Basis of Presentation (Policies)	3 Months Ended
Mar. 31, 2020
Basis of Presentation
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires the Company to make estimates and assumptions that affect the reported amounts of assets and liabilities, and disclosure of contingent assets and liabilities, as of the date of the financial statements as well as the reported amounts of revenues and expenses during the reporting period. Significant estimates include the fair value of marketable securities that are classified as level 2 of the fair value hierarchy, useful lives of fixed assets, the periods over which certain revenues will be recognized, including licensing and collaborative revenue recognized from non-refundable up-front and milestone payments, the determination of prepaid research and development, or R&D, clinical costs and accrued research projects, the amount of non-cash compensation costs related to share-based payments to employees and non-employees and the periods over which those costs are expensed, the incremental borrowing rate used in lease calculations and the likelihood of realization of deferred tax assets. The COVID-19 pandemic is expected to result in a slowdown of economic activity that is likely to interrupt business operations across the globe. Estimates and assumptions about future events and their effects cannot be determined with certainty and therefore require the exercise of judgment. As of the date of issuance of these condensed financial statements, the Company is not aware of any specific event or circumstance that would require the Company to update its estimates, assumptions and judgments or revise the reported amounts of assets and liabilities or the disclosure of contingent assets and liabilities. These estimates, however, may change as new events occur and additional information is obtained, and are recognized in the condensed financial statements as soon as they become known. Actual results could differ materially from the Company’s estimates and assumptions.
Significant Accounting Policies	Significant Accounting Policies There have been no material changes to the significant accounting policies previously disclosed in Note 2 to the Financial Statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019, except for the recent adoption of new accounting pronouncements as disclosed below.
Recent Accounting Pronouncements	Accounting Pronouncements Recently Adopted On January 1, 2020, the Company adopted Accounting Standards Update, or ASU, No. 2016-13, Financial Instruments—Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments, or ASU 2016-13, which replaces the incurred loss impairment methodology in prior GAAP that delays recognition of a credit loss until it is probable that such loss has been incurred, with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. The Company deems certain of its investments to be marketable securities if the investment, or in the case of money market funds, the securities underlying the money market fund, meet the definition of a debt security in Accounting Standards Codification, or ASC, section 320-10-20. The Company considers its marketable securities to be available-for-sale, which are its only financial instruments that are within the scope of ASU 2016-13 as of March 31, 2020. The Company’s investments in marketable securities, including U.S. Treasury securities, U.S. government agency obligations, corporate bonds, commercial paper and municipal bonds, are highly rated by Moody’s and S&P and have maturities primarily of less than one year but no longer than two years. Accordingly, credit risk associated with the Company’s available-for-sale debt security portfolio is mitigated. ASU 2016-13 modifies the prior other-than-temporary impairment model for available-for-sale debt securities by requiring (1) estimating expected credit losses (the portion of the amortized cost basis of a financial asset that the Company does not expect to collect) only when the fair value is below the amortized cost of the asset; (2) recording a credit loss without regard to the length of time a security has been in an unrealized loss position; (3) limiting the measurement of the credit loss to the difference between the security’s amortized cost basis and its fair value; and (4) presenting credit losses as an allowance rather than as a write-down, which will allow the Company to record reversals of credit losses in current period net income, a practice that was previously prohibited. In April and November 2019, respectively, codification improvements were issued to help clarify and correct certain portions of ASU 2016-13. The Company reviews each of its available-for-sale marketable securities for unrealized losses (declines in fair value below its amortized cost basis) at each balance sheet date presented in its financial statements and whenever events or changes in circumstances indicate that the amortized cost basis of an asset may not be recoverable. In accordance with the adoption of ASU 2016-13, the Company is required to determine whether any portion of the unrealized loss for any available-for-sale debt security is due to a credit loss, and if so, to measure the amount of the credit loss. The Company will rely on both qualitative and quantitative factors to determine whether the unrealized loss for each available-for-sale debt security at any balance sheet date is due to a credit loss. Qualitative factors may include a credit downgrade, severity of the decline in fair value below amortized cost and other adverse conditions related specifically to the security, as well as the intent to sell the security, or whether the Company will more likely than not be required to sell the security before recovery of its amortized cost basis. The Company’s assessment of whether a security is impaired could change in the future due to new developments or changes in assumptions related to any particular security. If material qualitative factors indicate that a credit loss has occurred, the Company will determine the magnitude of that credit loss using a discounted cash flow model or other quantitative method. If the Company intends to sell the security or it is more likely than not that the Company will be forced to sell the security before recovery of the amortized cost of the security, the entire unrealized loss is deemed to be a credit loss, which is recognized in net income (loss). Otherwise, the portion of the unrealized loss that is due to a credit loss will be recorded as an Allowance for Credit Loss, which will offset the balance of Marketable Securities on the Condensed Balance Sheets and as credit loss expense within other income, net on the Condensed Statements of Comprehensive Loss. The portion of the unrealized loss that is not due to a credit loss as well as all unrealized gains will be recorded in Accumulated Other Comprehensive Income (Loss), or AOCI, net of taxes, on the Condensed Balance Sheets. There was no cumulative effect adjustment as a result of the adoption of ASU 2016-13 on January 1, 2020 (see Note 3, Available-for-Sale Marketable Securities, and Note 5, Fair Value Measurements). Accrued interest receivables are excluded from the Company’s amortized cost bases for its available-for-sale marketable securities and are included within Other Receivables on the Company’s Condensed Balance Sheets. The Company’s policy is to not measure an allowance for credit losses on accrued interest receivable balances at each reporting period since it elects to write off uncollectible accrued interest receivable balances as credit loss expense in a timely manner, which is by maturity date for all categories of its debt securities. On January 1, 2020, the Company adopted ASU 2019-08, Compensation – Stock Compensation (Topic 718) and Revenue from Contracts with Customers (Topic 606), or ASU 2019-08, which requires the Company to measure and classify share-based payment awards granted to a customer by applying the guidance in Topic 718. The amount recorded as a reduction to the transaction price is required to be measured on the basis of the grant-date fair value of the share-based payment award in accordance with Topic 718. The grant date is the date at which a grantor (supplier) and a grantee (customer) reach a mutual understanding of the key terms and conditions of a share-based payment award. The classification and subsequent measurement of the award are subject to the guidance in Topic 718 unless the share-based payment award is subsequently modified and the grantee is no longer a customer. The adoption of ASU 2019-08 did not have a material effect on its results of operations, financial position or cash flows since the Company has not historically granted share-based payment awards to customers. On January 1, 2020, the Company adopted ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606, or ASU 2018-18, which clarifies the interaction between Topic 808 and Topic 606 by (1) clarifying that certain transactions between collaborative arrangement participants should be accounted for under Topic 606; (2) adding unit-of-account guidance in Topic 808 to align with the guidance in Topic 606; and (3) clarifying presentation guidance for transactions with a collaborative arrangement participant that are not accounted for under Topic 606. The adoption of ASU 2018-18 did not have any effect on its financial position, results of operations or cash flows since all three of its collaboration and licensing agreements are accounted for under Topic 606 (see Note 10, Collaboration and Licensing Agreements and Note 11, Revenue Recognition). On January 1, 2020, the Company adopted ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement, or ASU 2018-13, which modifies the disclosure requirements on fair value measurements in Topic 820 to remove the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, the policy for timing of transfers between levels, and the valuation processes for Level 3 fair value measurements. ASU 2018-13 also amends Topic 820 to clarify that the measurement uncertainty disclosure is to communicate information about the uncertainty in measurement as of the reporting date. ASU 2018-13 also requires additional disclosure for changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements held at the end of the reporting period as well as the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. Upon adoption of ASU 2018-13, the Company did not have any assets or liabilities that are included in Level 3 fair value measurements and no retrospective treatment was applicable. As a result, the adoption of ASU 2018-13 did not have a material effect on its results of operations, financial position or cash flows. Accounting Pronouncements Not Yet Adopted In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740), or ASU 2019-12, which removes specific exceptions to the general principles in Topic 740. ASU 2019-12 eliminates the need for an organization to analyze whether the following apply in a given period: i) exception to the incremental approach for intra-period tax allocation; ii) exceptions to accounting for basis differences when there are ownership changes in foreign investments; and iii) exception to the general methodology for calculating income taxes in an interim period when a year-to-date loss exceeds the anticipated loss. ASU 2019-12 also simplifies the accounting for income taxes for: i) franchise taxes that are partially based on income; ii) transactions with a government that result in a step up in the tax basis of goodwill; (iii) separate financial statements of legal entities that are not subject to tax; and iv) enacted changes in tax laws in interim periods. The amendments in ASU 2019-12 are effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2020. Early adoption of the amendments is permitted. An entity that elects to early adopt the amendments in an interim period should reflect any adjustments as of the beginning of the annual period that includes that interim period and must adopt all the amendments in the same period. The amendments in ASU 2019-12 related to separate financial statements of legal entities that are not subject to tax should be applied on a retrospective basis for all periods presented. The amendments related to changes in ownership of foreign equity method investments or foreign subsidiaries should be applied on a modified retrospective basis through a cumulative-effect adjustment to retained earnings as of the beginning of the fiscal year of adoption. The amendments related to franchise taxes that are partially based on income should be applied on either a retrospective basis for all periods presented or a modified retrospective basis through a cumulative-effect adjustment to retained earnings as of the beginning of the fiscal year of adoption. All other amendments should be applied on a prospective basis. As such, the Company expects to adopt ASU 2019-12 on January 1, 2021 and is currently evaluating the effect it will have on its results of operations, financial position and cash flows.

Basis of Presentation (Policies)

3 Months Ended

Mar. 31, 2020

The preparation of financial statements in conformity with GAAP requires the Company to make estimates and assumptions that affect the reported amounts of assets and liabilities, and disclosure of contingent assets and liabilities, as of the date of the financial statements as well as the reported amounts of revenues and expenses during the reporting period. Significant estimates include the fair value of marketable securities that are classified as level 2 of the fair value hierarchy, useful lives of fixed assets, the periods over which certain revenues will be recognized, including licensing and collaborative revenue recognized from non-refundable up-front and milestone payments, the determination of prepaid research and development, or R&D, clinical costs and accrued research projects, the amount of non-cash compensation costs related to share-based payments to employees and non-employees and the periods over which those costs are expensed, the incremental borrowing rate used in lease calculations and the likelihood of realization of deferred tax assets.

The COVID-19 pandemic is expected to result in a slowdown of economic activity that is likely to interrupt business operations across the globe. Estimates and assumptions about future events and their effects cannot be determined with certainty and therefore require the exercise of judgment. As of the date of issuance of these condensed financial statements, the Company is not aware of any specific event or circumstance that would require the Company to update its estimates, assumptions and judgments or revise the reported amounts of assets and liabilities or the disclosure of contingent assets and liabilities. These estimates, however, may change as new events occur and additional information is obtained, and are recognized in the condensed financial statements as soon as they become known.

Actual results could differ materially from the Company’s estimates and assumptions.

Significant Accounting Policies

There have been no material changes to the significant accounting policies previously disclosed in Note 2 to the Financial Statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019, except for the recent adoption of new accounting pronouncements as disclosed below.

Recent Accounting Pronouncements

Accounting Pronouncements Recently Adopted

On January 1, 2020, the Company adopted Accounting Standards Update, or ASU, No. 2016-13, Financial Instruments—Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments, or ASU 2016-13, which replaces the incurred loss impairment methodology in prior GAAP that delays recognition of a credit loss until it is probable that such loss has been incurred, with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates.

The Company deems certain of its investments to be marketable securities if the investment, or in the case of money market funds, the securities underlying the money market fund, meet the definition of a debt security in Accounting Standards Codification, or ASC, section 320-10-20. The Company considers its marketable securities to be available-for-sale, which are its only financial instruments that are within the scope of ASU 2016-13 as of March 31, 2020. The Company’s investments in marketable securities, including U.S. Treasury securities, U.S. government agency obligations, corporate bonds, commercial paper and municipal bonds, are highly rated by Moody’s and S&P and have maturities primarily of less than one year but no longer than two years. Accordingly, credit risk associated with the Company’s available-for-sale debt security portfolio is mitigated.

ASU 2016-13 modifies the prior other-than-temporary impairment model for available-for-sale debt securities by requiring (1) estimating expected credit losses (the portion of the amortized cost basis of a financial asset that the

Company does not expect to collect) only when the fair value is below the amortized cost of the asset; (2) recording a credit loss without regard to the length of time a security has been in an unrealized loss position; (3) limiting the measurement of the credit loss to the difference between the security’s amortized cost basis and its fair value; and (4) presenting credit losses as an allowance rather than as a write-down, which will allow the Company to record reversals of credit losses in current period net income, a practice that was previously prohibited. In April and November 2019, respectively, codification improvements were issued to help clarify and correct certain portions of ASU 2016-13.

The Company reviews each of its available-for-sale marketable securities for unrealized losses (declines in fair value below its amortized cost basis) at each balance sheet date presented in its financial statements and whenever events or changes in circumstances indicate that the amortized cost basis of an asset may not be recoverable. In accordance with the adoption of ASU 2016-13, the Company is required to determine whether any portion of the unrealized loss for any available-for-sale debt security is due to a credit loss, and if so, to measure the amount of the credit loss.

The Company will rely on both qualitative and quantitative factors to determine whether the unrealized loss for each available-for-sale debt security at any balance sheet date is due to a credit loss.

Qualitative factors may include a credit downgrade, severity of the decline in fair value below amortized cost and other adverse conditions related specifically to the security, as well as the intent to sell the security, or whether the Company will more likely than not be required to sell the security before recovery of its amortized cost basis. The Company’s assessment of whether a security is impaired could change in the future due to new developments or changes in assumptions related to any particular security. If material qualitative factors indicate that a credit loss has occurred, the Company will determine the magnitude of that credit loss using a discounted cash flow model or other quantitative method.

If the Company intends to sell the security or it is more likely than not that the Company will be forced to sell the security before recovery of the amortized cost of the security, the entire unrealized loss is deemed to be a credit loss, which is recognized in net income (loss). Otherwise, the portion of the unrealized loss that is due to a credit loss will be recorded as an Allowance for Credit Loss, which will offset the balance of Marketable Securities on the Condensed Balance Sheets and as credit loss expense within other income, net on the Condensed Statements of Comprehensive Loss. The portion of the unrealized loss that is not due to a credit loss as well as all unrealized gains will be recorded in Accumulated Other Comprehensive Income (Loss), or AOCI, net of taxes, on the Condensed Balance Sheets. There was no cumulative effect adjustment as a result of the adoption of ASU 2016-13 on January 1, 2020 (see Note 3, Available-for-Sale Marketable Securities, and Note 5, Fair Value Measurements).

Accrued interest receivables are excluded from the Company’s amortized cost bases for its available-for-sale marketable securities and are included within Other Receivables on the Company’s Condensed Balance Sheets. The Company’s policy is to not measure an allowance for credit losses on accrued interest receivable balances at each reporting period since it elects to write off uncollectible accrued interest receivable balances as credit loss expense in a timely manner, which is by maturity date for all categories of its debt securities.

On January 1, 2020, the Company adopted ASU 2019-08, Compensation – Stock Compensation (Topic 718) and Revenue from Contracts with Customers (Topic 606), or ASU 2019-08, which requires the Company to measure and classify share-based payment awards granted to a customer by applying the guidance in Topic 718. The amount recorded as a reduction to the transaction price is required to be measured on the basis of the grant-date fair value of the share-based payment award in accordance with Topic 718. The grant date is the date at which a grantor (supplier) and a grantee (customer) reach a mutual understanding of the key terms and conditions of a share-based payment award. The classification and subsequent measurement of the award are subject to the guidance in Topic 718 unless the share-based payment award is subsequently modified and the grantee is no longer a customer. The adoption of ASU 2019-08 did not

have a material effect on its results of operations, financial position or cash flows since the Company has not historically granted share-based payment awards to customers.

On January 1, 2020, the Company adopted ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606, or ASU 2018-18, which clarifies the interaction between Topic 808 and Topic 606 by (1) clarifying that certain transactions between collaborative arrangement participants should be accounted for under Topic 606; (2) adding unit-of-account guidance in Topic 808 to align with the guidance in Topic 606; and (3) clarifying presentation guidance for transactions with a collaborative arrangement participant that are not accounted for under Topic 606. The adoption of ASU 2018-18 did not have any effect on its financial position, results of operations or cash flows since all three of its collaboration and licensing agreements are accounted for under Topic 606 (see Note 10, Collaboration and Licensing Agreements and Note 11, Revenue Recognition).

On January 1, 2020, the Company adopted ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement, or ASU 2018-13, which modifies the disclosure requirements on fair value measurements in Topic 820 to remove the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, the policy for timing of transfers between levels, and the valuation processes for Level 3 fair value measurements. ASU 2018-13 also amends Topic 820 to clarify that the measurement uncertainty disclosure is to communicate information about the uncertainty in measurement as of the reporting date. ASU 2018-13 also requires additional disclosure for changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements held at the end of the reporting period as well as the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. Upon adoption of ASU 2018-13, the Company did not have any assets or liabilities that are included in Level 3 fair value measurements and no retrospective treatment was applicable. As a result, the adoption of ASU 2018-13 did not have a material effect on its results of operations, financial position or cash flows.

Accounting Pronouncements Not Yet Adopted

In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740), or ASU 2019-12, which removes specific exceptions to the general principles in Topic 740. ASU 2019-12 eliminates the need for an organization to analyze whether the following apply in a given period: i) exception to the incremental approach for intra-period tax allocation; ii) exceptions to accounting for basis differences when there are ownership changes in foreign investments; and iii) exception to the general methodology for calculating income taxes in an interim period when a year-to-date loss exceeds the anticipated loss. ASU 2019-12 also simplifies the accounting for income taxes for: i) franchise taxes that are partially based on income; ii) transactions with a government that result in a step up in the tax basis of goodwill; (iii) separate financial statements of legal entities that are not subject to tax; and iv) enacted changes in tax laws in interim periods. The amendments in ASU 2019-12 are effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2020. Early adoption of the amendments is permitted. An entity that elects to early adopt the amendments in an interim period should reflect any adjustments as of the beginning of the annual period that includes that interim period and must adopt all the amendments in the same period. The amendments in ASU 2019-12 related to separate financial statements of legal entities that are not subject to tax should be applied on a retrospective basis for all periods presented. The amendments related to changes in ownership of foreign equity method investments or foreign subsidiaries should be applied on a modified retrospective basis through a cumulative-effect adjustment to retained earnings as of the beginning of the fiscal year of adoption. The amendments related to franchise taxes that are partially based on income should be applied on either a retrospective basis for all periods presented or a modified retrospective basis through a cumulative-effect adjustment to retained earnings as of the beginning of the fiscal year of adoption. All other amendments should be applied on a prospective basis. As such, the Company expects to adopt ASU 2019-12 on January 1, 2021 and is currently evaluating the effect it will have on its results of operations, financial position and cash flows.