The Company’s board of directors approved the 2019 Equity Incentive Plan (the “2019 Plan”), which became effective in September 2019. The Company initially reserved 2,844,193 shares of common stock for issuance under the 2019 Plan. In addition, the number of shares of common stock reserved for issuance under the 2019 Plan will automatically increase on the first day of January for a period of up to ten years, commencing on January 1, 2020, in an amount equal to 4% of the total number of shares of the Company’s capital stock outstanding on the immediately preceding December 31, or a lesser number of shares determined by the Company’s board of directors. As of December 31, 2021, there were 1,428,304 shares of common stock available for issuance as future option grants under the 2019 Plan.

The board of directors determines the period over which options become exercisable and options generally vest over a four-year period, with 25% of options vesting on the first anniversary of employment, and thereafter, the remaining options vesting quarterly, over the following 36-month period. The options will expire within ten years from the date of grant. The exercise price of awards granted will not be less than the estimated fair value of the shares on the date of grant.

In October 2019, the Company adopted the 2019 Employee Stock Purchase Plan (“ESPP”), which became effective in November 2019. The Company initially reserved 225,188 shares of common stock for purchase under the ESPP. The number of shares of common stock reserved for issuance under the ESPP will automatically increase on the first day of January for a period of up to ten years, in an amount equal to 1% of the total number of shares of the Company’s common stock outstanding on the immediately preceding December 31, or a lesser number of shares determined by the Company’s board of directors. Purchases will be accomplished through the participation of discrete offering periods and each offering is expected to be 6 months long. For each offering period, ESPP participants will purchase shares of common stock at a price per share equal to 85% of the lesser of the fair market value of the Company’s common stock on (1) the first trading day of the applicable offering period or (2) the last trading day of the applicable offering period. The first six month offering period under the ESPP commenced on January 1, 2020.

As of December 31, 2021, there were 544,410 shares of common stock available for issuance under the ESPP.

The Company recorded stock-based compensation for the periods presented as follows (in thousands):

	Year Ended December 31,
	2021		2020
Research and development	$	2,966	$	1,250
General and administrative		5,712		2,557
Total stock-based compensation	$	8,678	$	3,807

The fair value of stock options granted for the periods presented was estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions:

	Year Ended December 31,
	2021		2020
Expected term (years)	5.5-6.1		4.0-6.1
Expected volatility	86.9-91.9%		86.4-97.6%
Risk-free interest rate	0.7-1.3%		0.2-1.5%
Expected dividend		—		—

The following table summarizes stock option activity for the year ended December 31, 2021:

						Weighted
				Weighted		Average
				Average		Remaining		Aggregate
	Number of			Exercise		Contractual		Intrinsic
	Options			Price		Term		Value
						(In years)		(In thousands)
Balance outstanding as of December 31, 2020		1,898,395		$	12.79		8.6	$	25,918
Granted		830,013			22.22
Exercised		(188,286	)		2.59
Cancelled and forfeited		(133,454	)		19.55
Balance outstanding as of December 31, 2021		2,406,668		$	16.46		8.1	$	9,970
Exercisable as of December 31, 2021		922,104		$	10.76		7.3	$	6,800

The estimated grant date fair value of stock options that vested during the year ended December 31, 2021, was $7.0 million. The weighted-average grant date fair value of stock options that were granted during the years ended December 31, 2021 and 2020 was $16.18 and $21.88 per share, respectively. As of December 31, 2021, there was $17.9 million of unrecognized stock-based compensation cost related to stock options granted under the 2019 Plan, which is expected to be recognized over a weighted-average period of 2.3 years.

In February 2021, the Company granted 56,545 service-based RSUs to its employees with a total grant date fair value of $1.3 million. The RSUs vest annually over a three-year period. During the year ended December 31, 2021, the Company recognized $0.3 million in expense related to the service-based RSUs. As of December 31, 2021, 11,651 RSUs were cancelled and 44,894 remain outstanding.

In February 2021, the Company granted 61,500 performance-based RSUs to certain executives, one-third of which vest on each one-year anniversary of the grant date subject to achievement of a development milestone and continued service to the Company. The performance-based RSUs have a total grant date fair value of $1.4 million. During the year ended December 31, 2021, the Company recognized $0.8 million in expense, using the accelerated attribution method, related to the performance-based RSUs. As of December 31, 2021, all of the performance-based RSUs remain outstanding. Subsequently, one-third of the performance-based RSUs vested in February 2022, upon achievement of the development milestone and satisfaction of the continued service condition.