In February 2021, the Company adopted the 2021 Incentive Award Plan (2021 Plan), which became effective on the completion of the Company’s IPO. The 2021 Plan provides for a variety of stock-based compensation awards, including stock options, restricted stock awards (RSAs), and restricted stock units (RSUs). In conjunction with adopting the 2021 Plan, the Company discontinued the 2018 Equity Incentive Plan with respect to new equity awards.

The 2021 Plan provides for an annual increase in the shares available for issuance thereunder, to be added on the first day of each year, beginning in 2022 and ending in 2031, equal to the lesser of (i) 5% of the outstanding shares of the Company’s common stock on the last day of the immediately preceding year, or (ii) such smaller number of shares determined by the board of directors or an authorized committee of the board of directors. As of December 31, 2021, the total number of shares available for future issuance of awards under the 2021 Plan was 13.2 million. As a result of the operation of this provision, on January 1, 2022, an additional 9.5 million shares became available for issuance under the 2021 Plan.

In February 2021, the Company adopted the 2021 Employee Stock Purchase Plan (2021 ESPP), which became effective on the completion of the Company’s IPO. The 2021 ESPP allows eligible employees to purchase shares of the Company’s common stock at a discount through payroll deductions of up to 15% of their earnings, subject to plan limitations. Unless otherwise determined by the Company’s board of directors, employees may purchase shares at 85% of the lower of the fair market value of the Company’s common stock on the first date of an offering or on the purchase date. The Company may specify offerings with durations of not more than 27 months and may specify shorter purchase periods within each offering.

The 2021 ESPP also provides for an annual share increase, to be added on the first day of each year, beginning in 2022 and ending in 2031, equal to the lesser of (i) 1% of the outstanding shares of the Company’s common stock on the last day of the immediately preceding year and (ii) such number of shares determined by the board of directors or authorized committee of the board of directors, provided that no more than 27.9 million shares may be issued under the 2021 ESPP. As of December 31, 2021, the total number of shares available for future issuance pursuant to the 2021 ESPP was 1.9 million. As a result of the operation of this provision, on January 1, 2022, an additional 1.9 million shares became available for issuance under the 2021 ESPP.

Stock-based compensation expense is recognized in the consolidated statements of operations as follows:

	Year Ended December 31,
	2021		2020		2019
	(in thousands)
Research and development	$	15,239	$	4,888	$	1,246
General and administrative		7,123		940		251
Total stock-based compensation expense	$	22,362	$	5,828	$	1,497

Unrecognized stock-based compensation costs related to unvested awards and the weighted-average period over which the costs are expected to be recognized as of December 31, 2021 are as follows:

	Stock Options		RSAs		RSUs
Unrecognized stock-based compensation expense (in thousands)	$	81,140	$	1,892	$	1,220
Weighted-average period costs expected to be recognized (in years)		3.2		1.5		1.5

	Stock Options (in thousands)			Weighted-Average Exercise Price per Share		Weighted-Average Remaining Contractual Life (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2020		15,677		$	4.52
Granted		4,165			22.57
Exercised		(1,586	)		2.00
Forfeited/Cancelled		(919	)		6.88
Outstanding as of December 31, 2021		17,337		$	8.96		8.7	$	141,718
Exercisable as of December 31, 2021		3,215		$	2.61		8.1	$	41,382

The fair value of stock options granted to employees, directors, and consultants was estimated on the date of grant using the Black-Scholes option pricing model using the following assumptions:

	Year Ended December 31,
Assumptions	2021	2020	2019
Risk free interest rate	0.46% - 1.38%	0.36% - 1.51%	1.53% - 2.62%
Expected volatility	70%	70%	70%
Expected term (years)	5.50 - 6.40	6.25 - 6.75	6.02 - 6.25
Expected dividend	0%	0%	0%

The following table summarizes additional information related to stock option activity:

	Year Ended December 31,
	2021		2020		2019
Weighted average grant date fair value per share for options granted	$	14.22	$	3.52	$	0.92
Aggregate intrinsic value of stock options exercised (in thousands)	$	29,880	$	605	$	25

	RSAs (in thousands)			RSAs Weighted-Average Grant Date Fair Value per Share		RSUs (in thousands)			RSUs Weighted-Average Grant Date Fair Value per Share
Unvested shares as of December 31, 2020		10,079		$	0.33		326		$	1.44
Granted		-			-		57			21.37
Vested		(5,691	)		0.25		(225	)		1.44
Forfeited		(23	)		1.00		(17	)		1.44
Unvested shares as of December 31, 2021		4,365		$	0.43		141		$	9.43

The fair value of vested RSAs was $1.5 million, $1.5 million and $1.0 million, respectively, for the years ended December 31, 2021, 2020 and 2019. The fair value of vested RSUs was $4.1 million for the year ended December 31, 2021 and immaterial for each of the years ended December 31, 2020 and 2019.