Equity Plans (Tables)

3 Months Ended

Mar. 31, 2020

Schedule of activity under equity incentive plans

		Weighted-		Weighted-
		Average		Average
		Exercise		Remaining	Aggregate
	Options	Price Per		Contractual	Intrinsic
	Outstanding	Share		Life (years)	Value (1)
					(in millions)
Balances at December 31, 2019	3,681,521	$	11.64	7.78
Options granted	977,600		7.78
Options exercised	(29,167)		1.50
Options forfeited	(52,367)		11.85
Balances at March 31, 2020	4,577,587	$	10.87	8.06	$	2.2
Options exercisable – March 31, 2020	2,167,161	$	11.90	6.84	$	2.2
Options vested and expected to vest – March 31, 2020	4,577,587	$	10.87	8.06	$	2.2

⁽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 March 31, 2020. The calculation excludes options with an exercise price higher than the closing price of the Company’s common stock on March 31, 2020.

Schedule of stock-based compensation expense

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

	Three Months Ended March 31,
	2020		2019
Research and development	$	1,066	$	1,122
General and administrative		982		857
Total stock-based compensation expense	$	2,048	$	1,979

Employee stock options

Black-Scholes option-pricing model assumptions

	Three Months Ended March 31,
	2020	2019
Expected term (in years)	5.27 - 6.08	6.02 - 6.08
Expected volatility	72.1% - 73.8%	62.2% - 62.6%
Risk-free interest rate	0.59% - 1.44%	2.45% - 2.58%
Dividend yield	—	—

Restricted stock units

Schedule of activity under equity incentive plans

		Weighted
		Average
	Number of	Grant Date
	Shares	Fair Value
Unvested at December 31, 2019	278,482	$	10.08
Granted	142,000		7.80
Vested	(131,147)		9.52
Forfeited	(9,338)		9.35
Unvested at March 31, 2020	279,997	$	9.21