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Stock-Based Compensation (Tables) | 9 Months Ended
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Sep. 30, 2021
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Stock-Based Compensation Expense on All Employee and Non-Employee Awards | The Company recognized stock-based compensation expense on all employee and non-employee awards as follows:
|  | For the Three Months Ended September 30, |  |  | For the Nine Months Ended September 30, |
(in thousands) |  | 2021 |  |  | 2020 |  |  | 2021 |  |  | 2020 |
Research and development |  |  | 535 |  |  |  | 522 |  |  |  | 2,115 |  |  |  | 1,587 |
General and administrative |  |  | 1,836 |  |  |  | 1,270 |  |  |  | 7,742 |  |  |  | 3,806 |
Stock-based compensation expense |  | $ | 2,371 |  |  | $ | 1,792 |  |  | $ | 9,857 |  |  | $ | 5,393 |
Fair Value of Stock Options Assumptions Using Black-Scholes Option Valuation Model | For the three and nine months ended September 30, 2021 and 2020, the fair value of stock options was estimated on the date of grant using a Black-Scholes option valuation model with the following assumptions:
|  | For the Three Months Ended September 30, |  |  | For the Nine Months Ended September 30, |
|  | 2021 |  |  | 2020 |  |  | 2021 |  |  | 2020 |
Risk-free interest rate |  | 0.89 - 0.96% |  |  | 0.36 - 0.39% |  |  | 0.50 - 1.15% |  |  | 0.36 - 1.68% |
Expected life in years |  | 6.00 - 6.25 |  |  | 5.75 - 6.25 |  |  | 5.50 - 6.25 |  |  | 5.75 - 6.25 |
Expected volatility |  | 72.53 - 72.84% |  |  | 73.59 - 74.18% |  |  | 72.53 - 74.80% |  |  | 71.11 - 74.18% |
Expected dividend yield |  |  | — | % |  |  | — | % |  |  | — | % |  |  | — | %
Stock Option Activity under Stock Option Plan | Stock option activity under the Company’s stock option plans for the nine months ended September 30, 2021 is as follows:
(in thousands, except share and per share data) |  | Number of Shares |  |  | Weighted- Average Exercise Price |  |  | Weighted- Average Contractual Term (Years) |  |  | Aggregate Intrinsic Value |
Outstanding, December 31, 2020 |  |  | 6,840,719 |  |  | $ | 3.81 |  |  |  |  |  |  |
Granted |  |  | 7,150,438 |  |  |  | 2.97 |  |  |  |  |  |  |
Exercised |  |  | (363,109 | ) |  |  | 2.86 |  |  |  |  |  |  |
Cancelled |  |  | (2,555,154 | ) |  |  | 3.54 |  |  |  |  |  |  |
Outstanding, September 30, 2021 |  |  | 11,072,894 |  |  | $ | 3.36 |  |  |  | 8.32 |  |  | $ | 473 |
Options exercisable, September 30, 2021 |  |  | 5,743,820 |  |  | $ | 4.08 |  |  |  | 7.24 |  |  | $ | — |
Options exercisable, December 31, 2020 |  |  | 3,596,315 |  |  | $ | 4.17 |  |  |  | 6.90 |  |  | $ | 598 |
Options available for future grant |  |  | 2,224,159 |  |  |  |  |  |  |  |  |  |
Summary of Unvested Restricted Stock | A summary of the status of unvested restricted stock for the nine months ended September 30, 2021 is as follows:
|  | Number of Shares |  |  | Weighted-Average Grant Date Fair Value |
Unvested, December 31, 2020 |  |  | 786,280 |  |  | $ | 3.08 |
Granted |  |  | 1,601,224 |  |  |  | 2.60 |
Vested |  |  | (466,436 | ) |  |  | 3.68 |
Cancelled |  |  | (410,413 | ) |  |  | 3.46 |
Unvested, September 30, 2021 |  |  | 1,510,655 |  |  | $ | 2.28 |

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- Definition Tabular disclosure of share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5047-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the changes in outstanding nonvested shares. + 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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