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Stock Plans (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 stock option activity under the Company’s stock plans and related information:
(In thousands, except exercise prices and years) | Options Outstanding |  | Weighted- Average Exercise Price |  | Weighted- Average Remaining Contract Life (in years) |  | Aggregate Intrinsic Value (a)
Balance at December 31, 2019 | 8,995 |  |  | $ | 7.19 |  |  | 7.6 |  | $ | 48,073 |
Granted | 4,126 |  |  | 18.52 |  |  |  |  |
Exercised | (2,043) |  |  | 6.21 |  |  |  |  |
Cancelled/forfeited | (646) |  |  | 10.67 |  |  |  |  |
Balance at December 31, 2020 | 10,432 |  |  | $ | 11.65 |  |  | 8.2 |  | $ | 31,626 |
Granted | 8,231 |  |  | 7.04 |  |  |  |  |
Exercised | (9) |  |  | 5.86 |  |  |  |  |
Cancelled/forfeited | (5,366) |  |  | 10.32 |  |  |  |  |
Balance at December 31, 2021 | 13,288 |  |  | $ | 9.34 |  |  | 6.9 |  | $ | 145 |
Vested and expected to vest as of December 31, 2021 | 13,288 |  |  | $ | 9.34 |  |  | 6.9 |  | $ | 145 |
Exercisable at December 31, 2021 | 5,579 |  |  | $ | 10.32 |  |  | 3.9 |  | $ | 145 |
(a)The aggregate intrinsic value is calculated as the difference between the stock option exercise price and the closing price of the Company’s common stock as quoted on a national exchange.
Schedule of Fair Value of Option Issued Valuation Assumptions | The fair value of each stock option issued was estimated at the date of grant using the Black-Scholes valuation model with the following weighted-average assumptions:
| Options |  | Employee Stock Purchase Plan
| Years ended December 31, |  | Years ended December 31,
| 2021 |  | 2020 |  |  |  | 2021 |  | 2020 |  |
Expected volatility | 91% |  | 87% |  |  |  | 122% |  | 89% |  |
Expected term (in years) | 6.0 |  | 6.0 |  |  |  | 1.21 |  | 0.5 |  |
Expected dividend yield | — |  | — |  |  |  | — |  | — |  |
Risk-free interest rate | 1.0% |  | 0.8% |  |  |  | 0.1% |  | 0.1% |  |
Summary of Restricted Stock Units Activity | The following table summarizes the RSU activity under the Company’s stock plans and related information:
(In thousands, except grant date fair value and years) | Number of Units (in thousands) |  | Weighted- Average Grant Date Fair Value (in dollars) |  | Weighted- Average Remaining Contractual Term (in years)
Balance at December 31, 2019 | 1,121 |  |  | $ | 4.59 |  |  | 0.9
Granted | 105 |  |  | 15.75 |  |  |
Vested and released | (590) |  |  | 4.49 |  |  |
Forfeited | (96) |  |  | 7.17 |  |  |
Balance at December 31, 2020 | 540 |  |  | $ | 6.41 |  |  | 0.8
Granted | 2,585 |  |  | 3.90 |  |  |
Vested and released | (305) |  |  | 5.26 |  |  |
Forfeited | (694) |  |  | 5.68 |  |  |
Balance at December 31, 2021 | 2,126 |  |  | $ | 3.76 |  |  | 1.44
Stock-Based Compensation Expense | The following table presents, by operating expense, the Company’s stock-based compensation expense:
| Years ended December 31,
| 2021 |  | 2020 |  |
| (In thousands)
Research and development | $ | 8,875 |  |  | $ | 7,120 |  |  |
General and administrative | 16,319 |  |  | 13,271 |  |  |
Total share-based compensation expense | $ | 25,194 |  |  | $ | 20,391 |  |  |

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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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