Stock-Based Compensation | 7. Stock-Based Compensation The Company has outstanding awards under its 2016 Stock Incentive Plan, as amended (the “2016 Plan”), but is no longer granting awards under this plan. The Company’s 2021 Stock Incentive Plan (the “2021 Plan” and, together with the 2016 Plan, the “Plans”) allows the Company to make equity-based and cash-based incentive awards to officers, employees, directors and consultants. The number of shares initially reserved under the 2021 Plan was 3,119,514 shares of the Company’s common stock. Additionally, shares of the Company’s common stock subject to outstanding awards under the 2016 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right will be added back to the shares of common stock available for issuance under the 2021 Plan. The 2021 Plan contains an “evergreen” provision, which allows for an annual increase in the number of shares of common stock available for issuance under the 2021 Plan on the first day of each fiscal year during the period beginning in fiscal year 2022. The annual increase in the number of shares shall be equal to 4 % of the number of shares of common stock outstanding on the immediately preceding December 31; or such lesser number of shares as determined by the Administrator as provided in the 2021 Plan. On January 1, 2024, the number of shares of common stock available for issuance under the 2021 Plan increased by 1,930,324 shares as a result of the automatic increase provision of the 2021 Plan. As of March 31, 2024 , 3,735,648 shares of common stock remain available for future issuance under the 2021 Plan. The vesting periods for equity awards, which generally are four years , are determined by the Company’s board of directors. The contractual term for stock option awards is ten years . The Company recorded stock-based compensation expense in the following expense categories of its condensed consolidated statements of operations and comprehensive loss (in thousands): Three Months Ended March 31, 2024 2023 Research and development $ 777 $ 1,072 General and administrative 961 928 Restructuring and other charges 263 — Total $ 2,001 $ 2,000 As of March 31, 2024 , the total unrecognized stock-based compensation balance for outstanding awards was $ 8.5 million, which is expected to be recognized over a weighted average period of 2.7 years. The following table summarizes stock option activity under the 2021 Plan for the three months ended March 31, 2024: Number of Weighted- Weighted- Aggregate Outstanding as of December 31, 2023 7,088,261 $ 6.35 6.92 $ 94 Granted 1,834,533 1.41 Exercised — — Cancelled or forfeited ( 226,602 ) 9.09 Outstanding as of March 31, 2024 8,696,192 $ 5.23 7.51 $ 96 Vested or expected to vest as of March 31, 2024 8,696,192 $ 5.23 7.51 $ 96 Options exercisable as of March 31, 2024 4,471,341 $ 6.33 6.14 $ 31 No options were exercised during the three months ended March 31, 2024 and 2023. The weighted-average grant date fair value of the stock options granted during the three months ended March 31, 2024 and 2023 was $ 1.08 per share and $ 1.96 per share, respectively. The fair value of each option award granted is estimated on the date of grant using the Black-Scholes option pric ing model and assumptions input into the model. The following table presents, on a weighted average basis, the assumptions used in the model to determine the grant-date fair value of stock options granted: Three Months Ended March 31, 2024 2023 Risk-free interest rate 4.17 % 3.92 % Expected dividend yield 0 % 0 % Expected option term (in years) 6.04 6.08 Expected stock price volatility 91.11 % 86.45 % |