In February 2021, the Company adopted the 2021 Incentive Award Plan (2021 Plan) and the 2021 Employee Stock Purchase Plan (2021 ESPP), both of which became effective on the completion of the Company’s initial public offering. The 2021 Plan provides for a variety of stock-based compensation awards, including stock options, restricted stock awards (RSAs), and restricted stock units (RSUs). The 2021 ESPP allows eligible employees to purchase shares of the Company’s common stock at a discount through payroll deductions of up to 15% of their earnings, subject to plan limitations. Unless otherwise determined by the Company’s board of directors, employees may purchase shares at 85% of the lower of the fair market value of the Company’s common stock on the first date of an offering period or on the purchase date. As of December 31, 2023, 17.8 million shares and 4.6 million shares were available for future issuance under the 2021 Plan and the 2021 ESPP, respectively.

Stock-based compensation expense is recognized in the consolidated statements of operations as follows:


	Year Ended December 31,
	2023		2022		2021
	(in thousands)
Research and development	$	23,240	$	26,583	$	15,239
General and administrative		12,293		11,754		7,123
Total stock-based compensation expense	$	35,533	$	38,337	$	22,362

Unrecognized stock-based compensation costs related to unvested awards and the weighted-average period over which the costs are expected to be recognized as of December 31, 2023 are as follows:


	Stock Options		RSUs
Unrecognized stock-based compensation expense (in thousands)	$	50,477	$	6,983
Weighted-average period costs expected to be recognized (in years)		2.1		2.2

A summary of the Company’s stock option activity is as follows:


	Stock Options (in thousands)			Weighted-Average Exercise Price per Share		Weighted-Average Remaining Contractual Life (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2022		23,686		$	7.79		8.3	$	10,628
Granted		8,434			3.95
Exercised		(1,146	)		1.78
Forfeited/Cancelled		(4,125	)		9.33
Outstanding as of December 31, 2023		26,849		$	6.60		8.0	$	11,235
Exercisable as of December 31, 2023		12,355		$	7.20		7.2	$	7,802

The fair value of stock options granted to employees, directors, and consultants was estimated on the date of grant using the Black-Scholes option pricing model using the following assumptions:


		Year Ended December 31,
Assumptions	2023	2022	2021
Risk free interest rate	3.36% – 4.98%	1.56% – 4.31%	0.46% – 1.38%
Expected volatility	70.0% – 72.5%	70.0%	70.0%
Expected term (years)	5.50 – 6.50	5.50 – 6.25	5.50 – 6.40
Expected dividend	0%	0%	0%

The following table summarizes additional information related to stock option activity:


			Year Ended December 31,
	2023		2022		2021
Weighted average grant date fair value per share for options granted	$	2.62	$	3.92	$	14.22
Aggregate intrinsic value of stock options exercised (in thousands)	$	3,532	$	6,133	$	29,880

A summary of the Company’s RSU activity is as follows:


	RSUs (in thousands)			RSUs Weighted-Average Grant Date Fair Value per Share
Unvested shares as of December 31, 2022		885		$	6.66
Granted		2,304			3.55
Vested		(431	)		6.88
Forfeited		(417	)		4.78
Unvested shares as of December 31, 2023		2,341		$	3.88

The fair value of vested RSAs was immaterial, $1.8 million, and $1.5 million for the years ended December 31, 2023, 2022, and 2021, respectively. The fair value of vested RSUs was $1.9 million, $0.5 million, and $4.1 million for the years ended December 31, 2023, 2022, and 2021, respectively.