In July 2021, our board of directors adopted and our stockholders approved the 2021 Equity Incentive Plan (the “2021 Plan“) that became effective on July 22, 2021. We reserved 5,200,000 shares of common stock for issuance under the 2021 Plan. In addition, 934,562 shares available for issuance under the 2013 Equity Incentive Plan, adopted in 2013 and amended and restated in 2019, were transferred into the 2021 Plan. Furthermore, any shares subject to awards under the 2013 Plan that terminate, expire, or lapse for any reason without the delivery of shares, or are reacquired or withheld (or not issued) to satisfy a tax withholding obligation or the purchase or exercise price, will be added to the 2021 Plan. The 2021 Plan also provides that the number of shares initially reserved and available for issuance will automatically increase each January 1, beginning on January 1, 2022 and ending on January 1, 2031, by an amount equal to the lesser of (a) 5% of the shares of common stock outstanding on the last day of the immediately preceding fiscal year and (b) such smaller number of shares of stock as determined by our Board. No more than 56,000,000 shares of stock may be issued upon the exercise of incentive stock options under the 2021 Plan. Options under the 2021 Plan may be granted for periods of up to 10 years at exercise prices no less than the fair market value of our common stock on the date of grant; provided, however, that the exercise price of an incentive stock option granted to a 10% stockholder may not be less than 110% of the fair market value of the shares on the date of grant and such option may not be exercisable after the expiration of five years from the date of grant. The grant date fair market value of all awards made under the 2021 Plan and all cash compensation paid by us to any non-employee director for services as a director in any fiscal year may not exceed $750,000, increased to $1,000,000 in the fiscal year of their initial service as a non-employee director. As of March 31, 2022, we had 6,783,690 shares available for issuance under the 2021 Plan.

The following table summarizes stock option activity under our equity incentive plans during the three months ended March 31, 2022:


	Shares Available to Grant			Stock Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (years)		Aggregate Intrinsic Value (in thousands)*

Outstanding at December 31, 2021		3,749,339			6,757,591		$	8.57		8.7	$	50,085
Addition to option pool		3,013,157
Options granted		(339,030	)		339,030			10.47
Options exercised		—			(389,855	)		1.61
Options cancelled or forfeited		420,224			(420,224	)		7.04
Outstanding at March 31, 2022		6,843,690			6,286,542		$	9.20		8.7	$	19,821
Exercisable at March 31, 2022					1,555,480		$	4.11		7.1	$	8,617
Vested and expected to vest at March 31, 2022					6,286,542		$	9.20		8.7	$	19,821

*The aggregate intrinsic value is calculated as the difference between the stock option exercise price and the estimated fair value of the underlying common stock at the end of each reporting period referenced above.

During the three months ended March 31, 2022 and 2021, we granted 339,030 and 1,561,079 stock options to employees and non-employees with a weighted-average grant date fair value of $6.68 and $2.73 per share, respectively.

As of March 31, 2022, there was $32.0 million of unrecognized stock-based compensation expense related to employee and non-employee stock options that is expected to be recognized over a weighted-average period of 3.0 years.

During the three months ended March 31, 2022, we granted 60,000 RSUs under the 2021 Plan. A summary of the status of and change in unvested RSUs as of March 31, 2022 was as follows:

As of March 31, 2022, the total unrecognized stock-based compensation expense related to unvested RSUs was $0.6 million, which is expected to be recognized over the remaining weighted-average vesting period of 3.8 years.

In July 2021, our board of directors adopted and our stockholders approved the ESPP, which became effective on July 22, 2021. The ESPP is intended to qualify as an employee stock purchase plan under Section 423 of the Internal Revenue Code of 1986, as amended (“Tax Code”). We reserved 511,000 shares of our common stock for employee purchases under the ESPP. The number of shares of common stock reserved for issuance under the ESPP will be automatically increased each January 1, beginning on January 1, 2022 and ending on January 1, 2031 by an amount equal to the lesser of (a) 1% of the shares of common stock outstanding on the last day of the immediately preceding fiscal year and (b) such smaller number of shares of stock as determined by our Board; provided that the maximum number of shares that may be issued under the ESPP is 10,000,000 shares. The ESPP allows an eligible employee to purchase shares of our common stock at a discount through payroll deductions of up to 15% of the employee’s eligible compensation. At the end of each purchase period, employees are able to purchase shares at 85% of the lower of the fair market value of our common stock at the beginning of the offering period or at the end of each applicable offering period. The first offering period commenced on August 16, 2021 and ended on February 15, 2022. We have issued 36,596 shares of common stock under the ESPP as of March 31, 2022. We recorded $0.1 million in accrued liabilities related to contributions withheld as of March 31, 2022.

We recorded stock-based compensation expense related to employee and non-employee equity-based awards in our condensed consolidated statements of operations and comprehensive loss for the three months ended March 31, 2022 and 2021 as follows (in thousands):


	Three Months Ended March 31,
	2022		2021
Research and development	$	1,100	$	197
General and administrative		1,924		146
Total	$	3,024	$	343

The above stock-based compensation expense related to the following equity-based awards (in thousands):


	Three Months Ended March 31,
	2022		2021
Stock options	$	2,930	$	343
ESPP		63		-
RSUs		31		-
Total	$	3,024	$	343

Stock-based compensation expense related to employees was $2.9 million and $0.3 million for the three months ended March 31, 2022 and 2021, respectively. Stock-based compensation expense related to non-employees was $0.1 million and less than $0.1 million for the three months ended March 31, 2022 and 2021, respectively.