The Company adopted the 2020 Equity Incentive Plan (the “2020 Plan”) on December 18, 2020. The 2020 Plan reserved 4,327,799 shares of common stock to grant stock-based compensation awards, including stock options and restricted stock awards, to employees and non-employees. As of April 9, 2021, the Company ceased granting awards under the 2020 Plan. However, 2020 Plan awards will remain subject to the terms of the 2020 Plan.

In April 2021, the Company adopted the 2021 Equity Incentive Plan (the “2021 Plan”). Options granted under the 2021 Plan expire no later than 10 years from the date of grant. The exercise price of options granted under the 2021 Plan must at least be equal to the fair market value of the Company’s common stock on the date of grant. With respect to any participant who owns more than 10% of the voting power of all classes of the Company’s outstanding stock, the term of an incentive stock option granted to such participant must not exceed five years and the exercise price must equal at least 110% of the fair market value on the grant date. Employee stock options generally vest 1/16th quarterly over four years subject to continued service to the Company.

Subject to adjustment in the case of certain capitalization events as provided in the 2021 Plan, the Company initially reserved 3,370,000 shares of the Company’s common stock for issuance pursuant to awards under the 2021 Plan. The 2021 Plan is administered by the Compensation Committee of the Company’s Board of Directors. The number of shares of the Company’s common stock available for issuance under the 2021 Plan will also include an annual increase on the first day of each fiscal year beginning in 2022 and ending in 2031, equal to the lesser of (i) 5% of the Company’s common stock outstanding at December 31 of the immediately preceding year, or (ii) such number of shares as determined by the Company’s Board of Directors. As of December 31, 2023, 799,485 shares of common stock remained available for issuance under the 2021 Plan. Effective January 1, 2024, the number of shares of common stock available under the 2021 Plan increased by 1,798,926 shares pursuant to the evergreen provision of the 2021 Plan.

On November 17, 2023, the Company's board of directors adopted the 2023 Inducement Equity Plan (Inducement Plan) pursuant to which the Company reserved 2,000,000 shares of common stock, to be used exclusively for grants of equity-based awards to individuals who were not previously employees or directors of the Company, as an inducement material to the individual’s entry into employment with the Company within the meaning of Rule 5635(c)(4) of the Nasdaq Listing Rules. The Inducement Plan provides for the grant of equity-based awards in the form of non-qualified stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance stock unit awards, unrestricted stock awards, dividend equivalent rights or Other Stock Based Awards (as defined in the Inducement Plan). The Inducement Plan was adopted by the board of directors without stockholder approval pursuant to Rule 5635(c)(4) of the Nasdaq Listing Rules. As of December 31, 2023, 2,000,000 shares of common stock remained available for issuance under the Inducement Plan.

The Company adopted a 2021 Employee Stock Purchase Plan (ESPP) in April 2021. The ESPP enables eligible employees of the Company and designated affiliates to purchase shares of common stock at a discount of 15%. Subject to adjustment in the case of certain capitalization events, a total of 306,000 common shares of the Company were available for purchase at adoption of the ESPP. Pursuant to the ESPP, the annual share increase pursuant to the evergreen provision is determined based on the lesser of (i) 1% of the Company’s common stock outstanding at December 31 of the immediately preceding year, or (ii) such number of shares as determined by the Company’s Board of Directors. As of December 31, 2023, 451,886 shares of common stock remained available for issuance under the ESPP. Effective January 1, 2024, the number of shares of common stock available under the ESPP increased by 359,785 shares pursuant to the evergreen provision of the ESPP.

Total stock-based compensation expense related to the 2020 Plan, 2021 Plan, and ESPP was recorded in the statements of operations and allocated as follows (in thousands):


	Year Ended December 31,
	2023		2022		2021
Research and development	$	6,933	$	4,678	$	2,637
General and administrative		7,198		5,658		3,597
Total stock-based compensation expense	$	14,131	$	10,336	$	6,234

As of December 31, 2023, there was $27.6 million of total unrecognized compensation cost related to stock options, restricted stock awards, and ESPP under the Plans. The unrecognized stock-based compensation cost is expected to be recognized over a weighted-average period of 2.3 years.

The following table summarizes stock option activity:


	Number of Options Outstanding			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (in thousands)
Balance as of December 31, 2022		5,341,975		$	8.79		8.6	$	6,875
Granted		2,441,795			10.66
Exercised		(125,192	)		7.48
Cancelled		(300,971	)		11.14
Balance as of December 31, 2023		7,357,607		$	9.34		8.1	$	42,071
Options exercisable as of December 31, 2023		3,471,556		$	8.87		7.7	$	20,783

The fair value of stock options was estimated using the following weighted-average assumptions:


	Year Ended December 31,
	2023			2022			2021
Expected term in years		6.0			6.0			6.0
Expected volatility		79.9	%		93.1	%		91.0	%
Risk-free interest rate		3.8	%		3.1	%		1.1	%
Dividend yield		—			—			—
Weighted average fair value of options granted	$	7.57		$	5.25		$	7.28

The aggregate intrinsic value of options exercised for the years ended December 31, 2023, 2022, 2021 was $1.7 million, $0.4 million, and $0.5 million, respectively. Intrinsic values are calculated as the difference between the exercise price of the underlying options and the fair value of the common stock on the date of exercise.

The fair value of ESPP was estimated using the following weighted-average assumptions:


	Year Ended December 31,
	2023			2022			2021
Expected term in years		1.3			1.3			1.3
Expected volatility		80.9	%		95.2	%		91.3	%
Risk-free interest rate		4.9	%		2.5	%		0.3	%
Dividend yield		—			—			—

The Company granted 824,429 restricted stock awards to employees and non-employees during the fourth quarter of 2020 that vest quarterly over four years. Restricted stock awards are share awards that entitle the holder to receive freely tradeable shares of the Company’s common stock. The underlying shares are outstanding as of the issuance date. Any unvested shares are subject to forfeiture in the case that the grantee’s service terminates prior to vesting of the restricted stock.

The following table summarizes the restricted stock activity:


	Number of Restricted Stock Awards			Weighted-Average Grant Date Fair Value
Balance, December 31, 2022		292,236		$	4.04
Granted		—			—
Released		(180,316	)		4.02
Forfeited		—			—
Balance, December 31, 2023		111,920		$	4.06