Equity Plans (Tables)

12 Months Ended

Dec. 31, 2017

Schedule of activity under equity incentive plans

			Options Outstanding
			Weighted-		Weighted-
			Average		Average
	Options		Exercise		Remaining	Aggregate
	Available for	Options	Price Per		Contractual	Intrinsic
	Grant	Outstanding	Share		Life (years)	Value (1)
						(in millions)
Balances at December 31, 2014	116,832	476,006	$	1.40	8.04
Additional options authorized	431,411	—
Options granted	(408,623)	408,623		1.24
Options exercised	—	(43,852)		1.30
Options forfeited	7,599	(7,599)		1.40
Balances at December 31, 2015	147,219	833,178		1.33	8.56
Additional options authorized	1,697,088	—
Options granted	(1,679,979)	1,679,979		14.24
Options exercised	—	(119,328)		1.28
Balances at December 31, 2016	164,328	2,393,829		10.39	8.79
Additional options authorized	668,891	—
Options granted	(493,500)	493,500		13.20
Options exercised	—	(257,858)		1.84
Options forfeited	191,320	(191,320)		15.00
Balances at December 31, 2017	531,039	2,438,151	$	11.51	8.26	$	23.3
Options exercisable – December 31, 2017		877,888	$	9.70	7.61	$	10.0
Options vested and expected to vest – December 31, 2017		2,438,151	$	11.51	8.26	$	23.3

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(1) The aggregate intrinsic values were calculated as the difference between the exercise price of the options and the closing price of the Company’s common stock on December 31, 2017. The calculation excludes options with an exercise price higher than the closing price of the Company’s common stock on December 31, 2017.

Schedule of stock-based compensation expense

Total stock-based compensation expense was as follows (in thousands):

	Year Ended December 31,
	2017		2016		2015
Research and development	$	2,008	$	1,080	$	39
General and administrative		2,233		1,050		60
Total stock-based compensation expense	$	4,241	$	2,130	$	99

Employee stock options

Schedule of valuation assumptions

	Year Ended December 31,
	2017	2016	2015
Expected term (in years)	5.50 – 6.08	4.16 – 5.95	5.89
Expected volatility	61.6% – 65.4%	62.5% – 64.8%	59.8%
Risk-free interest rate	1.88% – 2.24%	1.27% – 1.79%	1.57% – 1.58%
Dividend yield	—	—	—

Non-employee stock options

Schedule of valuation assumptions

	Year Ended December 31,
	2017	2016	2015
Expected term (in years)	6.97 – 7.40	6.59 – 9.97	6.80
Expected volatility	61.7% – 65.4%	62.5% – 62.8%	59.8%
Risk-free interest rate	2.17% – 2.33%	1.29% – 1.79%	1.95%
Dividend yield	—	—	—

ESPP rights

Schedule of valuation assumptions

	Year Ended December 31,
	2017	2016
Expected term (in years)	0.50	0.60
Expected volatility	52.43% – 52.44%	52.48%
Risk-free interest rate	0.89% – 1.16%	0.45%
Dividend yield	—	—