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Stock-Based Awards (Tables)
 6 Months Ended
Jun. 30, 2020
Share-based Payment Arrangement [Abstract]  
Summary of Stock Option Activity
The following table summarizes option award activity under the Company's 2017 Equity Incentive Plan and the 2015 Stock Incentive Plan: 
Number of
Options
Weighted-
Average
Exercise Price
Balance at December 31, 201911,640,734  $12.96  
Granted
3,244,298  24.78  
Exercised
(461,968) 7.67  
Forfeited
(232,093) 19.08  
Balance at June 30, 202014,190,971  $15.74  
Vested and expected to vest at June 30, 2020
12,446,239  $17.85  
Exercisable at June 30, 20205,318,846  $13.52  
Summary of Assumptions Used for Estimating the Fair Value of Stock Granted
The estimated fair value of stock options granted to employees were calculated using the Black-Scholes option-pricing model using the following assumptions:
Six Months Ended June 30,
20202019
Expected term (in years)
5.50 - 6.08
5.50 - 6.08
Volatility
65.2% - 67.1%
66.9% - 77.8%
Risk-free interest rate
0.4% - 1.7%
2.0% - 2.6%
Dividend yield
——
Restricted Stock Activity
Aggregated information regarding restricted stock for the six months ended June 30, 2020 is summarized below:
Share Awards & UnitsWeighted-Average Fair Value at Date of Grant per Share
Unvested at December 31, 2019882,636  $18.67  
Granted957,249  24.62  
Vested and released(118,421) 19.13  
Forfeited(48,412) 21.52  
Unvested at June 30, 20201,673,052  $21.98  
Expected to vest at June 30, 20201,673,052  $21.98  
Summary of Stock-Based Compensation Expense
The Company’s results of operations include expenses relating to stock-based compensation as follows (in thousands):
Three Months Ended June 30,Six Months Ended June 30,
2020201920202019
Research and development
$7,146  $5,295  $13,200  $9,277  
General and administrative
5,338  8,419  10,069  11,311  
Total
$12,484  $13,714  $23,269  $20,588