Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2018

Stock-Based Compensation

Schedule of total employee, director and non-employee stock-based compensation expense recognized

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2018		2017		2018		2017
General and administrative	$	1,688	$	1,416	$	3,305	$	2,884
Research and development		3,194		2,032		6,048		3,723
	$	4,882	$	3,448	$	9,353	$	6,607
	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2018		2017		2018		2017
Stock options	$	4,614	$	3,319	$	8,891	$	6,358
ESPP		190		129		353		249
Restricted stock units		78		—		109		—
	$	4,882	$	3,448	$	9,353	$	6,607

Summary of stock option activity

		Weighted		Weighted
		Average		Average
	Number of	Exercise		Remaining		Aggregate
	Shares subject	Price		Contractual		Intrinsic
	to outstanding	(Per		Term		Value
	options	Share)		(in years)		(in thousands)
Balances at December 31, 2017	5,093,442	$	15.32		7.62
Options granted	1,553,400	$	25.58
Options forfeited	(30,345)	$	21.90
Options exercised	(397,270)	$	7.34
Balance at June 30, 2018	6,219,227	$	18.36		7.90	$	116,282
Exercisable	2,941,950	$	13.76		6.79	$	68,393

Schedule of weighted average assumptions used for estimation of fair value of stock options

	Options				Options
	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2018		2017		2018		2017
Expected term (years)	5.9		6.1		6.1		6.1
Expected volatility	72.9	%	88.9	%	73.1	%	89.2	%
Risk-free interest rate	2.77	%	1.91	%	2.53	%	2.04	%
Expected dividend yield	—	%	—	%	—	%	—	%

Schedule of weighted average assumptions used for estimation of fair value of ESPP

	ESPP				ESPP
	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2018		2017		2018		2017
Expected term (years)	0.5 - 2.0		0.5 - 2.0		0.5 - 2.0		0.5 - 2.0
Expected volatility	61.2 - 71.4	%	67.8 - 79.8	%	61.2 - 71.4	%	67.8 - 79.8	%
Risk-free interest rate	1.47 - 2.41	%	.47 - 1.09	%	1.47 - 2.41	%	.47 - 1.09	%
Expected dividend yield	—	%	—	%	—	%	—	%

Summary of restricted stock unity activity

		Weighted
	Restricted	Average Grant
	Stock	Date Fair Value
	Units	(Per unit)
Unvested at December 31, 2017	—	$	—
Granted	33,933	$	27.64
Vested	—	$	—
Forfeited	—	$	—
Unvested at June 30, 2018	33,933	$	27.64