The 2021 Equity Incentive Plan (the “2021 Plan”) provides for an increase of shares 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. Effective January 1, 2023, the number of shares of common stock available under the 2021 Plan increased by 1,493,437 shares pursuant to the evergreen provision of the 2021 Plan. As of March 31, 2023, 1,288,702 shares of common stock remained available for issuance under the 2021 Plan.

Total stock-based compensation expense related to the Company’s equity incentive plan and employee stock purchase plan was recorded in the statements of operations and allocated as follows (in thousands):


	Three Months Ended
	March 31,
	2023		2022
Research and development	$	1,474	$	1,012
General and administrative		1,759		1,318
Total stock-based compensation expense	$	3,233	$	2,330

As of March 31, 2023, there was $30.5 million of total unrecognized stock-based compensation expense which is expected to be recognized over a weighted-average period of 2.7 years.

The Company estimates the fair value of stock options using the Black-Scholes option pricing model and the fair value of these stock options was estimated using the following weighted-average assumptions:


	Three Months Ended
	March 31,
	2023			2022
Expected term in years		6.0			6.0
Expected volatility		79	%		91	%
Risk-free interest rate		3.6	%		2.1	%
Dividend yield		—			—
Weighted average fair value of share-based awards granted	$	5.46		$	4.91

The following table summarizes stock option activity:


	Outstanding Options
	Shares			Weighted- Average Exercise		Weighted- Average Remaining Contractual Term (Years)
Balance, December 31, 2022		5,341,975		$	8.79		8.6
Granted		1,801,950		$	7.76
Exercised		(44,501	)	$	9.15
Cancelled		(150,343	)	$	9.29
Balance, March 31, 2023		6,949,081		$	8.51		8.8
Exercisable, March 31, 2023		2,118,829			8.77		8.3

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		(45,079	)		4.02
Forfeited		—			—
Balance, March 31, 2023		247,157		$	4.04