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Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Allocated Employee Stock-Based Compensation Expense
We recorded and allocated employee stock-based compensation expense for our equity incentive plans and our 2000 Employee Stock Purchase Plan (“ESPP”) as follows (in thousands):
 
Three Months Ended March 31,
 
2018
 
2017
Research and development
$
3,033

 
$
1,478

Selling, general and administrative
6,272

 
3,235

Total stock-based compensation
$
9,305

 
$
4,713
Weighted Average Grant-Date Fair Value of Stock Options and ESPP Purchases
We use the Black-Scholes Merton option pricing model to value our stock options and ESPP purchases. The weighted average grant-date fair value per share of our stock options and ESPP purchases was as follows:
 
Three Months Ended March 31,
 
2018
 
2017
Stock options
$
11.52

 
$
9.92

ESPP
$
7.39

 
$
3.71
Valuation Assumptions Used to Estimate Grant-Date Fair Value
The grant-date fair value of employee stock option grants and ESPP purchases was estimated using the following assumptions:
 
Three Months Ended March 31,
 
2018
 
2017
Stock options:
 
 
 
Risk-free interest rate
2.40
%
 
1.62
%
Dividend yield
—
%
 
—
%
Volatility
54
%
 
64
%
Expected life
4.0 years

 
4.0 years

ESPP:
 
 
 
Risk-free interest rate
1.53
%
 
0.62
%
Dividend yield
—
%
 
—
%
Volatility
53
%
 
68
%
Expected life
6 months

 
6 months
Stock Option Activity
Stock option activity for the three months ended March 31, 2018 was as follows (dollars in thousands, except per share amounts):
 
Shares
 
Weighted
Average
Exercise Price Per Share
 
Weighted
Average
Remaining Contractual
Term
 
Aggregate
Intrinsic
Value
Options outstanding at December 31, 2017
22,208,446

 
$
6.83

 
 
 
 
Granted
293,580

 
$
25.72

 
 
 
 
Exercised
(288,196
)
 
$
6.69

 
 
 
 
Forfeited
(18,484
)
 
$
12.04

 
 
 
 
Options outstanding at March 31, 2018
22,195,346

 
$
7.08

 
3.86 years
 
$
339,631

Exercisable at March 31, 2018
16,497,014

 
$
4.66

 
3.30 years
 
$
288,535
RSU Activity
RSU activity for the three months ended March 31, 2018 was as follows (dollars in thousands, except per share amounts):
 
Shares
 
Weighted
Average
Grant Date
Fair Value Per Share
 
Weighted
Average
Remaining
Contractual
Term
 
Aggregate
Intrinsic
Value
RSUs outstanding at December 31, 2017
3,762,990

 
$
17.76

 
 
 
 
Awarded
146,790

 
$
25.72

 
 
 
 
Vested and released
(197,884
)
 
$
6.30

 
 
 
 
Forfeited
(24,494
)
 
$
17.89

 
 
 
 
RSUs outstanding at March 31, 2018
3,687,402

 
$
18.69

 
1.83 years
 
$
81,676