In December 2019, the board of directors of the Company (the “Board”) adopted the 2019 Equity Incentive Plan (the “2019 Plan”). The 2019 Plan provides for the grant of share-based awards, including stock options and restricted stock units, to employees, directors, and non-employee service providers of the Company. In December 2019, the Board authorized 3,657,019 shares for future issuance under the 2019 Plan. In January 2021, the Board authorized an increase in shares available for issuance under the 2019 Plan to 9,496,699 shares. As of December 31, 2021, there are approximately 6,716,195 shares available for future grants.

The terms of the 2019 Plan permit certain option holders to exercise options before their options are vested. The shares of common stock granted upon early exercise that have not vested are subject to repurchase by the Company in the event of termination of the purchaser’s employment, at the price paid by the purchaser. While such shares have been issued, they are not considered outstanding for accounting purposes until they vest and are therefore excluded from shares used in determining loss per share until the repurchase right lapses and the shares are no longer subject to the repurchase feature. The liability is reclassified into common stock and additional paid-in capital as the shares vest and the repurchase right lapses. As of December 31, 2021 and 2020, there were no and 410,602 unvested shares, respectively, representing an early exercise liability of approximately $0 and $0.8 million, respectively.

In November 2021, the Company modified certain shares of an equity award that had previously been granted to the Company’s former Chief Executive Officer. The vesting of the unvested equity award was accelerated. During the year ended December 31, 2021, stock-based compensation expense of $0.3 million was recorded in connection with this modification and is included in general and administrative expenses in the accompanying consolidated statements of operations and comprehensive loss.

A summary of the Company’s stock option activity is as follows:


	Number of Shares			Weighted Average Exercise Price		Weighted Remaining Contract Term (in years)		Aggregate Intrinsic Value (in thousands)
Balance as of December 31, 2020		1,249,218		$	1.86		9.8		1,225
Granted		1,157,074			12.09
Exercised		(687,503	)		1.86
Forfeited		(30,000	)		12.20
Balance as of December 31, 2021		1,688,789		$	8.69		9.0	$	1,651
Exercisable and vested at December 31, 2021		493,459		$	7.26		8.9	$	771

The total intrinsic value of options exercised was $1.3 million and $0.4 million for the years ended December 31, 2021 and 2020, respectively.

The weighted average fair value of options to purchase common stock granted in the years ended December 31, 2021 and 2020 was $7.19 and $1.01, respectively.

The fair value of each stock option award is estimated on the grant-date using the Black-Scholes option pricing model. The inputs used below are subjective and require significant judgment to determine.


	Years Ended December 31,
	2021			2020
Expected term (in years)		5.70			5.40
Risk-free interest rate		0.76	%		0.32	%
Expected volatility		70.2	%		63.5	%
Dividend rate	—%			—%

Risk-free interest rate. The Company bases the risk‑free interest rate assumption on the U.S. Treasury’s rates for U.S. Treasury zero-coupon bonds with maturities similar to those of the expected term of the award being valued.

Expected volatility. Due to our limited operating history and lack of Company-specific historical or implied volatility, the expected volatility assumption was determined by examining the historical volatilities of a group of industry peers whose share prices are publicly available, and is calculated based on a period consistent with the expected term of the option.

Expected term. The expected term represents the period of time that options are expected to be outstanding. Because the Company does not have historical exercise behavior, it determines the expected life assumption using the simplified method, which is an average of the contractual term of the option and its vesting period.

Expected dividend yield. The Company bases the expected dividend yield assumption on the fact that it has never paid cash dividends and has no present intention to pay cash dividends.

The allocation of stock-based compensation expense was as follows (in thousands):


	Years Ended December 31,
	2021		2020
Research and development	$	2,545	$	600
General and administrative		1,573		233
Total stock-based compensation expense	$	4,118	$	833

At December 31, 2021, the unrecognized compensation cost related to outstanding time-based options was $5.0 million and is expected to be recognized as expense over approximately 1.4 years.

Common stock reserved for future issuance was as follow:


	As of December 31,
	2021		2020
Convertible preferred stock on an as-converted basis		—		20,549,478
Common stock options outstanding and early exercised stock options		1,688,789		1,249,218
Options and restricted stock available for future grants		6,716,195		754,369
Shares available under the 2021 Employee Stock Purchase Plan		388,703		—
Total common stock reserved for issuance		8,793,687		22,553,065