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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Summary of Stock-Based Compensation Expense | Total stock-based compensation expense recorded related to awards granted to employees and non-employees was as follows (in thousands):
|  | Year Ended December 31, |
|  | 2021 |  |  | 2020 |  |  | 2019 |
Research and development |  | $ | 3,492 |  |  | $ | 1,234 |  |  | $ | 956 |
General and administrative |  |  | 4,745 |  |  |  | 2,375 |  |  |  | 1,212 |
Total stock-based compensation expense |  | $ | 8,237 |  |  | $ | 3,609 |  |  | $ | 2,168 |
Summary of Activity under Plans | Activity under the Company’s 2015 and 2019 Plans is set forth below:
|  |  |  |  |  | Outstanding Options |  |  |  |  |  |  |  |  |
|  | Shares available for Grant |  |  | Shares |  |  | Weighted- Average Exercise Price |  |  | Weighted- Average Remaining Contractual Term (Years) |  |  | Aggregate Intrinsic Value (in thousands) |
Balance, January 1, 2021 |  |  | 975,135 |  |  |  | 2,591,456 |  |  | $ | 7.27 |  |  |  | 8.36 |  |  | $ | 15.70 |
Additional shares authorized |  |  | 1,181,488 |  |  |  | — |  |  |  |  |  |  |  |  |  |  |  |  |
Options granted |  |  | (1,560,425 | ) |  |  | 1,560,425 |  |  | $ | 20.52 |  |  |  |  |  |  |  |  |
Options exercised |  |  | — |  |  |  | (211,900 | ) |  | $ | 7.13 |  |  |  |  |  |  |  |  |
Options repurchased |  |  | — |  |  |  | — |  |  |  |  |  |  |  |  |  |  |  |  |
Options canceled |  |  | 326,628 |  |  |  | (319,315 | ) |  | $ | 14.33 |  |  |  |  |  |  |  |  |
Balance, December 31, 2021 |  |  | 922,826 |  |  |  | 3,620,666 |  |  | $ | 12.36 |  |  |  | 7.92 |  |  | $ | 41.40 |
Exercisable as of December 31, 2021 |  |  |  |  |  |  | 1,498,708 |  |  | $ | 6.58 |  |  |  | 6.60 |  |  | $ | 25.70 |
Vested and expected to vest as of December 31, 2021 |  |  |  |  |  |  | 3,620,666 |  |  | $ | 12.36 |  |  |  | 7.92 |  |  | $ | 41.40 |
Assumptions Used to Calculate Fair Values of Options | The fair values of options were calculated using the assumptions set forth below:
|  | Year Ended December 31, |
|  | 2021 |  |  | 2020 |  |  | 2019 |
Expected term |  | 5.5 - 6.1 years |  |  | 5.5 - 6.1 years |  |  | 5.4 - 6.1 years |
Expected volatility |  | 90.0% - 103.6% |  |  | 84.9 % - 98.3% |  |  | 77.2% - 85.5% |
Risk-free interest rate |  | 0.6% - 1.4% |  |  | 0.3% - 1.5% |  |  | 1.4% - 2.5 % |
Dividend yield |  | 0% |  |  | 0% |  |  | 0% |
Summary of Restricted Stock Activity | Restricted stock activity was as follows:
|  | Number of Shares Underlying Outstanding Restricted Stock Awards |  |  | Weighted Average Grant Date Fair Value |
Unvested, December 31, 2020 |  |  | 14,625 |  |  | $ | 0.82 |
Vested |  |  | (7,312 | ) |  | $ | 0.82 |
Repurchased |  |  | (7,313 | ) |  | $ | 0.82 |
Unvested, December 31, 2021 |  |  | — |  |  |  |  |

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- Definition Tabular disclosure of allocation of amount expensed and capitalized for award under share-based payment arrangement to statement of income or comprehensive income and statement of financial position. Includes, but is not limited to, corresponding line item in financial statement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=122041274&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5047-113901 + Details
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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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