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Stock Based Compensation
 6 Months Ended
Jun. 30, 2022
Share-based Payment Arrangement [Abstract]  
Stock Based Compensation Stock-Based Compensation
2021 Plan
The total shares of common stock authorized for issuance under the 2021 Stock Option and Incentive Plan (2021 Plan) increased from 3,986,270 as of December 31, 2021 to 5,252,471 as of June 30, 2022 primarily due to the automatic annual increase provision.
2021 Employee Stock Purchase Plan
The total shares of common stock authorized for issuance under the 2021 Employee Stock Purchase Plan (2021 ESPP) increased from 278,762 as of December 31, 2021 to 591,005 as of June 30, 2022 due to the automatic annual increase provision within the 2021 ESPP. No offering periods have commenced under the 2021 ESPP.
2016 Plan
The total shares of common stock authorized for issuance under the 2016 Plan as of June 30, 2022 and December 31, 2021 were 2,271,477 shares and 2,318,855 shares, respectively.
Stock-Based Compensation
Stock-based compensation expense recorded in the condensed consolidated statements of operations is as follows (in thousands):
Three Months Ended June 30,Six Months Ended June 30,
2022202120222021
Research and development expenses$974$106$1,678$162
General and administrative expenses1,5382202,628328
Total$2,512$326$4,306$490
Stock Option Valuation
The following table presents, on a weighted-average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of stock options granted during the six months ended June 30, 2022 and 2021:
June 30,
2022		June 30,
2021
Risk‑free interest rate1.77 %1.05 %
Expected volatility71 %74 %
Expected dividend yield— — 
Expected term (in years)6.036.01
Early Exercise of Unvested Stock Options
Shares purchased by employees pursuant to the early exercise of stock options are not deemed, for accounting purposes, to be outstanding shares until those shares vest according to their respective vesting schedules. Cash received from employee exercises of unvested options is included in current liabilities on the balance sheet. Amounts recorded are reclassified to common stock and additional paid-in capital as the shares vest. Vesting can occur in the year of exercise and thereafter. There were 81,794 and 111,756 unvested shares related to early exercises of stock options as of June 30, 2022 and December 31, 2021, respectively. As of June 30, 2022 and December 31, 2021, the liability associated with the unvested early exercise of stock options was $0.2 million and $0.3 million, respectively.
Stock Options
The following table summarizes the Company’s stock option activity during the six months ended June 30, 2022:
Number of
Shares		Weighted‑
Average
Exercise
Price
Outstanding as of December 31, 2021
3,461,870$10.38
Granted1,150,52311.04
Exercised(30,150)2.01
Forfeited(46,522)11.73
Outstanding as of June 30, 2022
4,535,721$10.59
Exercisable as of June 30, 2022 (1)
2,471,102$6.79
(1)This represents the number of vested and unvested options exercisable as of June 30, 2022.
The weighted-average grant-date fair value of stock options granted during the six months ended June 30, 2022 and 2021 was $7.00 per share and $5.35 per share, respectively. As of June 30, 2022, there was $24.7 million of unrecognized compensation cost related to unvested stock options, which is expected to be recognized over a weighted-average period of 3.16 years.
Restricted Stock Units
During the six months ended June 30, 2022, restricted stock units (RSUs) were granted to employees with vesting conditions based on continued service over time. Accordingly, stock-based compensation expense for such awards is recognized using a straight-line attribution model over the vesting term of each RSU. The fair value of each RSU is based on the closing price of the Company's common stock on the date of grant.
A summary of restricted stock activity during the six months ended June 30, 2022 is as follows:
SharesWeighted‑
Average
Grant‑Date
Fair Value
Unvested as of December 31, 2021
—$—
Issued302,27911.57
Forfeited(7,175)11.57
Unvested as of June 30, 2022
295,104$11.57
As of June 30, 2022, there was $3.1 million of unrecognized stock-based compensation expense related to restricted stock that is expected to vest. These costs are expected to be recognized over a weighted-average remaining vesting period of 3.42 years.