preferred stock.

After the payment of all preferential amounts to the holders of the Senior Preferred Stock, Series C-1 and Series C-2 preferred stock, then, to the extent available, the holders of Series B preferred stock would have been paid $0.4064 per share plus any dividends declared but unpaid on the Series B preferred stock, prior and in preference to any distributions to the holders of common stock and Series A and Series A-1 preferred stock.

After the payment of all preferential amounts to the holders of the Senior Preferred Stock, Series C-1, Series C-2 and Series B preferred stock, then, to the extent available, the holders of Series A and Series A-1 preferred stock would have been paid $1.00 and $1.30 per share, respectively, plus any dividends declared but unpaid on the Series A and A-1 preferred stock, prior and in preference to any distributions to the holders of common stock.

After payments have been made in full to the holders of the Redeemable Preferred Stock, then, to the extent available, the remaining amounts would have been distributed among the holders of the shares of preferred stock and common stock, pro rata based on the number of shares held by each holder, treating for this purpose all such securities as if they had been converted to common stock immediately prior to such dissolution, liquidation or winding up of the Company.

Unless 55% of the holders of the Senior Preferred Stock, voting together as a single class, elected otherwise, a Deemed Liquidation Event would have included a merger or consolidation (other than one in which stockholders of the Company own a majority by voting power of the outstanding shares of the surviving or acquiring corporation) or a sale, lease, transfer, exclusive license or other disposition of substantially all of the assets of the Company.

Conversion

Each share of Redeemable Preferred Stock was convertible, at the option of the holder, at any time, and without the payment of additional consideration, or would have automatically converted into shares of common stock at the applicable conversion ratio then in effect (i) upon the closing of a firm commitment underwritten public offering at a price per share to the public, which when multiplied by the total number of shares of common stock then outstanding or then issuable upon conversion of outstanding Redeemable Preferred Stock immediately prior to the consummation of the offering, exceeded $150,000 and with at least $50,000 of gross proceeds to the Company or (ii) upon the vote or written consent of the holders of at least 55% of the outstanding shares of the Senior Preferred Stock, voting together as a single class. All shares that were required to be surrendered per the provisions above would have been deemed to have been retired and canceled and would not be reissued as shares of preferred stock.

The conversion ratio of each series of Redeemable Preferred Stock was determined by dividing the Original Issue Price of each series of preferred stock by the Conversion Price of each series, except for Series D-1 preferred stock. The conversion ratio for Series D-1 was determined by dividing $11.722013 by the Series D Conversion Price. The Conversion Price was $99.37 for Series A, $129.181 for Series A-1, $4.038397 for Series B, $11.722013 for Series C-1, Series C-2, Series D, Series E and Series E-2, and $13.51432 for Series E-1, Series E-3 and Series F. The Conversion Price was subject to appropriate adjustment in the event of any stock dividend, stock split, combination or other similar recapitalization and other adjustments as set forth in the Company’s certificate of incorporation, as amended and restated.

Redemption Rights

At the written election of at least 55% of the holders of the Senior Preferred Stock, voting together as a single class, the shares of Redeemable Preferred Stock outstanding were redeemable, at any time on or after December 22, 2020, in three equal annual installments commencing 60 days after receipt of the required vote, in an amount equal to the Original Issue Price per share of each series of Redeemable Preferred Stock plus all declared but unpaid dividends thereon.

Common Stock	12 Months Ended
Common Stock	Dec. 31, 2017
Equity [Abstract]
Common Stock	9. Common Stock As of December 31, 2016, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue up to 143,500,000 shares of $0.001 par value common stock. On July 5, 2017, the Company filed the amended and restated certificate of incorporation which increased the authorized number of shares of common stock to 150,000,000 shares. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to receive dividends, as may be declared by the Company’s board of directors, if any, subject to the preferential dividend rights of the preferred stock. As of December 31, 2017 and 2016, no dividends had been declared. As of December 31, 2017, the Company had reserved 3,114,279 shares for the exercise of outstanding stock options and grant of future awards under the Company’s stock incentive plans (see Note 10).

Stock-Based Awards

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Awards

10. Stock-Based Awards

2017 Stock Incentive Plan

The Company’s 2017 Stock Incentive Plan (the “2017 Plan”) was approved by the Company’s stockholders on June 16, 2017 and became effective on June 28, 2017. Under the 2017 Plan, the Company may grant incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, awards of restricted stock units and other stock-based awards. The Company’s employees, officers, directors, consultants and advisors are eligible to receive awards under the 2017 Plan; however, incentive stock options may only be granted to employees. The 2017 Plan is administered by the board of directors or, at the discretion of the board of directors, by a committee of the board. The number of shares of common stock covered by options and the date those options become exercisable, type of options to be granted, exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or its committee if so delegated.

Stock options granted under the 2017 Plan with service-based vesting conditions generally vest over four years and may not have a duration in excess of ten years, although options have been granted with vesting terms of less than four years.

The total number of shares of common stock that may be issued under the 2017 Plan was 1,674,501 as of December 31, 2017, of which 868,046 shares remained available for grant. The Company initially reserved 1,244,816 shares of common stock plus the number of shares equal to the sum of the number of shares of common stock then available for issuance under the 2016 Plan, which was 424,601 shares, and the number of shares of common stock subject to outstanding awards under the 2006 Plan and the 2016 Plan that expire, terminate or are otherwise surrendered, canceled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right. The number of shares of common stock that may be issued under the 2017 Plan will automatically increase on January 1 of each year, beginning with the fiscal year ending December 31, 2018 and continuing for each fiscal year until, and including, the fiscal year ending December 31, 2027, equal to the least of (i) 1,244,816 shares of common stock, (ii) 4% of the outstanding shares of common stock on such date and (iii) an amount determined by the Company’s board of directors.

During the year ended December 31, 2017, pursuant to the terms of the 2017 Plan, the Company granted options to employees and directors to purchase 1,253,728 shares of common stock at a weighted average exercise price of $10.49 per share.

Shares that are expired, terminated, surrendered or canceled without having been fully exercised will be available for future awards. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for the grant of awards.

The exercise price for stock options granted may not be less than the fair market value of the common stock as of the date of grant.

2017 Employee Stock Purchase Plan

On June 16, 2017, the Company’s stockholders approved the 2017 Employee Stock Purchase Plan (the “2017 ESPP”), which became effective on June 28, 2017. A total of 150,000 shares of common stock are reserved for issuance under the 2017 ESPP. The number of shares of common stock that may be issued under the 2017 ESPP automatically increase on each January 1 and continuing for each fiscal year until, and including, the fiscal year ending December 31, 2027, equal to the least of (i) 622,408 shares, (ii) 1% of the outstanding shares of common stock on such date and (iii) an amount determined by the Company’s board of directors.

2016 Stock Incentive Plan

The Company’s 2016 Stock Incentive Plan (the “2016 Plan”) provided for the Company to grant incentive stock options or nonqualified stock options, restricted stock, restricted stock units and other equity awards to employees, directors and consultants of the Company. The 2016 Plan was administered by the board of directors or, at the discretion of the board of directors, by a committee of the board. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated.

Stock options granted under the 2016 Plan with service-based vesting conditions vest over four years and expire after ten years.

After the effective date of the 2017 Plan, no stock options or other awards were made under the 2016 Plan. No shares remained available for future issuance as of December 31, 2017.

Shares that are expired, terminated, surrendered or canceled without having been fully exercised will be available for future awards under the 2017 Plan. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for the grant of awards under the 2017 Plan.

2006 Stock Incentive Plan

The Company’s 2006 Stock Incentive Plan, as amended, (the “2006 Plan”) provided for the Company to grant incentive stock options or nonqualified stock options, restricted stock, restricted stock units and other equity awards to employees, directors and consultants of the Company. The 2006 Plan was administered by the board of directors or, at the discretion of the board of directors, by a committee of the board. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated.

Stock options granted under the 2006 Plan with service-based vesting conditions generally vest over four years and expire after ten years, although options have been granted with vesting terms of less than four years.

The 2006 Plan expired in 2016. No shares remained available for future issuance as of December 31, 2016.

Stock Option Valuation

The assumptions that the Company used to determine the grant-date fair value of the stock options granted to employees and directors during the year ended December 31, 2017, 2016 and 2015 were as follows, presented on a weighted average basis:

	Year Ended December 31,
	2017		2016		2015
Risk-free interest rate	2.16%		1.37%		2.01%
Expected term (in years)		6.1		6.1		6.1
Expected volatility	80.4%		80.5%		80.5%
Expected dividend yield	0%		0%		0%

Stock Options

The following table summarizes the Company’s stock option activity since January 1, 2017:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding at December 31, 2016		920,731		$	4.36		6.4	$	1,297
Granted		1,253,728			10.49
Exercised		(31,631	)		3.49
Forfeited		(46,595	)		5.14
Outstanding at December 31, 2017		2,096,233		$	8.02		8.0	$	7,332

Options exercisable at December 31, 2017		784,190		$	4.71		6.0	$	4,667
Options vested and expected to vest at December 31, 2017		2,030,629		$	7.96		7.9	$	7,198

The weighted average grant-date fair value of stock options granted during the year ended December 31, 2017, 2016 and 2015 was $7.49, $3.24 and $3.69, respectively.

The aggregate fair value of stock options that vested during the year ended December 31, 2017, 2016 and 2015 was $1,268, $784 and $442, respectively.

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised during the year ended December 31, 2017, 2016 and 2015 was $165, $12 and $300, respectively.

Stock-Based Compensation

The Company recorded stock-based compensation expense related to stock options in the following expense categories of its statements of operations and comprehensive loss:

	Year Ended December 31,
	2017		2016		2015
Research and development expenses	$	611	$	219	$	275
General and administrative expenses		1,138		469		351
	$	1,749	$	688	$	626

As of December 31, 2017, the Company had an aggregate of $8,776 of unrecognized stock-based compensation expense, which it expects to recognize over a weighted average period of 3.3 years.

Net Loss per Share

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Net Loss per Share

11. Net Loss per Share

Basic and diluted net loss per share attributable to common stockholders was calculated as follows:

	Year Ended December 31,
	2017			2016			2015
Numerator:
Net loss	$	(22,604	)	$	(18,123	)	$	(12,878	)
Accretion of redeemable convertible preferred stock to redemption value		(41	)		(75	)		(71	)
Net loss attributable to common stockholders	$	(22,645	)	$	(18,198	)	$	(12,949	)

Denominator:

Statement of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Series E-1 Redeemable Convertible Preferred Stock [Member]
Issuance of redeemable convertible preferred stock, Issuance costs		$ 17
Series F Redeemable Convertible Preferred Stock [Member]
Issuance of redeemable convertible preferred stock, Issuance costs	$ 32	$ 165

Preferred Stock	12 Months Ended
Preferred Stock	Dec. 31, 2017
Equity [Abstract]
Preferred Stock	8. Preferred Stock On July 5, 2017, in connection with the closing of the Company’s IPO, the Company filed its amended and restated certificate of incorporation, which authorizes the Company to issue up to 5,000,000 shares of preferred stock, $0.001 par value per share. As of December 31, 2017 and 2016, the Company had no shares of preferred stock issued or outstanding. The preferred stock was classified under stockholders’ equity as of December 31, 2017.