Equity Plans (Tables)

6 Months Ended

Jun. 30, 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	1,141,600		8.51
Options exercised	(113,808)		5.52
Options forfeited	(171,327)		10.88
Balances at June 30, 2020	4,537,986	$	11.04	7.90	$	32.8
Options exercisable – June 30, 2020	2,383,602	$	12.02	6.87	$	16.0
Options vested and expected to vest – June 30, 2020	4,537,986	$	11.04	7.90	$	32.8

(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 June 30, 2020. The calculation excludes options with an exercise price higher than the closing price of the Company’s common stock on June 30, 2020.

Schedule of stock-based compensation expense

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

	Three Months Ended June 30,				Six Months Ended June 30,
	2020		2019		2020		2019
Research and development	$	1,026	$	977	$	2,092	$	2,100
General and administrative		970		1,036		1,952		1,892
Total stock-based compensation expense	$	1,996	$	2,013	$	4,044	$	3,992

Employee stock options

Black-Scholes option-pricing model assumptions

	Three Months Ended June 30,		Six Months Ended June 30,
	2020	2019	2020	2019
Expected term (in years)	5.50 - 6.08	5.50 - 6.08	5.27 - 6.08	5.50 - 6.08
Expected volatility	74.3% - 74.5%	62.0% - 62.7%	72.1% - 74.5%	62.0% - 62.7%
Risk-free interest rate	0.39% - 0.42%	1.88% - 2.20%	0.39% - 1.44%	1.88% - 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	(26,727)		8.97
Unvested at June 30, 2020	262,608	$	9.24