As of March 31, 2021, the Company has granted service-based awards, which vest over a defined period of service, and performance-based and market-based awards, which vest upon the achievement of defined conditions. Service-based awards generally vest over a four-year period, with the first 25% vesting following twelve months of continued employment or service, and the remainder vesting in twelve quarterly installments over the following three years.

The 2020 Stock Option and Incentive Plan (the 2020 Plan), was adopted by the Company’s board of directors in October 2020, approved by the Company’s stockholders in October 2020 and became effective as of October 12, 2020. The 2020 Plan provides for the grant of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units, and other stock-based awards. The number of shares of the Company’s common stock initially reserved for issuance under the 2020 Plan was 1,043,402 shares. The number of shares reserved shall be annually increased on the first day of each calendar year beginning on January 1, 2021 and ending on and including January 1, 2030, equal to the lesser of 5% of the number of shares of common stock outstanding on the final day of the immediately preceding calendar year or such lesser number of shares determined by the compensation committee. As of January 1, 2021, 938,384 additional shares of common stock were reserved for issuance under the 2020 Plan.

The shares of the Company’s common stock subject to outstanding awards under the 2015 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right will be added back to the shares of common stock available for issuance under the 2020 Plan. As of March 31, 2021, there were 1,575,840 shares available for future issuance under the 2020 Plan.

The Company’s stock options expire after approximately ten years from the date of grant. As of March 31, 2021, the Company does not hold any treasury shares. Upon stock option exercise, the Company issues new shares and delivers them to the participant.

The Company’s 2020 Employee Stock Purchase Plan, (the ESPP) was adopted by our board of directors in October 2020, approved by the Company’s stockholders in October 2020 and became effective October 12, 2020. The ESPP initially provides participating employees with the opportunity to purchase up to an aggregate of 208,680 shares of the Company’s common stock. The number of shares of common stock reserved for issuance under the ESPP will automatically increase on each January 1st, beginning on January 1, 2021 and ending on January 1, 2030, by the lesser of (i) 834,720 shares of common stock, (ii) 0.5% of the outstanding shares of common stock on the immediately preceding December 31st or (iii) such lesser number of shares as determined by the administrator of the ESPP. There was no increase in the number of common stock reserved for issuance under the ESPP as of March 31, 2021.

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

	NUMBER OF SHARES			WEIGHTED AVERAGE EXERCISE PRICE PER SHARE		WEIGHTED AVERAGE REMAINING CONTRACTUAL TERM		AGGREGATE INTRINSIC VALUE (1)
						(In years)		(In thousands)
Outstanding as of December 31, 2020		4,543,318		$	8.52		7.22	$	108,048
Granted		813,000			26.84
Exercised		(46,807	)		6.76
Forfeited/Cancelled		(161,565	)		13.76
Outstanding as of March 31, 2021		5,147,946			11.26		6.99		29,108
Exercisable as of March 31, 2021		2,707,703			6.87		5.46		22,236
Vested and expected to vest as of March 31, 2021		5,147,946			11.26		6.99		29,108

⁽¹⁾	Aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock as of March 31, 2021 and December 31, 2020.

The weighted average grant date fair value per share of options granted during the three months ended March 31, 2021 and 2020 was $16.85 per share and $6.33 per share, respectively.

The aggregate intrinsic value of stock options exercised during the three months ended March 31, 2021 and 2020 was $1.0 million and less than $0.1 million, respectively.

The key assumptions used in the Black-Scholes option pricing model on the date of grant for options with service-based vesting conditions were as follows, presented on a weighted average basis:

	THREE MONTHS ENDED MARCH 31,
	2021			2020
Risk-free interest rate		0.65	%		1.63	%
Expected term (in years)	6.25			6.25
Expected volatility		70.25	%		66.90	%
Expected dividend yield		0.00	%		0.00	%
Fair value per share of common stock	$	16.85		$	6.33

The Company did not grant any performance-based awards during the three months ended March 31, 2021 and 2020.

The following table presents the components and classification of stock-based compensation expense (in thousands):

	THREE MONTHS ENDED MARCH 31,
	2021		2020
Research and development	$	1,058	$	1,033
General and administrative		1,215		717
	$	2,273	$	1,750
Employee	$	2,239	$	1,551
Non-employee		34		199
	$	2,273	$	1,750

The Company recognized less than $0.1 million during the three months ended March 31, 2021 related to performance-based awards that vested upon achievement of their underlying performance condition. The Company did not recognize expense related to performance-based awards during the three months ended March 31, 2020 because the associated performance conditions were not deemed probable of achievement.

As of March 31, 2021, the total unrecognized compensation expense related to the Company’s option awards was $24.6 million, which the Company expects to recognize over a weighted-average period of approximately 3.14 years.