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Stock Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Summary of Option Activity Under Company's Equity Incentive Plans | The following table summarizes stock option activity under the Company’s equity incentive plans and related information:
|  | Number of Shares Underlying Outstanding Options |  |  | Weighted- Average Exercise Price |  |  | Weighted- Average Remaining Contractual Term (Years) |  |  | Aggregate Intrinsic Value (thousands) |
Outstanding — December 31, 2017 |  |  | 821,741 |  |  | $ | 13.69 |  |  |  | 9.17 |  |  | $ | 6,376 |
Options granted |  |  | 612,050 |  |  | $ | 13.13 |  |  |  |  |  |  |  |  |
Options exercised |  |  | (12,716 | ) |  | $ | 4.17 |  |  |  |  |  |  | $ | 213 |
Options canceled/forfeited |  |  | (43,315 | ) |  | $ | 7.76 |  |  |  |  |  |  |  |  |
Options expired |  |  | (15,783 | ) |  | $ | 158.81 |  |  |  |  |  |  |  |  |
Outstanding — December 31, 2018 |  |  | 1,361,977 |  |  | $ | 12.04 |  |  |  | 8.71 |  |  | $ | 2,294 |
Options granted |  |  | 450,900 |  |  | $ | 7.85 |  |  |  |  |  |  |  |  |
Options exercised |  |  | (41,219 | ) |  | $ | 4.57 |  |  |  |  |  |  | $ | 159 |
Options forfeited |  |  | (182,722 | ) |  | $ | 8.36 |  |  |  |  |  |  |  |  |
Options expired |  |  | (11,395 | ) |  | $ | 96.49 |  |  |  |  |  |  |  |  |
Outstanding — December 31, 2019 |  |  | 1,577,541 |  |  | $ | 10.85 |  |  |  | 8.15 |  |  | $ | 1,350 |
Exercisable — December 31, 2019 |  |  | 740,356 |  |  | $ | 13.30 |  |  |  |  |  |  |  |  |
Shares available to be granted — December 31, 2019 |  |  | 1,014,126 |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Weighted Average Valuation Assumptions Used to Estimate Fair Value of Employee Stock Options | The fair value of employee stock options was estimated using the following weighted-average assumptions for the year ended December 31, 2019 and 2018:
|  | Year Ended December 31, |
|  | 2019 |  |  | 2018 |
Employee Stock Options: |  |  |  |  |  |  |  |
Expected term (in years) |  |  | 5.98 |  |  |  | 6.04 |
Risk-free interest rate |  |  | 2.35 | % |  |  | 2.67 | %
Dividend yield |  |  | — |  |  |  | — |
Volatility |  |  | 90.81 | % |  |  | 92.61 | %
Weighted-average fair value of stock options granted |  | $ | 5.85 |  |  | $ | 9.97 |
Summary of Stock-Based Compensation Recognized | Total stock-based compensation recognized was as follows (in thousands):
|  | Year Ended December 31, |
|  | 2019 |  |  | 2018 |
Research and development |  | $ | 1,052 |  |  | $ | 606 |
General and administrative (1) |  |  | 2,152 |  |  |  | 2,000 |
Total stock-based compensation |  | $ | 3,204 |  |  | $ | 2,606 |
(1) | Included $0.03 and $0.1 million related to modifications of certain Board member stock options in the years ended December 31, 2019 and 2018, respectively. These modifications extended the post-termination exercise period of certain options.
Also included in general and administrative stock-based compensation for the year ended December 31, 2019 and 2018 is expense related to 24,235 and 6,790 shares of common stock issued to certain board members in lieu of their cash compensation, respectively. As of December 31, 2019, 1,014,126 shares of common stock were available for future grant and 1,577,541 options to purchase shares of common stock were outstanding. As of December 31, 2019, the Company had unrecognized employee stock-based compensation expense of $5.3 million, related to unvested stock option awards, which is expected to be recognized over an estimated weighted-average period of 2.41 years.

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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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