Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2021

Summary of Stock Option Activity

The following table summarizes stock option activity:

	Number of Shares Underlying Options (in thousands)			Weighted- Average Exercise Price		Aggregate Intrinsic Value
						(In thousands)
Balance—December 31, 2020		5,514		$	13.24	$	38,433
Granted		1,371			15.49
Exercised		(281	)		6.17	$	2,864
Canceled		(395	)		29.27
Balance—September 30, 2021		6,209		$	13.04	$	25,980
Options exercisable—September 30, 2021		2,374		$	16.20	$	13,193
Options vested and expected to vest—September 30, 2021		6,209		$	13.04	$	25,980

Summary of Restricted Stock Unit or RSU Activity

The following table summarizes RSU activity:

	RSUs Outstanding
	Number of RSUs (in thousands)			Weighted- Average Grant Date Fair Value Per Share
Balance—December 31, 2020		441		$	14.51
Granted		523			15.18
Vested		(25	)		14.75
Forfeited		(37	)		14.69
Balance—September 30, 2021		902		$	14.89

Summary of Stock-Based Compensation

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

	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Research and development	$	1,522	$	151	$	4,282	$	334
General and administrative		1,765		164		5,078		380
Total stock-based compensation expense	$	3,287	$	315	$	9,360	$	714

2015 ESPP

Schedule of Black Scholes Option-Pricing Model

The fair value of our common stock to be issued under the 2015 ESPP is estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2021			2020			2021			2020
Expected term (in years)		—			—			0.5			—
Volatility		0.0	%		0	%		60.9	%		0	%
Risk-free interest rate		0.0	%		0	%		0.0	%		0	%
Dividend yield		—			—			—			—

Stock Options

Schedule of Black Scholes Option-Pricing Model

The fair value of stock options is estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumptions:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2021			2020			2021			2020
Expected term (in years)	6.2			0			6.1			6.1
Volatility		77.6	%		0.0	%		79.0	%		79.5	%
Risk-free interest rate		1.0	%		0.0	%		0.8	%		0.8	%
Dividend yield		—			—			—			—