| Stock-Based Compensation |
Note 10—Stock-Based Compensation Stock-based compensation expense includes the amortization of stock options granted to employees and non-employees and has been reported in our Condensed Consolidated Statements of Operations and Comprehensive Loss as follows: | | | | | | | | | | | | | | | Three Months Ended | | Nine Months Ended | | | September 30, | | September 30, | | | 2019 | | 2018 | | 2019 | | 2018 | | | (In thousands) | | (In thousands) | Research and development | | $ | 1,558 | | $ | 1,334 | | $ | 4,698 | | $ | 3,720 | Selling, general and administrative | | | 1,938 | | | 1,595 | | | 5,770 | | | 5,176 | Total | | $ | 3,496 | | $ | 2,929 | | $ | 10,468 | | $ | 8,896 |
The fair value of each option grant to employees, directors and non-employees is estimated on the date of grant using the Black-Scholes option-pricing model. The following assumptions were applied to all stock option grants: | | | | | | | | | | | | | | | | | Three Months Ended | | | Nine Months Ended | | | | September 30, | | | September 30, | | | | 2019 | | 2018 | | | 2019 | | 2018 | | Estimated weighted-average fair value | | $ | 11.81 | | $ | 16.30 | | | $ | 9.91 | | $ | 10.22 | | Weighted-average assumptions: | | | | | | | | | | | | | | | Expected volatility | | | 79 | % | | 78 | % | | | 81 | % | | 77 | % | Expected term, in years | | | 6.1 | | | 6.1 | | | | 6.0 | | | 6.0 | | Risk-free interest rate | | | 2.10 | % | | 2.77 | % | | | 2.44 | % | | 2.66 | % | Expected dividend yield | | | — | % | | — | % | | | — | % | | — | % |
Stock option activity for all stock plans and related information is as follows: | | | | | | | | | | | | | | | Weighted- | | | | | | | | | | Average | | | | Aggregate | | | | | Exercise | | Remaining | | Intrinsic | | | Options | | Price per | | Contractual Life | | Value | | | Outstanding | | Share | | (In years) | | (In thousands) | Balance at December 31, 2018 | | 10,313,138 | | $ | 11.22 | | | | | | Granted | | 1,956,205 | | | 14.17 | | | | | | Exercised | | (513,790) | | | 9.80 | | | | | | Forfeited | | (260,831) | | | 16.07 | | | | | | Balance at September 30, 2019 | | 11,494,722 | | $ | 11.67 | | 6.25 | | $ | 56,610 | Vested and expected to vest at September 30, 2019 | | 11,126,906 | | $ | 11.59 | | 6.17 | | $ | 55,668 | Exercisable at September 30, 2019 | | 8,117,842 | | $ | 10.68 | | 5.20 | | $ | 47,725 |
At September 30, 2019, there were 3,376,880 unvested options outstanding that will vest over a weighted-average period of 2.7 years and 5,396,836 shares were available to grant. The total estimated compensation expense yet to be recognized on outstanding options is $27.4 million.
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