Stock based compensation (Tables)

12 Months Ended

Dec. 31, 2018

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

Compensation cost recognized for all stock-based compensation awards

	Year ended December 31,
	2018		2017		2016
	(in thousands)
Research and development	$	4,717	$	5,367	$	4,296
General and administrative		10,993		3,871		2,014
Total stock-compensation expense	$	15,710	$	9,238	$	6,310

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, 2017	557,979	$	0.70
Issued	—
Vested	(319,891)	$	0.84
Repurchased	(2,794)	$	1.11
Unvested restricted common stock as of December 31, 2018	235,294	$	0.51

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, 2017	3,143,566	$	11.82
Granted	2,215,891	$	20.78
Exercised	(384,186)	$	10.13
Cancelled or forfeited	(750,119)	$	16.13
Outstanding at December 31, 2018	4,225,152	$	15.91	8.4	$	578
Exercisable at December 31, 2018	1,531,832	$	12.55	7.3	$	507
Vested and expected to vest at December 31, 2018	4,225,152	$	15.91	8.4	$	578

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,
	2018		2017		2016
Risk-free interest rate	2.8	%	2.0	%	1.5	%
Expected dividend yield	—	%	—	%	—	%
Expected term (in years)	6.0		6.0		6.0
Expected volatility	74.4	%	73.7	%	73.1	%

Non-employees stock options

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

Assumptions

	As of December 31,
	2018		2017		2016
Risk-free interest rate	2.6	%	2.4	%	2.1	%
Expected dividend yield	—	%	—	%	—	%
Expected term (in years)	7.6		8.5		9.1
Expected volatility	73.2	%	76.2	%	83.3	%