As of each balance sheet date, the Company had reserved shares of common stock for issuance in connection with the following:

	March 31, 2021		December 31, 2020
Conversion of outstanding shares of convertible preferred stock		—		16,079,230
Options outstanding under incentive award plans		2,544,778		2,466,670
Shares available for future grant under incentive award plans		2,327,433		17,556
Shares available for future grant under employee stock purchase plans		240,000		—
Total shares reserved		5,112,211		18,563,456

Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to receive dividends, if any, as may be declared by the Company’s board of directors, subject to the preferential dividend rights of the convertible preferred stock. Through March 31, 2021, no cash dividends have been declared or paid by the Company.

The Company has two stock-based compensation plans, the 2017 Incentive Award Plan (the “2017 Plan”) and the 2021 Incentive Award Plan (the “2021 Plan”) which was adopted in February 2021. Each plan, while effective, authorizes the granting of equity awards to employees and directors of the Company, as well as non-employee consultants. Stock options granted to employees and nonemployees under the plans generally vest over four years and allows the holder of the option to purchase common stock at a stated exercise price. Options granted under the plans generally expire ten years after the date of grant. The Company recognizes the stock-based compensation expense over the requisite service period of the individual grantees, which generally equals the vesting period.

In January 2021, the Company's board of directors approved the 2021 Plan which permits the granting of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance bonus awards, performance stock unit awards and other stock awards to employees, directors, officers and consultants. In February 2021, 2,400,007 shares were authorized for issuance under the 2021 Plan. The 2021 Plan is successor to the 2017 Incentive Award Plan and no additional awards may be issued from the 2017 Plan. However, the 2017 Plan will continue to govern the terms and conditions of the outstanding awards granted under this plan. Shares of common stock subject to awards granted under the 2017 Plan that are forfeited or lapse unexercised and which following the effective date of the 2021 Plan are not issued under the 2017 Plan will be available for issuance under the 2021 Plan. As of March 31, 2021, 2,327,433 shares of the Company’s common stock were available for future grants under the 2021 Plan.

The 2021 Employee Stock Purchase Plan (the “2021 ESPP”) was approved by the Company’s board of directors in January 2021. In February 2021, a total of 240,000 shares were initially reserved for issuance under this plan, which shall be cumulatively increased on the first day of each year beginning in 2022 and ending in 2031 equal to the lesser of (i) 1% of the shares outstanding (on an as converted basis) on the last day of the immediately preceding fiscal year and (ii) such number of shares as may be determined by the Board.

The following table summarizes the stock option activity for all stock plans during the quarter ended March 31, 2021:

	Number of Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)		(in thousands)
Outstanding as of December 31, 2020		2,466,670		$	8.03		9.61	$	21,678
Granted		106,000			20.12				—
Forfeited		(27,892	)		7.18				—
Outstanding as of March 31, 2021		2,544,778		$	8.57		8.64	$	33,976
Exercisable, March 31, 2021		2,009,995		$	8.16		8.67	$	27,742
Vested and expected to vest, March 31, 2021		2,544,778		$	8.57		8.64	$	33,976

The aggregate intrinsic value of stock 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 Company’s common stock.

As of March 31, 2021, there was $26.0 million of unrecognized stock-based compensation expense related to unvested stock options which is estimated to be recognized over a period of 3.53 years.

The Company estimated the fair value of options granted using a Black-Scholes option pricing model with the following assumptions presented on a weighted average basis:

	Three Months Ended March 31,
	2021			2020
Expected option life		5.95			5.88
Expected volatility		66.70	%		59.95	%
Risk-free interest rate		0.76	%		0.58	%
Expected dividend yield		—	%		—	%
Fair value of underlying common stock	$	20.13		$	9.24
Fair value of option	$	11.94		$	5.67

Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

The following table summarizes the stock award activity for all stock plans during the quarter ended March 31, 2021:

	Number of Shares			Grant-Date Fair Value
Unvested restricted common stock as of December 31, 2020		83,334		$	1.96
Forfeited		(11,905	)		1.96
Unvested restricted common stock as of March 31, 2021		71,429		$	1.96

As of March 31, 2021, unrecognized stock-based compensation expense related to restricted stock granted by the Company was nominal and is estimated to be recognized over a period of 0.02 years.

Stock-based compensation expense was classified in the condensed consolidated statements of operations and comprehensive loss as follows:

	Three Months Ended March 31,
(in thousands)	2021		2020
Research and development expense	$	482	$	78
General and administrative expense		1,350		124
Total stock-based compensation expense	$	1,832	$	202