Stock based compensation (Tables)

12 Months Ended

Dec. 31, 2016

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation cost recognized for all stock-based compensation awards

	Year ended December 31,
	2016		2015		2014
	(in thousands)
Research and development	$	4,296	$	3,218	$	297
General and administrative		2,014		809		128
Total stock-compensation expense	$	6,310	$	4,027	$	425

Summary of status and changes in unvested restricted stock

		Weighted
		Average
		Grant Date
		Fair Value
	Shares	Per Share
Unvested restricted common stock as of December 31, 2015	1,818,261	$	0.76
Issued	—
Vested	(601,501)	$	0.79
Repurchased	(48,776)	$	1.11
Unvested restricted common stock as of December 31, 2016	1,167,984	$	0.76

Summary of status and changes in stock options

		Weighted		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value
	Shares	Price		(in years)	(in thousands)
Outstanding at December 31, 2015	1,022,617	$	8.35
Granted	989,560	$	11.90
Exercised	(65,432)	$	7.85
Cancelled or forfeited	(75,508)	$	9.62
Outstanding at December 31, 2016	1,871,237	$	10.21	8.9	$	5,232
Exercisable at December 31, 2016	458,737	$	9.02	8.5	$	1,759
Vested and expected to vest at December 31, 2016	1,871,237	$	10.21	8.9	$	5,232

Assumptions

The assumptions utilized to value the warrants to purchase equity securities on the acquisition date, September 2, 2016 and as of December 31, 2016 are as follows:

	As of September 2,		As of December 31,
	2016		2016
Risk-free interest rate	1.2	%	1.8	%
Expected dividend yield	—	%	—	%
Expected term (in years)	5.0		4.7
Expected volatility	92.3	%	97.5	%

Employees and directors stock options

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Assumptions

The fair value of each option issued to employees and directors was estimated at the date of grant using the Black‑Scholes option pricing model with the following weighted‑average assumptions:

	Year ended December 31,
	2016		2015
Risk-free interest rate	1.5	%	1.6	%
Expected dividend yield	—	%	—	%
Expected term (in years)	6.0		6.0
Expected volatility	73.1	%	78.6	%

Non-employees stock options

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Assumptions

	As of December 31,
	2016		2015
Risk-free interest rate	2.1	%	2.0	%
Expected dividend yield	—	%	—	%
Expected term (in years)	9.1		10.0
Expected volatility	83.3	%	84.0	%