In December 2015, the Company’s board of directors adopted and approved the 2015 Stock Incentive Plan (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 February 2021, the Company’s board of directors adopted and the 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. As of March 31, 2021, the Company had 2,693,301 shares 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 the remaining 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:

	Three Months Ended March 31,
	2021			2020
Fair value of common stock	$	22.51		$	2.18
Expected term (in years)		6.0			5.9
Expected volatility		79.7	%		75.9	%
Risk-free interest rate		0.7	%		0.7	%
Dividend yield		—			—

The following table summarizes the Company’s stock option activity for three months ended March 31, 2021:

	Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding at January 1, 2021		5,084,679		$	3.79		9.42	$	32,086
Granted		394,934		$	22.51
Exercised		(38,216	)	$	1.31
Outstanding at March 31, 2021		5,441,397		$	5.16		9.24	$	206,544
Exercisable at March 31, 2021		989,860		$	1.80		8.85	$	40,772

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 end of the period.

The weighted-average grant-date fair value of stock options granted during the three months ended March 31, 2021 and 2020 was $15.27 and $1.42 per share, respectively. As March 31, 2021, total unrecognized compensation expense related to stock options was $13.9 million which is expected to be recognized over a weighted-average period of 3.11 years.

As of March 31, 2021, options for 359,719 shares with a weighted average exercise price of $1.35 were exercised and unvested. The underlying proceeds from the unvested exercises of $0.7 million is recorded in other current liabilities on the consolidated balance sheet.

	Three Months Ended March 31,
(in thousands)	2021		2020
Research and development	$	449	$	47
General and administrative		442		65
Total stock-based compensation expense	$	891	$	112