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Stock-Based Compensation (Tables) | 3 Months Ended
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Mar. 31, 2020
Share-based Payment Arrangement [Abstract] |
Summary of Option Activity Plan | A summary of option activity under the Plans for the three months ended March 31, 2020 follows:
2006 and 2016 Plans (in thousands, except for share and per share amounts) | Number of Shares |  | Weighted Average Exercise Price at Grant Date |  | Weighted Average Remaining Term (Years) |  | Aggregate Intrinsic Value
Outstanding at December 31, 2019 | 4,495,145 |  |  | $ | 12.21 |  |  | 5.58 |  | $ | 21,148 |
Granted | 487,500 |  |  | 11.42 |  |  |  |  |
Forfeited | (225,000 | ) |  | 18.83 |  |  |  |  |
Expired | (10,104 | ) |  | 11.78 |  |  |  |  |
Exercised | (172,500 | ) |  | 4.96 |  |  |  |  | 890 |
Outstanding at March 31, 2020 | 4,575,041 |  |  | 12.07 |  |  | 5.81 |  | 4,818 |
Exercisable at March 31, 2020 | 3,379,139 |  |  | 11.02 |  |  | 4.63 |  | 4,818 |
Vested and expected to vest at March 31, 2020 | 4,340,916 |  |  | 11.99 |  |  | 5.60 |  | 4,818 |
Summary of RSU Activity for 2006 Plan and 2016 Plan | A summary of RSU activity under the Plans for the three months ended March 31, 2020 follows:
2006 and 2016 Plans | Number of Shares Underlying RSUs |  | Weighted Average Grant Date Fair Value
Unvested at December 31, 2019 | 1,649,285 |  |  | $ | 18.04 |
Granted | 745,818 |  |  | 11.32 |
Forfeited | (136,091 | ) |  | 18.65 |
Vested | (498,505 | ) |  | 16.73 |
Unvested at March 31, 2020 | 1,760,507 |  |  | 15.52 |
Stock-Based Compensation Expense | Stock-based compensation expense recognized for the three months ended March 31, 2020 and 2019 was comprised of the following:
| Three Months Ended
(in thousands) | March 31, 2020 |  | March 31, 2019
Research and development | $ | 1,111 |  |  | $ | 728 |
Selling, general and administrative | 2,833 |  |  | 2,554 |
Total stock-based compensation expense | $ | 3,944 |  |  | $ | 3,282 |
Black-Scholes-Merton Option Pricing Model for Employee and Director Stock Options Granted | Assumptions used in the Black-Scholes-Merton option pricing model for employee and director stock options granted during the three months ended March 31, 2020 and 2019 were as follows:
| Three Months Ended
| March 31, 2020 |  | March 31, 2019
Expected dividend yield | 0 | % |  | 0 | %
Weighted average expected volatility | 52 | % |  | 58 | %
Weighted average expected term (years) | 6.09 |  |  | 5.92 |
Weighted average risk-free rate | 1.37 | % |  | 2.51 | %

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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=116856206&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=116856206&loc=d3e5047-113901 + Details
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- Definition Tabular disclosure of the number and weighted-average grant date fair value for restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock units that were granted, vested, or forfeited during the year. + 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)(1) -URI http://asc.fasb.org/extlink&oid=116856206&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=116856206&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=116856206&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=116856206&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=116856206&loc=d3e5070-113901 + Details
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