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


	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2022		2021		2022		2021
Research and development	$	4,267	$	4,600	$	13,572	$	13,909
General and administrative		6,465		7,896		20,313		22,338
Total	$	10,732	$	12,496	$	33,885	$	36,247

No income tax benefits for stock-based compensation expense have been recognized for the three and nine months ended September 30, 2022 and 2021 as a result of the Company’s full valuation allowance applied to net deferred tax assets and net operating loss carryforwards.

In July 2018, the Board of Directors adopted the 2018 Equity Incentive Plan (the “2018 Plan”). The 2018 Plan provides for the grant of incentive stock options, non-statutory stock options, restricted stock awards, restricted stock units (“RSUs”), stock appreciation rights, performance units and performance shares. The number of shares of common stock that may be issued under the 2018 Plan will automatically increase on each January 1, beginning with the fiscal year ending December 31, 2019, equal to the least of (i) 5,000,000 shares, (ii) 5% of the outstanding shares of common stock as of the last day of the preceding fiscal year or (iii) such other amount as the

Board of Directors may determine. Stock options and RSUs granted under the 2018 Plan generally vest over four years and expire no more than 10 years from the date of grant.

Following the IPO and upon the effectiveness of the 2018 Plan, the Company’s 2012 Equity Incentive Plan, as amended, (the “2012 Plan”), terminated and no further awards will be granted thereunder. All outstanding awards under the 2012 Plan will continue to be governed by their existing terms. Any shares subject to awards granted under the 2012 Plan that, on or after the termination of the 2012 Plan, expire or terminate and shares previously issued pursuant to awards granted under the 2012 Plan that, on or after the termination of the 2012 Plan, are forfeited or repurchased by the Company will be transferred into the 2018 Plan. As of September 30, 2022, the maximum number of shares that may be added to the 2018 Plan pursuant to the preceding sentence is 2,757,163 shares.

Prior to its termination, the 2012 Plan provided for the grant of stock options, stock appreciation rights, restricted stock and restricted stock units to employees, directors and consultants. Stock options granted under the 2012 Plan generally vest over four years and expire no more than 10 years from the date of grant.

The following weighted-average assumptions were used to calculate the fair value of stock options granted during the periods indicated:


	Three Months Ended						Nine Months Ended
	September 30,						September 30,
	2022			2021			2022			2021
Risk-free interest rate		3.11	%		0.89	%		2.81	%		0.89	%
Expected volatility		74.33	%		70.09	%		73.81	%		70.02	%
Expected dividend yield		—			—			—			—
Expected term (in years)		5.94			6.08			5.81			5.97

The Company’s stock option activity during the nine months ended September 30, 2022 is summarized as follows (number of shares in thousands):


				Weighted-
				Average
	Options			Exercise
	Outstanding			Price
Balance at December 31, 2021		5,530		$	21.51
Granted		1,356		$	3.85
Exercised		(405	)	$	1.63
Expired		(868	)	$	26.32
Forfeited		(248	)	$	64.43
Balance at September 30, 2022		5,365		$	15.79
Options exercisable		3,769		$	15.27
Options vested and expected to vest		5,323		$	15.79

During the three and nine months ended September 30, 2022 and 2021, the Company did not grant any stock options with performance-based or market-based vesting conditions.

As of September 30, 2022, total unrecognized stock-based compensation expense relating to unvested stock options was $14.9 million. This amount is expected to be recognized over a weighted-average period of 2.5 years.

RSU activity under the 2018 Plan during the nine months ended September 30, 2022 is summarized as follows (in thousands, except per share data):


				Weighted-
				Average
				Grant Date
	Shares			Fair Value
Balance at December 31, 2021		1,506		$	93.14
Granted		4,812		$	5.37
Vested		(180	)	$	101.60
Forfeited		(1,142	)	$	46.78
Balance at September 30, 2022		4,996		$	18.90

The weighted-average fair value of RSUs granted during the nine months ended September 30, 2022 and 2021 was $5.37 and $100.16, respectively.

As of September 30, 2022, total unrecognized stock-based compensation expense relating to unvested RSUs was $80.8 million and the weighted-average remaining vesting period was 2.7 years.

PSU activity under the 2018 Plan during the nine months ended September 30, 2022 is summarized as follows (in thousands, except per share data):


				Weighted-
				Average
				Grant Date
	Shares			Fair Value
Balance at December 31, 2021		113		$	79.60
Granted		4,124		$	5.57
Forfeited		(678	)	$	14.87
Balance at September 30, 2022		3,559		$	6.16

As of September 30, 2022, total unrecognized stock-based compensation expense relating to unvested PSUs was $21.9 million.

In July 2018, the Company’s Board of Directors and stockholders approved the 2018 Employee Stock Purchase Plan (the “2018 ESPP”). At inception, the 2018 ESPP had a maximum number of 500,000 shares of common stock available. The number of shares of common stock that may be issued under the 2018 ESPP shall automatically increase on each January 1, beginning with the fiscal year ending December 31, 2019, equal to the least of (i) 1,000,000 shares, (ii) 1% of the outstanding shares of common stock as of the last day of the immediately preceding fiscal year and (iii) such other amount determined by the 2018 ESPP administrator. Under the 2018 ESPP, employees may purchase shares of the Company’s common stock at a price per share equal to 85% of the lower of the fair market value of the common stock on the first trading day of the offering period or on the exercise date. The 2018 ESPP provides for consecutive, overlapping 24-month offering periods, each of which will include four 6-month purchase periods. The Company’s first offering period under the 2018 ESPP commenced on July 18, 2018. During three and nine months ended September 30, 2022, stock-based compensation expense related to the 2018 ESPP was $0.1 million and $0.6 million, respectively. During the three and nine months ended September 30, 2021, stock-based compensation expense related to the 2018 ESPP was $0.3 million and $0.8 million, respectively.