In December 2015, the Company’s board of directors adopted and approved the 2015 Stock Incentive Plan (as amended to date, the “2015 Plan”). The 2015 Plan provided for the granting of incentive stock options, non-statutory stock options, restricted stock awards and other stock-based awards to eligible employees, officers, directors, consultants and advisors as determined by the Company’s board of directors.

In October 2020, the Company granted an option to purchase 294,117 shares of its common stock to an advisor outside of the 2015 Plan, with a strike price of $28.29 per share. Such grant was made outside of the 2015 Plan but is subject to the terms and conditions of such plan.

In February 2021, the Company’s board of directors adopted and stockholders approved the 2021 Equity Incentive Plan (the “2021 Plan”). The 2021 Plan became effective on February 5, 2021, following which no further grants were or will be made under the 2015 Plan. The 2021 Plan provides for the grant of stock options, non-statutory stock options, restricted stock awards, restricted stock unit awards, stock appreciation rights, performance stock awards and other forms of stock compensation to our employees, consultants and directors.

The number of shares of our common stock reserved for issuance under our 2021 Plan will automatically increase on January 1 of each year through January 1, 2031, by 4.0% of the total number of shares of common stock outstanding on December 31 of the preceding calendar year. Any grants that expire or are canceled, terminated,

forfeited, or fail to vest are allowed to be reissued under 2021 Plan. As of December 31, 2021, the Company had 2,643,187 shares of its common stock available for future issuance under the 2021 Plan.

The Company’s stock options generally vest over 48 months with 25% vesting after one year followed by ratable monthly vesting over three years and have a contractual term of 10 years. The weighted-average assumptions used principally in determining the fair value of options granted were as follows:


	Year Ended December 31,
	2021			2020
Fair value of common stock	$	23.63		$	4.29
Expected term (in years)		6.0			6.0
Expected volatility		78.4	%		77.3	%
Risk-free interest rate		0.9	%		0.4	%
Dividend yield		—			—

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


	Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2020		5,084,679		$	3.79		9.42	$	32,086
Granted		730,884		$	19.46
Vested and exercised		(442,947	)	$	2.02
Forfeited		(285,698	)	$	4.10
Expired		(126	)	$	18.00
Outstanding at December 31, 2021		5,086,792		$	6.17		8.00	$	38,000
Exercisable at December 31, 2021		1,669,852		$	4.15		7.49	$	14,384

The aggregate intrinsic value of 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 common stock as of the respective date.

The weighted-average grant-date fair value of stock options granted during the years ended December 31, 2021 and 2020 was $13.14 and $2.19 per share, respectively. As of December 31, 2021, total unrecognized compensation expense related to stock options was $13.1 million which is expected to be recognized over a weighted-average period of 2.51 years. The intrinsic value of stock options exercised was $3.1 million for the year ended December 31, 2021.

During the year ended December 31, 2020, options for 159,197 shares with a weighted-average exercise price of $2.59 and a weighted-average grant date fair value of $1.90 were exercised but unvested at the time of exercise. As of December 31, 2021 and 2020, options for 200,687 and 388,157 shares with weighted average exercise prices of $1.90 and $1.86 were exercised and unvested, respectively. The underlying proceeds from the unvested exercises of $0.4 million and $0.7 million is recorded in other current liabilities as of December 31, 2021 and 2020, on the consolidated balance sheet.

Stock-based compensation expense was allocated as follows:


	Year Ended December 31,
(in thousands)	2021		2020
Research and development	$	2,281	$	665
General and administrative		2,034		677
Total stock-based compensation expense	$	4,315	$	1,342