Equity Plans (Tables)

12 Months Ended

Dec. 31, 2021

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, 2020	4,648,120	$	11.87	7.61	$	40.0
Options granted	1,949,940		29.96
Options exercised	(474,801)		11.34
Options forfeited	(232,719)		17.84
Balances at December 31, 2021	5,890,540	$	17.66	7.47	$	102.7
Options exercisable – December 31, 2021	3,224,575	$	13.13	6.38	$	68.1
Options vested and expected to vest – December 31, 2021	5,890,540	$	17.66	7.47	$	102.7

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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, 2021. The calculation excludes options with an exercise price higher than the closing price of the Company’s common stock on December 31, 2021.

Schedule of stock-based compensation expense

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

	Year Ended December 31,
	2021		2020		2019
Research and development	$	8,996	$	4,121	$	4,350
General and administrative		7,399		3,778		4,003
Total stock-based compensation expense	$	16,395	$	7,899	$	8,353

Options to Purchase Common Stock

Black-Scholes option-pricing model assumptions

	Year Ended December 31,
	2021	2020	2019
Expected term (in years)	5.27- 6.08	5.27- 6.08	5.00 - 6.08
Expected volatility	87.4% - 95.2%	72.1% - 87.5%	61.0% - 64.8%
Risk-free interest rate	0.11% - 1.35%	0.23% - 1.44%	1.42% - 2.58%
Dividend yield	—	—	—

Restricted stock units

Schedule of activity under equity incentive plans

		Weighted
		Average
	Number of	Grant Date
	Shares	Fair Value
Unvested RSUs at December 31, 2020	244,545	$	9.31
Granted	302,250		24.40
Vested	(78,165)		10.13
Forfeited	(62,658)		18.65
Unvested RSUs at December 31, 2021	405,972	$	20.13

Performance Stock Units

Schedule of performance stock unit activity

		Weighted
		Average
	Number of	Grant Date
	Shares	Fair Value
Unvested PSUs at December 31, 2020	—
Granted	110,500	$	23.57
Vested	—		—
Forfeited	(5,000)		23.57
Unvested PSUs at December 31, 2021	105,500	$	23.57

ESPP shares

Black-Scholes option-pricing model assumptions

	Year Ended December 31,
	2021	2020	2019
Expected term (in years)	0.50	0.50	0.50
Expected volatility	50.9% - 69.7%	89.1% - 120.4%	58.9% - 65.3%
Risk-free interest rate	0.06%	0.12% - 0.43%	1.89% - 2.32%
Dividend yield	—	—	—