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Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2020
Share-based Payment Arrangement [Abstract]  
Summary of Option Activity
A summary of option activity under the Plans for the nine months ended September 30, 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, 20194,495,145 $12.21 5.58$22,148 
Granted627,500 11.27 
Forfeited(225,000)18.83 
Expired(557,604)14.21 
Exercised(568,973)7.39 2,155 
Outstanding at September 30, 20203,771,068 12.09 5.773,331 
Exercisable at September 30, 20202,639,153 11.17 4.423,331 
Vested and expected to vest at September 30, 20203,604,062 12.05 5.613,331 
Summary of RSU Activity
A summary of RSU activity under the Plans for the nine months ended September 30, 2020 follows:
2006 and 2016 Plans
Number of
Shares
Underlying RSUs
Weighted
Average
Grant Date Fair Value
Unvested at December 31, 20191,649,285 $18.04 
Granted828,162 11.28 
Forfeited(247,799)17.35 
Vested(571,069)16.53 
Unvested at September 30, 20201,658,579 15.29 
Stock-Based Compensation Expense
Stock-based compensation expense recognized for the three and nine months ended September 30, 2020 and 2019 was comprised of the following:
 
 Three Months EndedNine Months Ended
(in thousands)September 30,
2020		September 30,
2019		September 30,
2020		September 30,
2019
Research and development$853 $827 $2,864 $2,311 
Selling, general and administrative2,282 2,580 7,284 7,479 
Total stock-based compensation expense$3,135 $3,407 $10,148 $9,790 
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 nine months ended September 30, 2020 and 2019 were as follows:
 
 Nine Months Ended
September 30, 2020September 30, 2019
Expected dividend yield0 %0 %
Weighted average expected volatility51 %58 %
Weighted average expected term (years)6.075.94
Weighted average risk-free rate1.14 %2.29 %