Shares of common stock reserved for issuance under the Company’s equity incentive plans were as follows:

A summary of the Company’s stock option activity (excluding performance-based stock option activity summarized further below) under the 2011 Plan, 2019 Plan, and Inducement Plan for the six months ended June 30, 2022 was as follows:

	Outstanding Options
(in thousands, except share and per share data)	Number of Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Balance—December 31, 2021		5,002,419		$	10.66		6.89	$	28,308
Options granted		221,601			6.31
Options exercised		(475,044	)		2.06
Options forfeited or expired		(118,396	)		13.92
Balance—June 30, 2022		4,630,580		$	11.25		6.72	$	550
Options vested and exercisable as of June 30, 2022		2,967,710		$	8.33		5.81	$	549

The weighted-average grant date fair value of options granted was $3.19 and $12.34 per share for the three months ended June 30, 2022 and 2021, and $3.99 and $13.55 per share for the six months ended June 30, 2022 and 2021, respectively. As of June 30, 2022, the unrecognized stock-based compensation cost of unvested options was $14.3 million, which is expected to be recognized over a weighted-average period of 2.1 years.

The Company estimated the fair value of stock options (excluding performance-based stock options discussed below) using the Black-Scholes option-pricing model. The fair value of stock options is recognized on a straight-line basis over the requisite service periods of the awards. The fair value of stock options was estimated using the following weighted-average assumptions:

	Three Months Ended June 30,		Six Months Ended June 30,
	2022	2021	2022	2021
Expected term (in years)	5.50 - 6.08	5.50 - 6.27	5.50 - 6.08	5.50 - 6.27
Volatility	69.70 - 71.56%	69.39 - 69.90%	68.37 - 71.56%	68.06 - 69.90%
Risk-free interest rate	2.82 - 3.39%	0.94 - 1.06%	1.62 - 3.39%	0.62 - 1.06%
Dividend yield	–%	–%	–%	–%

No performance-based stock option activity occurred during the six months ended June 30, 2022. As of June 30, 2022, stock options for 421,000 shares with an exercise price of $5.10 and a remaining contractual term of 7.71 years remained outstanding. The stock options are fully vested and exercisable and had no intrinsic value as of June 30, 2022.

A summary of the Company’s RSU activity under the 2019 Plan and Inducement Plan for the six months ended June 30, 2022 was as follows:

	Unvested Restricted Stock Units
(in thousands, except share and per share data)	Number of Shares			Weighted- Average Grant Date Fair Value		Aggregate Fair Value
Balance—December 31, 2021		1,679,696		$	16.35	$	23,969
RSUs granted		232,015			6.65
RSUs vested		(307,127	)		14.93		1,621
RSUs forfeited		(56,017	)		17.35
Balance—June 30, 2022		1,548,567		$	15.14	$	5,343

As of June 30, 2022, the unrecognized stock-based compensation cost of unvested RSUs was $21.3 million, which is expected to be recognized over a weighted-average period of 2.5 years.

During the six months ended June 30, 2022 and 2021, 204,775 and 57,001 shares of common stock were purchased under the ESPP, respectively. The fair value of stock purchase rights granted under the ESPP was estimated using the following assumptions:

	Three and Six Months Ended June 30,
	2022			2021
Expected term (in years)		0.50			0.49
Volatility		82.35	%		74.88	%
Risk-free interest rate		1.49	%		0.04	%
Dividend yield	–%			–%
Grant-date fair value per share	$	2.23		$	8.21

The following is a summary of stock-based compensation expense by award type (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2022		2021		2022		2021
Stock options	$	2,008	$	2,325	$	4,404	$	4,503
RSUs		2,227		1,011		4,395		1,655
ESPP		180		264		432		499
Total stock-based compensation expense	$	4,415	$	3,600	$	9,231	$	6,657

The following is a summary of stock-based compensation expense by function (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2022		2021		2022		2021
Cost of revenue	$	445	$	332	$	903	$	620
Research and development		1,319		900		2,660		1,628
Selling, general and administrative		2,651		2,368		5,668		4,409
Total stock-based compensation expense	$	4,415	$	3,600	$	9,231	$	6,657