Stock based compensation (Tables)

6 Months Ended

Jun. 30, 2016

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

Compensation cost recognized for all stock-based compensation awards

Total compensation cost recognized for all stock‑based compensation awards in the statements of operations and comprehensive loss is as follows:

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2016		2015		2016		2015
	(in thousands)				(in thousands)
Research and development	$	931	$	634	$	2,048	$	859
General and administrative		387		268		691		345
Total stock-compensation expense	$	1,318	$	902	$	2,739	$	1,204

Summary of status and changes in unvested restricted stock

A summary of the status of and changes in unvested restricted stock unit activity under the Company’s equity award plan was as follows:

		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	(332,369)	$	0.76
Repurchased	(35,540)		—
Unvested restricted common stock as of June 30, 2016	1,450,352	$	0.75

Summary of status and changes in stock options

The following is a summary of stock option activity for the six months ended June 30, 2016:

		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	806,010	$	11.58
Exercised	(12,910)	$	7.27
Cancelled or forfeited	(31,766)	$	9.93
Outstanding at June 30, 2016	1,783,951	$	9.80	9.3	$	2,817
Exercisable at June 30, 2016	266,194	$	8.34	8.7	$	709
Vested and expected to vest at June 30, 2016	1,783,951	$	9.80	9.3	$	2,817

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:

	Three Months Ended		Six Months Ended
	June 30, 2016
Risk-free interest rate	1.3	%	1.5	%
Expected dividend yield	—	%	—	%
Expected term (in years)	6.0		6.0
Expected volatility	73.5	%	73.1	%

Non-employees stock options

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

Assumptions

The fair value of each option issued to non‑employees was estimated at each vesting and reporting date using the Black‑Scholes option pricing model. The reporting date fair value was determined using the following weighted‑average assumptions:

	Six Months Ended
	June 30, 2016
Risk-free interest rate	1.5	%
Expected dividend yield	—	%
Expected term (in years)	10.0
Expected volatility	84.1	%