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Equity-based Compensation and Equity Incentive Plans (Tables) | 12 Months Ended
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Dec. 31, 2018
Schedule of Restricted Common Stock Activity | The following table shows restricted common stock activity:
|  | Restricted stock awards |
|  |  |  |  |  | Weighted average grant date |
|  | Number of |  |  | fair value |
|  | shares |  |  | (per share) |
Unvested at December 31, 2016 |  |  | — |  |  | $ | — |
Awards exchanged upon 2017 Reorganization |  |  | 1,059,912 |  |  |  | 13.53 |
Granted |  |  | 2,884 |  |  |  | 19.01 |
Vested |  |  | (671,204 | ) |  |  | 13.54 |
Forfeited |  |  | (16,113 | ) |  |  | 13.53 |
Unvested at December 31, 2017 |  |  | 375,479 |  |  | $ | 13.55 |
Granted |  |  | — |  |  |  | — |
Vested |  |  | (186,332 | ) |  |  | 13.54 |
Forfeited |  |  | (70,468 | ) |  |  | 13.61 |
Unvested at December 31, 2018 |  |  | 118,679 |  |  | $ | 13.54 |
Schedule of Equity-based Compensation Expenses | The following table summarizes equity‑based compensation expense within the Company’s consolidated statements of operations and comprehensive loss for the years ended December 31, 2018 and 2017 (in thousands):
|  | Years ended December 31, |
|  | 2018 |  |  | 2017 |
Research and development |  | $ | 1,333 |  |  | $ | 1,410 |
General and administrative |  |  | 2,984 |  |  |  | 1,242 |
|  | $ | 4,317 |  |  | $ | 2,652 |
Schedule of Equity-based Compensation Expenses by Award Type | The following table summarizes equity‑based compensation expense by type of award for the years ended December 31, 2018 and 2017 (in thousands):
|  | Years ended December 31, |
|  | 2018 |  |  | 2017 |
Stock options |  | $ | 3,361 |  |  | $ | 1,956 |
Restricted stock awards |  |  | 956 |  |  |  | 508 |
Incentive units |  |  | — |  |  |  | 132 |
Restricted common units |  |  | — |  |  |  | 56 |
|  | $ | 4,317 |  |  | $ | 2,652 |
Restricted Common Units |
Schedule of Restricted Common Unit Activity | The following table shows the restricted common unit activity for the year ended December 31, 2017, prior to the 2017 Reorganization, as adjusted for the Merger:
|  | Restricted common units |
|  |  |  |  |  | Grant date |
|  | Number of |  |  | fair value |
|  | units |  |  | (per unit) |
Unvested at December 31, 2016 |  |  | 219,087 |  |  | $ | 1.48 |
Granted |  |  | — |  |  |  | — |
Vested |  |  | (37,770 | ) |  |  | 1.48 |
Forfeited |  |  | — |  |  |  | — |
Exchanged as part of 2017 Reorganization |  |  | (181,317 | ) |  |  | 1.48 |
Unvested at December 31, 2017 |  |  | — |  |  | $ | — |
2015 and 2017 Plan |
Schedule of Weighted Average Assumption Used Black-Scholes Option-pricing Model for Stock Options Issued to Employees and Nonemployees | The weighted average assumptions used in the Black-Scholes option-pricing model for stock options issued to employees under its two active equity plans, the 2015 Plan and the 2017 Plan, during the years ended December 31, 2018 and 2017 and to nonemployees during the year ended December 31, 2017 were:
|  | Year ended December 31, |  |
Employees: |  | 2018 |  |  | 2017 |  |
Expected term |  | 6.2 years |  |  | 6.2 years |  |
Weighted-average, risk-free interest rate |  |  | 2.8 | % |  |  | 2.1 | % |
Expected volatility |  |  | 70.8 | % |  |  | 70.4 | % |
Dividend yield |  |  | — |  |  |  | — |  |
Schedule of Stock Option Activity | The following table summarizes stock option activity, as adjusted for the Exchange Ratio under the 2015 and 2017 Plans.
|  | Stock options outstanding |
|  |  |  |  |  |  |  |  |  | Weighted |  |  |  |  |
|  |  |  |  |  |  |  |  |  | average |  |  |  |  |
|  |  |  |  |  | Weighted |  |  | remaining |  |  | Aggregate |
|  |  |  |  |  | average |  |  | contractual |  |  | intrinsic |
|  | Number of |  |  | exercise |  |  | term |  |  | value (a) |
|  | options |  |  | price |  |  | (in years) |  |  | (in thousands) |
Outstanding at December 31, 2017 |  |  | 1,267,221 |  |  | $ | 13.62 |  |  |  | 9.6 |  |  | $ | — |
Granted |  |  | 919,496 |  |  |  | 9.71 |  |  |  |  |  |  |  |  |
Exercised |  |  | (19,830 | ) |  |  | 12.28 |  |  |  |  |  |  |  |  |
Forfeited |  |  | (394,136 | ) |  |  | 12.09 |  |  |  |  |  |  |  |  |
Expired |  |  | (32,867 | ) |  |  | 13.53 |  |  |  |  |  |  |  |  |
Outstanding at December 31, 2018 |  |  | 1,739,884 |  |  |  | 11.92 |  |  |  | 9.0 |  |  | $ | — |
Vested or expected to vest at December 31, 2018 |  |  | 1,739,884 |  |  |  | 11.92 |  |  |  | 9.0 |  |  | $ | — |
Exercisable at December 31, 2018 |  |  | 489,236 |  |  |  | 13.20 |  |  |  | 8.4 |  |  | $ | — |
(a) | The aggregate intrinsic value is calculated as the difference between the exercise price of the options and the fair market value of the underlying common stock for the options that were in the money at December 31, 2018 . No options were in the money at December 31, 2018 .
Plan 2015 |
Schedule of Stock Option Activity | The following table represents a summary of incentive unit activity, as adjusted for the Merger, under the 2015 LLC Plan:
|  | Incentive units |
|  |  |  |  |  | Weighted- |  |  | Weighted- |  |  | Weighted- |
|  |  |  |  |  | average |  |  | average |  |  | average |
|  | Number of |  |  | strike |  |  | threshold |  |  | grant date |
|  | units |  |  | price |  |  | price |  |  | fair value |
Non-vested units at December 31, 2016 |  |  | 971,906 |  |  | $ | 5.22 |  |  | $ | 5.93 |  |  | $ | 1.01 |
Granted |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |
Vested |  |  | (73,719 | ) |  |  | 4.01 |  |  |  | 5.53 |  |  |  | 0.87 |
Forfeited |  |  | (260,145 | ) |  |  | 4.19 |  |  |  | 5.57 |  |  |  | 1.05 |
Non-vested units cancelled upon 2017 Reorganization |  |  | (638,042 | ) |  |  | 5.78 |  |  |  | 6.15 |  |  |  | 1.05 |
Non-vested units at December 31, 2017 |  |  | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |
Vested or expected to vest at December 31, 2017 |  |  | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |

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- Definition Tabular disclosure of components of a stock option or other award plan under which equity-based compensation is awarded to employees, typically comprised of the amount of unearned compensation (deferred compensation cost), compensation expense, and changes in the quantity and fair value of the shares (or other type of equity) granted, exercised, forfeited, and issued and outstanding pertaining to that plan. Disclosure may also include nature and general terms of such arrangements that existed during the period and potential effects of those arrangements on shareholders, effect of compensation cost arising from equity-based payment arrangements on the income statement, method of estimating the fair value of the goods or services received, or the fair value of the equity instruments granted, during the period, cash flow effects resulting from equity-based payment arrangements and, for registrants that accelerate vesting of out of the money share options, reasons for the decision to accelerate. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5047-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the allocation of equity-based compensation costs to a given line item on the balance sheet and income statement for the period. This may include the reporting line for the costs and the amount capitalized and expensed. + 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=109197908&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=109197908&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=109197908&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=109197908&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=109197908&loc=d3e5070-113901 + Details
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