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Stock-Based Compensation | 6 Months Ended
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Jun. 30, 2019
Share-based Payment Arrangement [Abstract] |
Stock-Based Compensation | Stock-Based Compensation Employee Stock Purchase Plan In May 2017, the Company’s stockholders approved the 2016 Employee Stock Purchase Plan (the 2016 ESPP). The 2016 ESPP is structured as a qualified employee stock purchase plan under Section 423 of the Internal Revenue Code of 1986, as amended (IRC), and is not subject to the provisions of the Employee Retirement Income Security Act of 1974. The Company reserved 800,000 shares of common stock for issuance under the 2016 ESPP. The 2016 ESPP allows eligible employees to purchase shares of the Company’s common stock at a discount through payroll deductions of up to 10% of their eligible compensation, subject to any plan limitations. The 2016 ESPP provides for six-month offering periods ending on May 31 and November 30 of each year. At the end of each offering period, employees are able to purchase shares at 85% of the fair market value of the Company’s common stock on the last day of the offering period. During the six months ended June 30, 2020, 18,304 shares of common stock were purchased under the 2016 ESPP for net proceeds to the Company of approximately $0.3 million. Employee Stock Option Plans Effective February 2003, the Company implemented the 2003 Equity Incentive Plan (2003 Plan), and it was amended and approved by the Company's stockholders in 2005. Stock options granted under the 2003 Plan may be either incentive stock options as defined by the IRC, or non-qualified stock options. In 2013, the 2003 Plan was terminated, and no further awards may be issued under the plan. Any shares of common stock subject to awards under the 2003 Plan that expire, terminate, or are otherwise surrendered, canceled, forfeited or repurchased without having been fully exercised, or resulting in any common stock being issued, will become available for issuance under the 2013 Stock Incentive Plan (2013 Plan), up to a specified number of shares. As of June 30, 2020, under the 2003 Plan, there were options to purchase an aggregate of 281,024 shares of common stock outstanding at a weighted average exercise price of $2.79 per share. In October 2013, the Company implemented the 2013 Plan. The 2013 Plan provides for the grant of stock options and other stock-based awards, as well as cash-based performance awards. The number of shares of common stock reserved for issuance under the 2013 Plan will automatically increase on January 1 of each year from January 1, 2014 through and including January 1, 2023, by the lesser of (a) 1,960,168 shares, (b) 4.0% of the total number of shares of common stock outstanding on December 31 of the preceding calendar year, or (c) the number of shares of common stock determined by the Company's Board of Directors. During the six months ended June 30, 2020, the maximum number of shares of common stock authorized to be issued by the Company under the 2013 Plan was increased to 11,896,613. If an option expires or terminates for any reason without having been fully exercised, if any shares of restricted stock are forfeited, or if any award terminates, expires or is settled without all or a portion of the shares of common stock covered by the award being issued, such shares are available for the grant of additional awards. However, any shares that are withheld (or delivered) to pay withholding taxes or to pay the exercise price of an option are not available for the grant of additional awards. As of June 30, 2020, there were options to purchase an aggregate of 6,993,516 shares of common stock outstanding at a weighted average exercise price of $21.81 per share under the 2013 Plan. The following stock-based compensation expense was recognized for the periods indicated (in thousands):
| Three Months Ended June 30, |  |  |  |  |  | Six Months Ended June 30, |  |
| 2020 |  | 2019 |  |  |  | 2020 |  | 2019
Research and development | $ | 2,589 |  |  | $ | 2,453 |  |  |  |  | $ | 5,023 |  |  | $ | 4,267 |
General and administrative | 2,506 |  |  | 2,431 |  |  |  |  | 4,564 |  |  | 4,416 |
Total stock-based compensation expense | $ | 5,095 |  |  | $ | 4,884 |  |  |  |  | $ | 9,587 |  |  | $ | 8,683 |
The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table for options issued during the period indicated:
| Six Months Ended June 30, |  |
| 2020 |  | 2019
Expected dividend yield | 0% |  | 0%
Expected volatility | 67.3% - 108.8% |  | 73.7% - 75.3%
Risk-free interest rate | 0.5% - 1.8% |  | 1.9% - 2.6%
Expected term | 6.25 years |  | 6.25 years
For periods through December 31, 2019, the computation of expected volatility is based on the historical volatility of several public entities of similar size, complexity and stage of development, as the Company did not have sufficient history of its own volatility. As of December 31, 2019, the Company had sufficient company-specific historical and implied volatility information. As such, beginning the first quarter of 2020, the computation of expected volatility is based only on the historical volatility of the Company’s common stock. The following table summarizes stock option activity during the six months ended June 30, 2020:
| Shares |  | Weighted- Average Exercise Price |  | Weighted-Average Remaining Contractual Term (Years) |  | Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2019 | 6,706,994 |  |  | $ | 22.33 |  |  | 6.9 |  |
Granted | 1,547,627 |  |  | 12.67 |  |  |  |  |
Exercised | (329,592) |  |  | 7.31 |  |  |  |  |
Forfeited or expired | (650,489) |  |  | 20.90 |  |  |  |  |
Outstanding, June 30, 2020 | 7,274,540 |  |  | 21.07 |  |  | 7.2 |  | $ | 54,434 |
As of June 30, 2020: |  |  |  |  |  |  |
Exercisable | 4,134,978 |  |  | 23.32 |  |  | 5.9 |  | 23,276 |
Vested and expected to vest | 6,954,436 |  |  | 21.17 |  |  | 7.1 |  | 51,498 |
The weighted-average grant-date fair value of options granted during the six months ended June 30, 2020 and 2019 was $8.30 and $14.38, respectively. The total intrinsic value of options exercised during each of the six month periods ended June 30, 2020 and 2019 was approximately $2.6 million. The total cash received for options exercised during the six months ended June 30, 2020 and 2019 was approximately $2.4 million and $0.6 million, respectively. The total fair value of shares vested in the six months ended June 30, 2020 and 2019 was approximately $7.4 million and $6.5 million, respectively. As of June 30, 2020, the total unrecognized compensation expense related to unvested stock options, net of related forfeiture estimates, was approximately $31.2 million, which the Company expects to recognize over a weighted-average period of approximately 2.4 years. Restricted Stock Units During 2019, the Company awarded restricted stock units (RSUs) under the 2013 Plan to all employees with at least six months of service as of the date of grant except executive officers. Each RSU entitles the holder to receive one share of the Company's common stock when the RSU vests. The RSUs vest in two equal installments on the first and second anniversary of the grant date. Compensation expense is recognized on a straight-line basis. The following table summarizes RSU activity during the six months ended June 30, 2020:
| Shares |  | Weighted-Average Grant Date Fair Value
Outstanding, December 31, 2019 | 452,500 |  |  | $ | 15.32 |
Granted | 15,000 |  |  | 23.68 |
Exercised | — |  |  | — |
Forfeited | (29,100) |  |  | 15.32 |
Outstanding, June 30, 2020 | 438,400 |  |  | 15.61 |
At June 30, 2020, there was $3.7 million of total unrecognized compensation cost related to unvested RSUs, which the Company expects to recognize over a remaining weighted-average period of 1.2 years.

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