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Stock-Based Compensation (Tables) | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2019 | Dec. 31, 2018
Stock-Based Compensation |  |
Summary of stock option activity |
|  | Options Outstanding and Exercisable
|  |  |  |  |  |  | Weighted |  |  |
|  |  |  | Weighted |  | Average |  | Aggregate
|  |  |  | Average |  | Remaining |  | Intrinsic
|  | Number of |  | Exercise |  | Contractual |  | Value(1)
|  | Shares |  | Price |  | Life (Years) |  | (in thousands)
Outstanding as of December 31, 2018 |  | 2,056,891 |  | $ | 4.74 |  | 9.4 |  | $ | 5,946
Granted |  | 247,275 |  | $ | 18.18 |  | 10.0 |  |  |
Exercised |  | (53,963) |  | $ | 1.20 |  | 8.1 |  |  |
Forfeitures |  | (1,250) |  | $ | 7.63 |  | 9.5 |  |  |
Outstanding as of September 30, 2019 |  | 2,248,953 |  | $ | 6.30 |  | 9.1 |  | $ | 55,305
Vested and expected to vest as of September 30, 2019 |  | 2,248,953 |  | $ | 6.30 |  |  |  | $ | 55,305
Exercisable as of September 30, 2019 |  | 450,866 |  | $ | 1.15 |  |  |  | $ | 12,335
The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at September 30, 2019 and December 31, 2018. | A summary of stock option activity for awards under the 2015 Plan is presented below:
|  | Options Outstanding and Exercisable
|  |  |  |  |  |  | Weighted |  |  |
|  |  |  | Weighted |  | Average |  | Aggregate
|  |  |  | Average |  | Remaining |  | Intrinsic
|  | Number of |  | Exercise |  | Contractual |  | Value(1)
|  | Shares |  | Price |  | Life (Years) |  | (in thousands)
Outstanding as of January 1, 2017 |  | 189,189 |  | $ | 0.48 |  | 8.6 |  | $ | 137
Granted |  | 317,397 |  |  | 1.21 |  |  |  |  |
Outstanding as of December 31, 2017 |  | 506,586 |  |  | 0.94 |  | 9.0 |  |  | 137
Granted |  | 1,570,136 |  |  | 5.92 |  | 9.9 |  |  | 2,684
Exercised |  | (5,599) |  |  | 0.84 |  |  |  |  |
Forfeitures |  | (14,232) |  |  | 1.11 |  |  |  |  |
Outstanding as of December 31, 2018 |  | 2,056,891 |  |  | 4.74 |  | 9.4 |  |  | 5,946
Vested and expected to vest as of December 31, 2018 |  | 2,056,891 |  |  | 4.74 |  | 9.4 |  |  | 5,946
Exercisable as of December 31, 2018 |  | 242,079 |  |  | 0.88 |  | 7.9 |  |  | 1,634
| (1) |  | The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at December 31, 2018 and 2017.
Summary of stock based compensation expense recorded | Stock‑based compensation expense recorded as research and development and general and administrative expenses is as follows (in thousands):
|  | Three Months Ended |  | Nine Months Ended |
|  | September 30, |  | September 30, |
|  | 2019 |  | 2018 |  | 2019 |  | 2018 |
Research and development |  | $ | 182 |  | $ | 41 |  | $ | 482 |  | $ | 62 |
General and administrative |  |  | 342 |  |  | 49 |  |  | 837 |  |  | 76 |
Total stock-based compensation expense |  | $ | 524 |  | $ | 90 |  | $ | 1,319 |  | $ | 138 |
| Stock‑based compensation expense recorded as research and development and general and administrative expenses is as follows (in thousands):
|  | December 31,
|  | 2018 |  | 2017
Research and development |  | $ | 85 |  | $ | 35
General and administrative |  |  | 178 |  |  | 40
Total stock-based compensation expense |  | $ | 263 |  | $ | 75

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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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