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Stock-Based Compensation
 3 Months Ended
Mar. 31, 2020
Share-based Payment Arrangement [Abstract]  
Stock-Based Compensation Stock-Based Compensation
The Company recognized stock-based compensation expense related to the 2003 Stock Option and Restricted Stock Plan (the 2003 Plan), the 2013 Equity Incentive Plan (the 2013 Plan), and the 2013 Employee Stock Purchase Plan (the 2013 ESPP) in the consolidated statements of operations and comprehensive loss during the three months ended March 31, 2020 and 2019, respectively, as follows (in thousands):
 
Three Months Ended 
 March 31,
 
2020
 
2019
Research and development
$
3,141

 
$
3,500

Selling, general and administrative
3,538

 
3,492

 
$
6,679

 
$
6,992


Stock Options
The fair value of each option issued to employees was estimated on the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:
 
Three Months Ended 
 March 31,
 
2020
 
2019
Expected volatility
55.1
%
 
59.1
%
Expected term (in years)
6.0

 
6.0

Risk-free interest rate
1.6
%
 
2.6
%
Expected dividend yield
—
%
 
—
%

The following table summarizes the stock option activity under the Company’s stock option plans during the three months ended March 31, 2020 (in thousands, except per share amounts and years):
 
Number
of Stock Options
 
Weighted-
Average
Exercise
Price
Per Share
 
Weighted-
Average
Contractual
Life (in years)
 
Aggregate
Intrinsic
Value(1)
Outstanding at December 31, 2019
3,820

 
$
36.26

 
6.81
 
 

Granted
835

 
$
54.55

 
 
 
 

Exercised
(296
)
 
$
28.69

 
 
 
 

Canceled or forfeited
(127
)
 
$
41.03

 
 
 
 

Outstanding at March 31, 2020
4,232

 
$
40.25

 
7.07
 
$
210,031

Exercisable at March 31, 2020
2,328

 
$
34.86

 
5.48
 
$
128,039

(1)
The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at March 31, 2020.
The aggregate intrinsic value of options exercised during the three months ended March 31, 2020 was $15.0 million. 
As of March 31, 2020, there was $44.6 million of unrecognized compensation expense related to unvested stock options that is expected to be recognized over a weighted-average period of 2.84 years. 
Restricted Stock Units
The following table summarizes the restricted stock unit (RSU) activity under the 2013 Plan during the three months ended March 31, 2020 (in thousands, except per share amounts):
 
Number
of Stock Units
 
Weighted-
Average
Grant Date Fair Value Per Share
Unvested balance at December 31, 2019
397

 
$
39.20

Granted
197

 
$
54.31

Vested
(86
)
 
$
38.69

Forfeited
(26
)
 
$
41.36

Unvested balance at March 31, 2020
482

 
$
45.36


As of March 31, 2020, there was approximately $18.2 million of related unrecognized compensation cost, which the Company expects to recognize over a remaining weighted-average period of 2.05 years.
Performance-Based Restricted Stock Units
On January 22, 2020, the Company granted performance-based restricted stock units (PSU) whereby vesting depends upon the occurrence of certain milestone events before December 31, 2022. As of March 31, 2020, none of the PSU milestones had been achieved. When achievement of a milestone becomes probable, compensation cost will be recognized from the grant date over the requisite service period. As of March 31, 2020, no related compensation cost had been recognized. The following table summarizes PSU activity under the 2013 Plan during the three months ended March 31, 2020 (in thousands, except per share amounts):
 
Number
of Stock Units
 
Weighted-
Average
Grant Date Fair Value Per Share
Unvested balance at December 31, 2019
—

 
$
—

Granted (1)
78

 
$
52.99

Vested
—

 
$
—

Forfeited
(3
)
 
$
52.99

Unvested balance at March 31, 2020
75

 
$
52.99


(1)
Pursuant to the terms of the awards granted on January 22, 2020, the actual number of awards earned could range between 0% and 200% of the number of awards granted.
As of March 31, 2020, there was approximately $4.0 million of related unrecognized compensation cost, which the Company expects to recognize over a remaining weighted-average period of 2.75 years. Depending on the actual number of awards earned, the actual expense recognized could range between 0% and 200% of this number.