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Stock-Based Awards (Tables) | 9 Months Ended
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Sep. 30, 2020
Summary of Stock Option Plans Activity | Activity under the Company’s stock option plans is set forth below:
|  | Number of Options |  |  | Weighted Average Exercise Price |  |  | Weighted Average Remaining Contractual Term |  |  | Aggregate Intrinsic Value |
|  |  |  |  |  |  |  |  |  | (Years) |  |  | (in thousands) |
Outstanding at December 31, 2019 |  |  | 7,186,298 |  |  | $ | 5.25 |  |  |  | 8.5 |  |  |  |  |
Granted |  |  | 4,175,235 |  |  | $ | 32.53 |  |  |  |  |  |  |  |  |
Exercised |  |  | (1,387,629 | ) |  | $ | 2.55 |  |  |  |  |  |  |  |  |
Forfeited |  |  | (514,889 | ) |  | $ | 13.78 |  |  |  |  |  |  |  |  |
Outstanding at September 30, 2020 |  |  | 9,459,015 |  |  | $ | 17.22 |  |  |  | 8.7 |  |  | $ | 174,230 |
Vested and expected to vest at September 30, 2020 |  |  | 9,459,015 |  |  | $ | 17.22 |  |  |  | 8.7 |  |  | $ | 174,230 |
Vested and exercisable at September 30, 2020 |  |  | 2,194,929 |  |  | $ | 4.52 |  |  |  | 7.8 |  |  | $ | 65,422 |
Summary of Restricted Stock Activity | The following table summarizes restricted stock activity:
|  | Number of Shares |  |  | Weighted Average Fair Value at Date of Grant per Share |
Unvested as of December 31, 2019 |  |  | 2,075,511 |  |  | $ | 0.95 |
Vested |  |  | (1,791,880 | ) |  |  | 0.91 |
Unvested as of September 30, 2020 |  |  | 283,631 |  |  | $ | 1.15 |
Summary of Stock-based Compensation Expense | The following table sets forth the total stock-based compensation expense for all awards granted to employees and non-employees, including shares sold through the issuance of non-recourse promissory notes of which all the shares are considered to be options for accounting purposes in the Company’s consolidated statements of operations:
|  | Three Months Ended September 30, |  |  | Nine Months Ended September 30, |
|  | 2020 |  |  | 2019 |  |  | 2020 |  |  | 2019 |
|  | (in thousands) |  |  | (in thousands) |
Research and development |  | $ | 4,181 |  |  | $ | 926 |  |  | $ | 8,369 |  |  | $ | 1,925 |
General and administrative |  |  | 4,401 |  |  |  | 1,262 |  |  |  | 8,930 |  |  |  | 4,115 |
Total stock-based compensation |  | $ | 8,582 |  |  | $ | 2,188 |  |  | $ | 17,299 |  |  | $ | 6,040 |
Stock Option |
Summary of Assumptions Used for Estimating the Fair Value of Stock Options Granted | The fair value of stock options granted to employees was estimated on the date of grant using the Black-Scholes option-pricing model using the following assumptions:
|  | Three Months Ended September 30, |  |  | Nine Months Ended September 30, |
|  | 2020 |  |  | 2019 |  |  | 2020 |  |  | 2019 |
Expected term of options (in years) |  | 5.0 - 6.1 |  |  |  | 6.0 |  |  | 5.0 - 6.1 |  |  | 5.9 - 6.0 |
Expected stock price volatility |  | 99.7% - 108.6% |  |  | 88.5% |  |  | 88.8% - 108.6% |  |  | 88.5% - 89.4% |
Risk-free interest rate |  | 0.3% - 0.4% |  |  | 1.9% |  |  | 0.3% - 1.2% |  |  | 1.9% - 2.5% |
Expected dividend yield |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |

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- Definition Tabular disclosure of the changes in outstanding nonvested restricted stock units. + 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 (c)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + 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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