On July 23, 2021, the Company’s board of directors adopted and its stockholders approved the 2021 Stock Option and Incentive Plan (the "2021 Plan"), which became effective on July 28, 2021. The 2021 Plan provides for the grant of incentive stock options, non-qualified stock options, stock appreciation rights, restricted stock awards, restricted stock units, unrestricted stock awards, cash-based awards and dividend equivalent rights. The number of shares of Class A common stock reserved for issuance under the 2021 Plan is subject to increase on each January 1 thereafter by 5.0% of the number of shares of the Company’s Class A and Class B

common stock outstanding on the immediately preceding December 31 or such lesser number of shares determined by the Company’s board of directors or compensation committee of the board of directors. The shares of Class A common stock underlying any awards that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by the Company prior to vesting, satisfied without the issuance of stock, expired or are otherwise terminated (other than by exercise) under the 2021 Plan and the Company’s previously outstanding 2017 Stock Option and Grant Plan will be added back to the shares of Class A common stock available under the 2021 Plan. As of December 31, 2023, 5,691,917 shares of Class A common stock remained available for future issuance under the 2021 Plan. The number of authorized shares reserved for issuance was increased by 3,203,257 shares effective as of January 1, 2024, in accordance with the provisions of the 2021 Plan described above.

On July 23, 2021, the Company’s board of directors adopted and its stockholders approved the 2021 Employee Stock Purchase Plan, which became effective on July 28, 2021. The Company’s board of directors approved the amendment and restatement of the 2021 Employee Stock Purchase Plan in its entirety on June 16, 2022 (as amended and restated, the “ESPP”). The ESPP permits eligible employees to purchase shares of Class A common stock at a discount and consists of consecutive, overlapping 12-month offering periods, each consisting of two six-month purchase periods beginning in December and June of each year. On the first day of each offering period, the Company will grant to each employee who is enrolled in the ESPP an option to purchase up to a whole number of shares of Class A common stock. The purchase price of each of the shares purchased in a given purchase period will be 85% of the closing price of a share of the Class A common stock on the first day of the offering period or the last day of the purchase period, whichever is lower. The number of shares of Class A common stock that may be issued under the ESPP will automatically increase on each January through January 1, 2031, by the least of (i) 473,064 shares of Class A common stock, (ii) 1% of the number of shares of the Company’s Class A and Class B common stock outstanding on the immediately preceding December 31 or (iii) such lesser number of shares as determined by the administrator of the ESPP.

The initial offering period of the ESPP commenced on December 1, 2022. During the year ended December 31, 2023, 13,187 shares were sold under the ESPP. As of December 31, 2023, 1,406,005 shares remained available for issuance and sale under the ESPP. The number of authorized shares reserved for issuance under the ESPP was increased by 473,064 shares effective as of January 1, 2024, in accordance with the provisions of the ESPP described above.

The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted:


	Year Ended December 31,
	2023			2022
Risk-free interest rate		3.8	%		2.1	%
Expected volatility		79.9	%		77.5	%
Expected dividend yield		—			—
Expected term (in years)		6.0			6.0

The following table summarizes the Company’s option activity since December 31, 2022:


				Weighted		Weighted
				Average		Average		Aggregate
	Number			Exercise		Contractual		Intrinsic
	of Shares			Price		Term		Value
						(in years)		(in thousands)
Outstanding as of December 31, 2022		6,453,741		$	9.94		8.49	$	128,285
Granted		2,823,230			32.58
Exercised		(1,222,294	)		11.28
Forfeited		(19,922	)		21.61
Outstanding as of December 31, 2023		8,034,755		$	17.66		8.06	$	449,403
Vested and expected to vest as of December 31, 2023		8,034,755		$	17.66		8.06	$	449,403
Options exercisable as of December 31, 2023		3,426,961		$	10.23		7.50	$	217,126

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2023 and 2022 was $52.3 million and $10.7 million, respectively.

The weighted average grant-date fair value of stock options granted during the years ended December 31, 2023 and 2022 was $23.01 per share and $11.54 per share, respectively.

The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands):


	Year Ended December 31,
	2023		2022
Research and development expenses	$	11,600	$	4,214
General and administrative expenses		13,963		6,111
	$	25,563	$	10,325

As of December 31, 2023, total unrecognized compensation cost related to common stock awards was $70.0 million, which is expected to be recognized over a weighted average period of 2.7 years.