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 June 30, 2023, 15.6 million shares and 4.8 million shares were available for future issuance under the 2021 Plan and the 2021 ESPP, respectively.

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


	Three Months Ended June 30,				Six Months Ended June 30,
	2023		2022		2023		2022
	(in thousands)
Research and development	$	6,693	$	7,408	$	12,665	$	13,120
General and administrative		3,155		2,503		5,934		4,546
Total stock-based compensation expense	$	9,848	$	9,911	$	18,599	$	17,666

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 June 30, 2023 are as follows:


	Stock Options		RSUs
Unrecognized stock-based compensation expense (in thousands)	$	74,944	$	6,653
Weighted-average period costs expected to be recognized (in years)		2.5		2.8

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		7,959			3.93
Exercised		(590	)		1.76
Forfeited/Cancelled		(759	)		7.82
Outstanding as of June 30, 2023		30,296		$	6.89		8.3	$	36,614
Exercisable as of June 30, 2023		11,245		$	7.33		7.1	$	15,381

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:


	Six Months Ended June 30,
Assumptions	2023	2022
Risk free interest rate	3.36% – 4.28%	1.56% – 3.36%
Expected volatility	70.0%	70.0%
Expected term (years)	5.50 – 6.50	5.50 – 6.25
Expected dividend	0%	0%

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


	Six Months Ended June 30,
	2023		2022
Weighted average grant date fair value per share for options granted	$	2.60	$	3.83
Aggregate intrinsic value of stock options exercised (in thousands)	$	2,058	$	3,917

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		1,269			3.77
Vested		(261	)		6.67
Forfeited		(117	)		5.12
Unvested shares as of June 30, 2023		1,776		$	4.69