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


	Year Ended December 31,
	2023		2022
Research and development	$	17,599	$	17,399
General and administrative		23,624		25,759
Total	$	41,223	$	43,158

No income tax benefits for stock-based compensation expense have been recognized for the years ended December 31, 2023 and 2022, 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-based awards ("PSUs") and other stock-based awards. 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 and (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 December 31, 2023, the maximum number of shares that may be added to the 2018 Plan pursuant to the preceding clause is 2,546,977 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.

As of December 31, 2023, the number of shares available for issuance under the 2018 Plan was 2,036,811.

Stock option activity under the 2018 Plan and the 2012 Plan during the year ended December 31, 2023 is summarized as follows (in thousands, except per share data):


				Weighted-		Weighted-
				Average		Average		Aggregate
	Options			Exercise		Remaining		Intrinsic
	Outstanding			Price		Years		Value
Balance at December 31, 2022		5,423		$	15.41		6.4	$	22,657
Granted		3,458		$	5.79
Exercised		(206	)	$	3.26
Expired		(267	)	$	48.35
Forfeited		(440	)	$	7.99
Balance at December 31, 2023		7,968		$	10.85		6.9	$	3,168
Options exercisable		4,216		$	13.62		5.0	$	3,165
Options vested and expected to vest		7,776		$	10.95		6.9	$	3,168

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


	Year Ended December 31,
	2023			2022
Risk-free interest rate		3.88	%		2.95	%
Expected volatility		99.96	%		75.94	%
Expected dividend yield		—			—
Expected term (in years)		5.96			5.84

The weighted-average fair value of options granted during the years ended December 31, 2023 and 2022 was $4.61 and $2.76 per share, respectively.

The aggregate fair value of stock options that vested during the years ended December 31, 2023 and 2022 was $6.7 million and $15.2 million, respectively.

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2023 and 2022 was $0.1 million and $1.8 million, respectively.

As of December 31, 2023, total unrecognized stock-based compensation expense relating to unvested stock options was $16.9 million. This amount is expected to be recognized over a weighted-average period of 2.4 years.

Each time-based restricted stock unit (“RSU”) represents one equivalent share of our common stock to be awarded after satisfying the applicable continued service-based vesting criteria over a specified period. The fair value for these RSUs is based on the closing price of our common stock on the date of grant. The RSUs do not entitle

RSU activity under the 2018 Plan is summarized as follows (in thousands, except per share data):


				Weighted-
				Average
	Number			Grant Date
	of Shares			Fair Value
Balance at December 31, 2022		4,478		$	16.90
Granted		3,860		$	6.52
Vested		(1,734	)	$	18.29
Forfeited		(983	)	$	12.23
Balance at December 31, 2023		5,621		$	10.16

The weighted-average fair value of RSUs granted during the years ended December 31, 2023 and 2022 was $6.52 and $5.38, respectively.

As of December 31, 2023, total unrecognized stock-based compensation expense relating to unvested RSUs was $48.4 million and the weighted-average remaining vesting period was 2.3 years.

The aggregate intrinsic value of RSUs is calculated as the closing price per share of the Company’s common stock on the last trading day of the fiscal period, multiplied by the number of RSUs expected to vest as of December 31, 2023. As of December 31, 2023, the aggregate intrinsic value of RSUs was $15.3 million.

During the year ended December 31, 2023, no restricted stock units with performance-based vesting (“PSUs”) from the 2018 Plan were granted, however, 2.8 million PSUs were outstanding as of December 31, 2023. Each PSU represents one equivalent share of our common stock to be awarded upon vesting at the end of the performance periods, if specific performance goals set by the Board of Directors are achieved. No PSUs will vest if the performance goals are not met. The fair value of these PSUs is based on the closing price of our common stock on the date of grant. The Company assesses the probability of achieving the performance goals on a quarterly basis. Changes in our assessment of the probability results in adjustments to stock-based compensation, which may include either a cumulative catch-up of expense or a reduction of expense depending on whether the likelihood of vesting has increased or decreased, that is recognized in the period such determination is made. The PSUs do not entitle participants to the rights of holders of common stock, such as voting rights, until the shares are issued. PSUs that are expected to vest are net of estimated future forfeitures.

The following table summarizes the PSUs activity for the year ended December 31, 2023 (in thousands, except per share data):


				Weighted-
				Average
	Number			Grant Date
	of Shares			Fair Value
Balance at December 31, 2022		3,276		$	6.21
Granted		—		$	—
Forfeited		(431	)	$	5.58
Balance at December 31, 2023		2,845		$	6.30

No stock-based compensation expense related to the PSUs has been recognized during the years ended December 31, 2023 and 2022 as the achievement of the performance conditions were not deemed probable and the remaining vesting period is undeterminable. As of December 31, 2023, total unrecognized compensation expense relating to PSUs was $17.9 million and the aggregate intrinsic value of PSUs at December 31, 2023 was $7.8 million.

In July 2018, the Company’s Board of Directors and stockholders approved the 2018 Employee Stock Purchase Plan (the “2018 ESPP”). 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. As of December 31, 2023, the number of shares available for issuance under the 2018 ESPP was 2,781,902.

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 purchase periods. The first offering period commenced on July 18, 2018.

During the years ended December 31, 2023 and 2022, stock-based compensation related to the 2018 ESPP was $0.8 million and $0.8 million, respectively.

The following weighted-average assumptions were used to calculate the fair value of ESPP shares during the periods indicated:


	Year Ended December 31,
	2023			2022
Risk-free interest rate		5.14	%		2.20	%
Expected volatility		127.37	%		75.83	%
Expected dividend yield		—			—
Expected term (in years)		1.32			1.67

As of December 31, 2023, total unrecognized compensation expense relating to shares to be purchased under the 2018 ESPP was $0.3 million and the weighted-average remaining vesting period was 1.1 years.