In December 2015, the Company’s board of directors adopted and approved the 2015 Stock Incentive Plan (as amended to date, 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 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 September 30, 2021, the Company had 2,480,665 shares of its common stock 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 September 30,						Nine Months Ended September 30,
	2021			2020			2021			2020
Fair value of common stock	$	13.12		$	2.87		$	20.27		$	2.73
Expected term (in years)		6.0			6.0			6.0			6.0
Expected volatility		76.0	%		77.2	%		78.8	%		77.0	%
Risk-free interest rate		0.9	%		0.4	%		0.8	%		0.4	%
Dividend yield		—			—			—			—

During the nine months ended September 30, 2021 and 2020, the Company granted stock options to purchase 637,884 shares and 3,457,481 shares of its common stock, respectively, with aggregate grant date fair values of $8.7 million and $6.7 million, respectively.

As of September 30, 2021, total unrecognized compensation expense related to stock options was $14.0 million, which is expected to be recognized over a weighted-average period of 2.69 years.

As of September 30, 2021, options for 229,132 shares of Company common stock with a weighted average exercise price of $1.88 were exercised and unvested. The underlying proceeds from the unvested exercises of $0.4 million is recorded in other current liabilities on the condensed consolidated balance sheet.

Stock-based compensation expense was allocated as follows:


	Three Months Ended September 30,				Nine Months Ended September 30,
(in thousands)	2021		2020		2021		2020
Research and development	$	630	$	254	$	1,621	$	374
General and administrative		524		229		1,491		374
Total stock-based compensation expense	$	1,154	$	483	$	3,112	$	748