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 September 30, 2024, 20.9 million shares and 6.2 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 September 30,				Nine Months Ended September 30,
	2024		2023		2024		2023
	(in thousands)
Research and development	$	6,481	$	5,722	$	19,452	$	18,387
General and administrative		4,289		3,281		11,790		9,215
Total stock-based compensation expense	$	10,770	$	9,003	$	31,242	$	27,602

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 September 30, 2024 are as follows:


	Stock Options		RSUs
Unrecognized stock-based compensation expense (in thousands)	$	60,322	$	25,801
Weighted-average period costs expected to be recognized (in years)		2.5		3.2

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, 2023		26,849		$	6.60		8.0	$	11,235
Granted		7,538			8.82
Exercised		(2,472	)		3.72
Forfeited/Cancelled		(3,432	)		8.06
Outstanding as of September 30, 2024		28,483		$	7.26		7.8	$	7,966
Exercisable as of September 30, 2024		14,635		$	7.19		6.9	$	6,450

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:


	Nine Months Ended September 30,
Assumptions	2024	2023
Risk free interest rate	3.7% – 4.7%	3.4% – 4.6%
Expected volatility	72.5%	70.0% – 72.5%
Expected term (years)	5.50 – 6.25	5.50 – 6.50
Expected dividend	0%	0%

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


	Nine Months Ended September 30,
	2024		2023
Weighted average grant date fair value per share for options granted	$	5.98	$	2.62
Aggregate intrinsic value of stock options exercised (in thousands)	$	7,632	$	2,426

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


	RSUs (in thousands)			RSUs Weighted-Average Grant Date Fair Value per Share
Unvested as of December 31, 2023		2,341		$	3.88
Granted		3,345			8.75
Vested		(524	)		4.76
Forfeited		(673	)		6.46
Unvested as of September 30, 2024		4,489		$	7.03