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Stock Plans and Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Summary of Option Activity Includes Options Granted | The following table, which includes options granted under the Company’s Stock Plans, summarizes option activity:
|  | Shares Subject to Outstanding Options |  |  | Weighted Average Exercise Price Per Share |  |  | Weighted Average Remaining Contractual Term (in years) |  |  | Aggregate Intrinsic Value (in thousands) |
Outstanding at December 31, 2019 |  |  | 4,738,004 |  |  | $ | 9.00 |  |  |  |  |  |  |  |  |
Options granted |  |  | 5,784,550 |  |  | $ | 17.36 |  |  |  |  |  |  |  |  |
Options exercised |  |  | (392,523 | ) |  | $ | 8.54 |  |  |  |  |  |  |  |  |
Options forfeited or canceled |  |  | (237,334 | ) |  | $ | 10.38 |  |  |  |  |  |  |  |  |
Outstanding at December 31, 2020 |  |  | 9,892,697 |  |  | $ | 13.88 |  |  |  | 8.81 |  |  | $ | 122,232 |
Options vested and expected to vest as of December 31, 2020 |  |  | 9,892,697 |  |  | $ | 13.88 |  |  |  | 8.81 |  |  | $ | 122,232 |
Options exercisable as of December 31, 2020 |  |  | 2,793,050 |  |  | $ | 10.28 |  |  |  | 8.12 |  |  | $ | 44,012 |
Summary of Restricted Stock Units Activity | The Company granted restricted stock units (RSUs) to its employees and directors under the 2018 Plan. The shares subject to the RSUs vest annually or quarterly over four years for employees and annually for directors.
|  | Total Restricted Stock Units |  |  | Weighted Average Grant Date Fair Value |
Nonvested at December 31, 2019 |  |  | - |  |  | $ | - |
RSUs granted |  |  | 758,950 |  |  |  | 27.77 |
RSUs vested |  |  | (6,250 | ) |  |  | 17.00 |
RSUs forfeited or canceled |  |  | (14,050 | ) |  |  | 29.05 |
Nonvested at December 31, 2020 |  |  | 738,650 |  |  | $ | 27.84 |
Summary of Employee and Non-Employee Stock-based Compensation Expense | The following table summarizes employee and non-employee stock-based compensation expense for the years ended December 31, 2020, 2019 and 2018, and also the allocation within the consolidated statements of operations and comprehensive loss (in thousands):
|  | Year Ended December 31, |
|  | 2020 |  |  | 2019 |  |  | 2018 |
Research and development |  | $ | 11,195 |  |  | $ | 4,152 |  |  | $ | 2,255 |
General and administrative |  |  | 10,630 |  |  |  | 4,829 |  |  |  | 1,619 |
Total stock-based compensation |  | $ | 21,825 |  |  | $ | 8,981 |  |  | $ | 3,874 |
Schedule of Assumptions Used to Calculate Fair Value of Stock-Based Compensation | The following assumptions were used to calculate the fair value of stock-based compensation for the years ended December 31, 2020, 2019, and 2018:
|  | Stock Options |
|  | Year Ended December 31, |
|  | 2020 |  |  | 2019 |  |  | 2018 |
Risk-free interest rate |  | 0.4% - 0.5% |  |  | 1.6% - 2.3% |  |  | 1.2% - 3.1% |
Expected term (in years) |  | 6.02 |  |  | 6.02 |  |  | 5.16-9.95 |
Volatility |  | 76.5% - 78.5% |  |  | 71.8% - 74.6% |  |  | 58.7%-75.5% |
Dividend yield |  | 0% |  |  | 0% |  |  | 0% |
|  | ESPP |
|  | Year Ended December 31, |
|  | 2020 |  |  | 2019 |  |  | 2018 |
Risk-free interest rate |  | 0.1% - 0.2% |  |  | 1.6% - 2.3% |  |  | 2.1% - 2.6% |
Expected term (in years) |  | 0.5-2.0 |  |  | 0.5-2.0 |  |  | 0.5-2.0 |
Volatility |  | 66.6% - 136.0% |  |  | 64.8% - 77.1% |  |  | 54.3% - 65.5% |
Dividend yield |  | 0% |  |  | 0% |  |  | 0% |

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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=115993241&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 employee stock purchase plans, including, but not limited to: (a) expected term, (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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