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Stock-Based Compensation (Tables)
9 Months Ended
Sep. 30, 2020
Share-based Payment Arrangement [Abstract]  
Stock-Based Compensation Expense
The following stock-based compensation expense was recognized for the periods indicated (in thousands):
Three Months Ended September 30,Nine Months Ended September 30,
2020201920202019
Research and development$3,059 $2,765 $8,082 $7,032 
General and administrative2,773 2,633 7,337 7,049 
Total stock-based compensation expense$5,832 $5,398 $15,419 $14,081 
Valuation Assumptions Using the Black-Scholes Option-Pricing Model
The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table for options issued during the period indicated:
Nine Months Ended September 30,
20202019
Expected dividend yield0%0%
Expected volatility
67.3% - 109%
73.7% - 76.6%
Risk-free interest rate
0.4% - 1.8%
1.4% - 2.6%
Expected term6.25 years6.25 years
Stock Option and Restricted Stock Unit Activity
The following table summarizes stock option activity during the nine months ended September 30, 2020:
SharesWeighted-
Average
Exercise Price
Weighted-Average
Remaining
Contractual Term
(Years)
Aggregate
Intrinsic
Value
(in thousands)
Outstanding, December 31, 20196,706,994 $22.33 6.9
Granted1,816,277 14.97 
Exercised(448,732)9.93 
Forfeited or expired(735,322)20.78 
Outstanding, September 30, 20207,339,217 21.42 7.0$39,014 
As of September 30, 2020:
Exercisable4,407,713 23.03 5.918,806 
Vested and expected to vest7,027,237 21.50 7.036,973 
Schedule of RSU Activity
The following table summarizes RSU activity during the nine months ended September 30, 2020:

Shares
Weighted-Average
Grant Date Fair Value
Outstanding, December 31, 2019452,500 $15.32 
Granted15,000 23.68 
Vested(210,950)15.32 
Forfeited(40,950)15.32 
Outstanding, September 30, 2020215,600 15.90