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Stock-based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2021
Share-based Payment Arrangement [Abstract] |
Summary of Stock Options Activity | The following table summarizes the stock options activity for the year ended December 31, 2021:
|  |  |  |  | Options Outstanding |  |  |  |  |  |  |
|  | Number of Shares Available for Grant |  |  | Number of Shares Underlying Outstanding Options |  |  | Weighted- Average Exercise Price |  |  | Weighted- Average Remaining Contractual Term (in years) |  |  | Aggregate Intrinsic Value (in thousands) |
Balances at December 31, 2020 |  |  | 2,606,546 |  |  |  | 3,194,113 |  |  | $ | 11.05 |  |  |  | 8.46 |  |  | $ | 97,105 |
Options authorized |  |  | 1,334,099 |  |  |  | — |  |  |  |  |  |  |  |  |  |
Options granted |  |  | (1,566,150 | ) |  |  | 1,566,150 |  |  |  | 34.83 |  |  |  |  |  |  |
Options exercised |  |  | — |  |  |  | (563,387 | ) |  |  | 6.26 |  |  |  |  |  |  |
Options expired |  |  | 4,324 |  |  |  | (4,324 | ) |  |  | 36.33 |  |  |  |  |  |  |
Options forfeited |  |  | 311,278 |  |  |  | (311,278 | ) |  |  | 29.17 |  |  |  |  |  |  |
Balances at December 31, 2021 |  |  | 2,690,097 |  |  |  | 3,881,274 |  |  | $ | 19.86 |  |  | $ | 7.97 |  |  | $ | 24,666 |
Shares exercisable, December 31, 2021 |  |  |  |  |  | 1,592,139 |  |  | $ | 12.75 |  |  | $ | 7.07 |  |  | $ | 17,280 |
Shares vested and expected to vest, December 31, 2021 |  |  |  |  |  | 3,881,274 |  |  | $ | 19.86 |  |  | $ | 7.97 |  |  | $ | 24,666 |
Summary of Stock Compensation Expense for Employees and Nonemployees by Function | The following table is a summary of stock compensation expense for employees and nonemployees by function (in thousands):
|  | Year Ended December 31, |  |
|  | 2021 |  |  | 2020 |  |
Research and development |  | $ | 6,847 |  |  | $ | 2,670 |  |
General and administrative |  |  | 6,952 |  |  |  | 2,314 |  |
Total stock-based compensation expense |  | $ | 13,799 |  |  | $ | 4,984 |  |
Schedule of Assumptions Used in Black-Scholes Valuation Model to Estimate Fair Value of Stock Options | The Company estimates the fair value of employee and nonemployee stock options using the Black-Scholes option pricing model. The fair value of employee and nonemployee options is recognized on a straight-line basis over the requisite service period of the awards. The fair value of the Company's stock options was estimated using the following assumptions for the years ended December 31, 2021 and 2020.
|  | Year Ended December 31, 2021 |  | Year Ended December 31, 2020
|  | Employee |  |  | Nonemployee |  | Employee |  |  | Nonemployee
Expected term |  | 5.8 – 6.3 years |  |  | 6.3 years |  | 5.5 – 6.3 years |  |  | 6.0 years
Expected volatility |  | 80.0% – 84.3% |  |  | 83.5% |  | 82.1% – 83.8% |  |  | 82.1% – 83.1%
Risk-free interest rate |  | 0.6% – 1.3% |  |  | 0.6% |  | 0.4% – 0.7% |  |  | 0.4% – 0.7%
Expected dividend yield |  | 0% |  |  | 0% |  | 0% |  |  | 0%

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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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