The Company’s 2020 Stock Option and Grant Plan, or the 2020 Plan, provided for the Company to grant stock options, restricted stock and other stock awards, to employees, non-employee directors, and consultants. Upon the effectiveness of the 2021 Plan (as defined below), no further issuances were made under the 2020 Plan.

The Company’s 2021 Stock Option and Incentive Plan, or the 2021 Plan, was approved by the Company’s board of directors on May 28, 2021 and the Company’s stockholders on June 17, 2021 and became effective on the date immediately prior to the date on which the registration statement for the Company’s IPO was declared effective. The 2021 Plan provides for the grant of incentive stock options, non-qualified stock options, stock appreciation rights, restricted stock units, restricted stock awards, unrestricted stock awards, cash-based awards and dividend equivalent rights to the Company’s officers, employees, directors and consultants. The number of shares initially reserved for issuance under the 2021 Plan was 4,903,145. Under the evergreen provision of the 2021 Plan, the shares available for issuance under the 2021 Plan will be automatically increased each January 1st by 5% of the outstanding number of shares of the Company’s common stock on the immediately preceding December 31st or such lesser number of shares as may be determined by the Company’s compensation, nomination and corporate governance committee. Effective January 1, 2023 the number of shares available under the 2021 Plan automatically increased by 2,466,176 shares pursuant to the evergreen provision of the 2021 Plan. As of September 30, 2023, 3,227,124 shares were available for issuance under the 2021 Plan.

The Company’s 2021 Employee Stock Purchase Plan, or the 2021 ESPP, was approved by the Company’s board of directors on May 28, 2021 and the Company’s stockholders on June 17, 2021 and became effective on the date immediately prior to the date on which the registration statement for the Company’s IPO was declared effective. A total of 439,849 shares of the Company’s common stock were initially reserved for issuance under the 2021 ESPP. The shares available for issuance under the 2021 ESPP will be automatically increased on each January 1st, through January 1, 2031, by the least of (i) 439,849 shares of the Company’s common stock, (ii) 1% of the outstanding number of shares of the Company’s common stock on the immediately preceding December 31st or (iii) such lesser number of shares of the Company’s common stock as determined by the plan administrator of the 2021 ESPP. Effective January 1, 2023 the number of shares available under the 2021 ESPP automatically increased by 439,849 shares pursuant to the evergreen provision of the 2021 ESPP. As of September 30, 2023, 1,215,623 shares were available for issuance under the 2021 ESPP.

The following summarizes stock option activity:


	Number of options			Weighted average exercise price		Weighted average remaining contractual term (years)		Aggregate intrinsic value (in thousands)
Outstanding—December 31, 2022		7,436,339		$	9.14		8.5	$	12,440
Granted		3,241,800			7.54		—		—
Exercised		(549,508	)		3.67		—		—
Forfeited		(639,512	)		8.37		—		—
Outstanding—September 30, 2023		9,489,119		$	8.93		8.1	$	3,401
Vested or expected to vest—September 30, 2023		9,489,119		$	8.93		8.1	$	3,401
Exercisable—September 30, 2023		3,700,455		$	9.02		7.2	$	2,452

The aggregate intrinsic value of options granted is calculated as the difference between the exercise price of the options and the estimated fair value of the Company’s common stock.

Unvested restricted stock awards were granted to employees under the 2020 Plan. Restricted stock awards generally vest over a four year period provided the individual remains in continuous service of the Company.

The following summarizes restricted stock award activity:


	Number of shares			Weighted average grant date fair value
Unvested restricted stock as of December 31, 2022		122,271		$	1.04
Vested		(97,064	)	$	0.75
Forfeited		(5,720	)	$	2.00
Unvested restricted stock as of September 30, 2023		19,487		$	2.18

The aggregate fair value of restricted stock awards that vested during the nine months ended September 30, 2023 and 2022 was $0.2 million and $1.2 million, respectively. The weighted average grant date fair value of restricted stock awards that vested during the nine months ended September 30, 2023 and 2022 was $0.75 and $0.77, respectively.

Starting in 2022, the Company granted restricted stock units, or RSUs, to employees under the 2021 Plan. Each of the RSUs represents the right to receive one share of the Company’s common stock upon vesting. The RSUs will vest over two years provided the individual remains in continuous service of the Company. Accordingly, stock-based compensation expense for each RSU is recognized on a straight-line basis over the vesting term. The fair value of each RSU is based on the closing price of the Company’s common stock on the date of grant.

The following summarizes restricted stock unit activity:


	Number of shares			Weighted average grant date fair value
Unvested restricted stock units as of December 31, 2022		91,000		$	10.11
Granted		206,665		$	7.55
Vested		(43,600	)	$	10.11
Forfeited		(3,800	)	$	10.11
Unvested as of September 30, 2023		250,265		$	8.00

The aggregate fair value of restricted stock units that vested during the nine months ended September 30, 2023 was $0.3 million. The weighted average grant date fair value of restricted stock units that vested during nine months ended September 30, 2023 was $10.11. No restricted stock units vested during the nine months ended September 30, 2022.

Stock-based compensation expense is classified as follows (in thousands):


	Nine months ended September 30,
	2023		2022
Research and development	$	6,729	$	4,289
General and administrative		5,866		4,049
Total stock-based compensation expense	$	12,595	$	8,338

As of September 30, 2023 total unrecognized stock–based compensation cost related to unvested stock options and restricted stock units was $33.1 million and $1.6 million, respectively. The Company expects to recognize this remaining cost over a weighted average period of 2.5 years and 1.5 years, respectively.