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Stock Plan and 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 Includes Options Granted | The following table, which includes options granted under the 2015 Plan, 2018 Plan and the Non-Plan Option, summarizes option activity:
|  | Shares Subject to Outstanding Options |  |  | Weighted Average Exercise Price Per Share |  |  | Weighted Average Remaining Contractual Term (in years) |  |  | Aggregate Intrinsic Value (in thousands) |
Outstanding at December 31, 2018 |  |  | 1,458,080 |  |  | $ | 7.55 |  |  |  |  |  |  |  |  |
Options granted |  |  | 3,967,746 |  |  | $ | 9.66 |  |  |  |  |  |  |  |  |
Options exercised |  |  | (41,762 | ) |  | $ | 4.85 |  |  |  |  |  |  |  |  |
Options forfeited or canceled |  |  | (646,060 | ) |  | $ | 10.01 |  |  |  |  |  |  |  |  |
Outstanding at December 31, 2019 |  |  | 4,738,004 |  |  | $ | 9.00 |  |  |  | 8.83 |  |  | $ | 8,174 |
Options vested and expected to vest as of December 31, 2019 |  |  | 4,738,004 |  |  | $ | 9.00 |  |  |  | 8.83 |  |  | $ | 8,174 |
Options exercisable as of December 31, 2019 |  |  | 1,145,303 |  |  | $ | 8.52 |  |  |  | 7.97 |  |  | $ | 2,851 |
Summary of Non-Vested Restricted Stock | A summary of the Company’s non-vested restricted stock for the periods presented is as follows:
|  | Number of Shares |  |  | Weighted Average Grant Date Fair Value |  |  | Remaining Contractual Term (Years) |
Balance, December 31, 2018 |  |  | 289,352 |  |  | $ | 0.000396 |  |  |  | 0.4 |
Vested during the year |  |  | 289,352 |  |  | $ | 0.000396 |  |  |  |  |
Balance, December 31, 2019 |  |  | - |  |  | $ | - |  |  |  | - |
Summary of Employee and Non-Employee Stock-based Compensation Expense | The following table summarizes employee and non-employee stock-based compensation expense for the years ended December 31, 2019 and 2018, and also the allocation within the consolidated statements of operations and comprehensive loss (in thousands):
|  | Year Ended December 31, |
|  | 2019 |  |  | 2018 |
Research and development |  | $ | 4,152 |  |  | $ | 2,255 |
General and administrative |  |  | 4,829 |  |  |  | 1,619 |
Total stock-based compensation |  | $ | 8,981 |  |  | $ | 3,874 |
Schedule of Assumptions Used to Calculate Fair Value of Stock-Based Compensation | The following assumptions were used to calculate the fair value of stock-based compensation for the years ended December 31, 2019, and 2018:
|  | Stock Options |  |  | ESPP |
|  | Year Ended December 31, |  |  | Year Ended December 31, |
|  | 2019 |  |  | 2018 |  |  | 2019 |  |  | 2018 |
Risk-free interest rate |  | 1.6% - 2.3% |  |  | 1.2% - 3.1% |  |  | 1.6% - 2.3% |  |  | 2.1% - 2.6% |
Expected term (in years) |  | 6.02 |  |  | 5.16-9.95 |  |  | 0.5-2.0 |  |  | 0.5-2.0 |
Volatility |  | 71.8% - 74.6% |  |  | 58.7%-75.5% |  |  | 64.8% - 77.1% |  |  | 54.3% - 65.5% |
Dividend yield |  | 0% |  |  | 0% |  |  | 0% |  |  | 0% |

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- Definition Tabular disclosure of allocation of amount expensed and capitalized for award under share-based payment arrangement to statement of income or comprehensive income and statement of financial position. Includes, but is not limited to, corresponding line item in financial statement. + 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 (h)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5047-113901 + Details
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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + 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 (d) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of employee stock purchase plans, including, but not limited to: (a) expected term, (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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