	Three Months Ended March 31,
	2021		2020
	(In thousands)
Research and development	$	4,363	$	3,111
General and administrative		4,437		3,531
Total stock-based compensation	$	8,800	$	6,642

	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted common stock as of December 31, 2020		186,425		$	6.95
Vested		(85,544	)		6.95
Unvested restricted common stock as of March 31, 2021		100,881		$	6.95

As of March 31, 2021, total unrecognized stock-based compensation related to unvested restricted common stock was $0.5 million, which the Company expects to recognize over a remaining weighted-average period of 0.6 years.

On January 1, 2021, the Company added 3,965,813 shares to the shares reserved for issuance under the 2019 Equity Incentive Plan (2019 Plan). As of March 31, 2021, the Company had reserved 19,178,644 shares of common stock for issuance under the 2019 Plan, of which 6,544,211 shares were available for issuance.

Activity for the options to purchase common stock shown below (in thousands, except share and per share amounts):

	Number of Options			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (In years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2020		12,654,998		$	13.96
Granted		521,748			16.02
Exercised		(415,386	)		9.33
Forfeited		(126,927	)		16.03
Outstanding as of March 31, 2021		12,634,433		$	14.18		8.6	$	84,326
Exercisable as of March 31, 2021		3,893,199		$	13.27		8.0	$	28,021
Vested and expected to vest as of March 31, 2021		12,634,433		$	14.18		8.6	$	84,326

The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying stock options and the fair value of the Company’s common stock for stock options that were in-the-money. As of March 31, 2021, total unrecognized stock-based compensation related to unvested stock options was $79.4 million, which the Company expects to recognize over a remaining weighted-average period of 2.3 years.

The 2019 Employee Stock Purchase Plan (2019 ESPP) enables eligible employees of the Company to purchase shares of common stock at a discount. As of March 31, 2021, the Company has reserved for issuance 2,524,063 shares of common stock pursuant to the 2019 ESPP. Each offering period is approximately six months long. 2019 ESPP participants will purchase shares of common stock at a price per share equal to 85% of the lesser of (1) the fair market value per share of the common stock on the first trading day of the offering period or (2) the fair market value of the common stock on the purchase date.