Equity Incentive Plans, Employee Stock Purchase Plan and Stock-Based Compensation (Tables)
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12 Months Ended |
Dec. 31, 2020 |
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| Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
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| Summary of Option Activity |
The following table summarizes option activities under the Company’s 2004 Plan and 2018 Plan:
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|
Outstanding
Options
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|
|
Weighted-
Average
Exercise Price
|
|
|
Weighted-
Average
Remaining
Contract Term
(Years)
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Aggregate
Intrinsic Value
(in thousands)
|
|
|
Balances at December 31, 2019
|
|
|
3,872,664
|
|
|
$
|
12.89
|
|
|
|
7.88
|
|
|
$
|
2,119
|
|
|
Granted
|
|
|
1,856,078
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|
|
$
|
9.70
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|
|
|
|
|
|
|
|
|
|
Exercised
|
|
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(171,354
|
)
|
|
$
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10.86
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|
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|
|
|
|
|
|
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Canceled/Forfeited
|
|
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(118,093
|
)
|
|
$
|
10.00
|
|
|
|
|
|
|
|
|
|
|
Balances at December 31, 2020
|
|
|
5,439,295
|
|
|
$
|
11.93
|
|
|
|
7.75
|
|
|
$
|
53,202
|
|
|
Exercisable at December 31, 2020
|
|
|
2,628,592
|
|
|
$
|
12.47
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|
|
|
6.84
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|
|
$
|
24,285
|
|
|
| Schedule of Employee Stock Options Valuation |
For determining stock-based compensation expense under the Plan, the fair-value-based measurement of our share-based payments were estimated as of the date of grant using the Black-Scholes option pricing model with assumptions as follows:
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Year Ended December 31,
|
|
|
|
|
2020
|
|
|
2019
|
|
|
2018
|
|
|
Expected term (in years)
|
|
3.1-7.0
|
|
|
4.5-7.0
|
|
|
5.3-6.1
|
|
|
Expected volatility
|
|
73.2%-87.4%
|
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|
72.7%-74.9%
|
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|
54.6%-62.4%
|
|
|
Risk-free interest rate
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|
0.2%-1.6%
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|
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1.4%-2.6%
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|
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2.7%-3.1%
|
|
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Expected dividend
|
|
|
-
|
|
|
|
-
|
|
|
|
-
|
|
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| Summary of Status and Activity of Non-vested RSUs |
A summary of the status and activity of non-vested RSUs during the years ended December 31, 2020, 2019 and 2018 is as follows:
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Number of
Shares
|
|
|
Weighted
Average
Grant-Date
Fair Value
|
|
|
Non-vested December 31, 2019
|
|
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335,799
|
|
|
$
|
13.49
|
|
|
Granted
|
|
|
530,250
|
|
|
$
|
8.64
|
|
|
Released
|
|
|
(151,976
|
)
|
|
$
|
14.03
|
|
|
Canceled
|
|
|
(47,698
|
)
|
|
$
|
8.86
|
|
|
Non-vested December 31, 2020
|
|
|
666,375
|
|
|
$
|
9.83
|
|
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| Schedule of Fair Value of ESPP Shares Using Option Pricing Model |
For the years ended December 31, 2020, 2019 and 2018, the fair value of ESPP shares was estimated using the following assumptions:
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Year Ended December 31,
|
|
|
|
|
|
|
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2020
|
|
|
2019
|
|
|
2018
|
|
|
Expected term (in years)
|
|
0.5
|
|
|
0.5
|
|
|
0.5
|
|
|
Expected volatility
|
|
63.0%-111.4%
|
|
|
63.0%-83.2%
|
|
|
|
55.3
|
%
|
|
Risk-free interest rate
|
|
0.1%-1.9%
|
|
|
1.9%-2.5%
|
|
|
|
2.4
|
%
|
|
Expected dividend
|
|
|
-
|
|
|
|
-
|
|
|
|
-
|
|
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| Schedule of Stock-Based Compensation Expense Recognized |
Total stock-based compensation expense recognized was as follows:
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|
Year Ended December 31,
|
|
|
|
|
2020
|
|
|
2019
|
|
|
2018
|
|
|
|
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(in thousands)
|
|
|
|
|
|
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Research and development expense:
|
|
|
|
|
|
|
|
|
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|
|
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|
Stock options
|
|
$
|
1,405
|
|
|
$
|
903
|
|
|
$
|
425
|
|
|
Restricted stock units
|
|
|
770
|
|
|
|
623
|
|
|
|
95
|
|
|
ESPP
|
|
|
512
|
|
|
|
389
|
|
|
|
99
|
|
|
Subtotal
|
|
|
2,687
|
|
|
|
1,915
|
|
|
|
619
|
|
|
General and administrative expense:
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Stock options
|
|
|
7,098
|
|
|
|
6,815
|
|
|
|
1,895
|
|
|
Restricted stock units
|
|
|
2,021
|
|
|
|
1,464
|
|
|
|
323
|
|
|
ESPP
|
|
|
111
|
|
|
|
118
|
|
|
|
35
|
|
|
Subtotal
|
|
|
9,230
|
|
|
|
8,397
|
|
|
|
2,253
|
|
|
Total
|
|
$
|
11,917
|
|
|
$
|
10,312
|
|
|
$
|
2,872
|
|
|
| Summary of the Status of Call Options |
A summary of the status of the call options at December 31, 2020 and 2019 is as follows:
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|
December 31, 2020
|
|
|
December 31, 2019
|
|
|
|
|
|
Shares
|
|
|
Shares
|
|
|
|
Options vested and exercised
|
|
|
257,834
|
|
|
|
191,153
|
|
|
|
Options vested and outstanding
|
|
-
|
|
|
-
|
|
|
|
Options unvested and outstanding
|
|
|
8,890
|
|
|
|
75,571
|
|
|
|
Total options granted
|
|
|
266,724
|
|
|
|
266,724
|
|
|
|
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