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Summary of Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2020
Accounting Policies [Abstract] |
Summary of Fair Value Measurement Financial Assets | Financial assets measured at fair value on a recurring basis were as follows (in thousands):
| Fair Value Measurement at December 31, 2020
|  |  | Quoted Prices in Active Markets for Identical Assets |  | Significant Other Observable Inputs |  | Significant Unobservable Inputs
| Total |  | Level 1 |  | Level 2 |  | Level 3
Assets: |  |  |  |  |  |  |
Money market funds | $ | 49,004 |  |  | $ | 49,004 |  |  | $ | — |  |  | $ | — |
U.S Treasury securities | 60,623 |  |  | — |  |  | 60,623 |  |  | — |
Corporate debt securities | 33,776 |  |  | — |  |  | 33,776 |  |  | — |
Total assets measured at fair value (a) | $ | 143,403 |  |  | $ | 49,004 |  |  | $ | 94,399 |  |  | $ | — |
| Fair Value Measurement at December 31, 2019
|  |  | Quoted Prices in Active Markets for Identical Assets |  | Significant Other Observable Inputs |  | Significant Unobservable Inputs
| Total |  | Level 1 |  | Level 2 |  | Level 3
Assets: |  |  |  |  |  |  |
Money market funds | $ | 46,149 |  |  | $ | 46,149 |  |  | $ | — |  |  | $ | — |
Government-sponsored enterprises | 13,222 |  |  | — |  |  | 13,222 |  |  | — |
Corporate debt securities | 103,135 |  |  | — |  |  | 103,135 |  |  | — |
Total assets measured at fair value (b) | $ | 162,506 |  |  | $ | 46,149 |  |  | $ | 116,357 |  |  | $ | — |
(a) Total assets measured at fair value at December 31, 2020 includes approximately $52.0 million reported in cash and cash equivalents on the balance sheet. (b) Total assets measured at fair value at December 31, 2019 includes approximately $73.2 million reported in cash and cash equivalents on the balance sheet.
Summary of Percentage of Customer Concentration | The following table includes those collaborators that represent more than 10% of total revenue earned in the periods indicated:
| Year Ended December 31,
| 2020 |  | 2019 |  | 2018
Incyte Corporation (Incyte) | 47% |  | 35% |  | 68%
Janssen Biotech, Inc. (Janssen) | 19% |  | * |  | *
Zai Lab Limited (Zai Lab) | 11% |  | 29% |  | *
Les Laboratoires Servier and Institut de Recherches Servier (Servier) | * |  | 18% |  | *
* Balance is less than 10% The following table includes those counterparties that represent more than 10% of accounts receivable at the date indicated:
| December 31,
| 2020 |  | 2019
Janssen | 87% |  | *
Incyte | * |  | 62%
Zai Lab | * |  | 23%
Estimated Useful Lives | Depreciation and amortization are computed using the straight-line method over the following estimated useful lives:
Computer equipment | 3 years
Software | 3 years
Furniture | 10 years
Laboratory and office equipment | 5 years
Leasehold improvements | Shorter of lease term or useful life
Schedule of Stock Options and RSUs Excluded from the Calculation of Net Loss Per Share | The following table presents the number of stock options and RSUs that were excluded from the calculation of net loss per share:
| Year Ended December 31,
| 2020 |  | 2019 |  | 2018
Stock options and RSUs | 7,467,603 |  | 7,159,494 |  | 5,273,964

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- Definition Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19279-110258 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2C -URI http://asc.fasb.org/extlink&oid=117815213&loc=SL7498348-110258 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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