2.	Summary of Significant Accounting Policies

The significant accounting policies and estimates used in preparation of the condensed consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2017, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K. There have been no material changes to the Company’s significant accounting policies during the nine months ended September 30, 2018 except for the adoption of the new revenue standard discussed in Note 8, Collaboration Revenue.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, the accrual of research and development expenses and the valuation of stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.

Net Loss per Share

Basic net loss per share is computed using the weighted average number of common shares outstanding during the period. Diluted net loss per share is computed using the sum of the weighted average number of common shares outstanding during the period and, if dilutive, the weighted average number of potential shares of common stock, including the assumed exercise of stock options and warrants and unvested restricted stock.

The restricted stock units granted by the Company entitle the holder of such awards to dividends declared or paid by the board of directors, regardless of whether such awards are unvested, as if such shares were outstanding common shares at the time of the dividend. However, the unvested restricted stock units are not entitled to share in the residual net assets (deficit) of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive.

The following potential common shares, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:

	Three and Nine Months Ended September 30,
	2018		2017
Stock options to purchase common stock		7,444,040		6,133,596
Unvested restricted stock units		289,908		401,900
		7,733,948		6,535,496

Recently Issued Accounting Standards

In May 2014, the Financial Accounting Standards Board (the “FASB”) issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, which delayed the effective date of the new standard from January 1, 2017 to January 1, 2018. The FASB also agreed to allow entities to choose to adopt the standard as of the original effective date. In March 2016, the FASB issued ASU 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations, which clarifies the implementation guidance on principal versus agent considerations. In April 2016, the FASB issued ASU 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing, which clarifies how a company identifies promised goods or services and clarifies whether an entity’s promise to grant a license provides a customer with either a right to use the entity’s intellectual property (which is satisfied at a point in time) or a right to access the entity’s intellectual property (which is satisfied over time). In May 2016, the FASB issued ASU No. 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients related to disclosures of remaining performance obligations, as well as other amendments to guidance on collectability, non-cash consideration and the presentation of sales and other similar taxes collected from customers. In December 2016 the FASB issued ASU No. 2016-20, Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers, which amends certain narrow aspects of the guidance issued in ASU 2014-09 including guidance related to the disclosure of remaining performance obligations and prior-period performance obligations, as well as other amendments to the guidance on loan guarantee fees, contract costs, refund liabilities, advertising costs and the clarification of certain examples. ASU 2016-08, ASU 2016-10 and ASU 2016-12 have the same effective dates and transition requirements as ASU 2014-09, all of which collectively are herein referred to as “ASC 606.” The Company has adopted this standard as of the required effective date of January 1, 2018, using the modified retrospective transition method. See Note 8, “Collaboration Revenue,” for discussion of the impact of adoption of this standard.

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), (“ASU 2016-02”), which establishes principles that lessees and lessors shall apply to report useful information to users of financial statements about the amount, timing and uncertainty of cash flows arising from a lease. The most notable change will be lessees recognizing an asset and liability on their balance sheet for operating leases. In 2018, the FASB issued ASU 2018-01, and ASU 2018-11, which collectively add two practical expedients, provide a second modified retrospective transition method which does not require retrospective adjustment of prior periods, and provide certain narrow scope improvements to the new lease guidance. ASU 2016-02 and the amending ASUs are effective for the Company for annual periods beginning after December 15, 2018 and interim periods therein, with early adoption permitted. The Company will adopt the new guidance for its fiscal year beginning January 1, 2019 and expects to use the modified retrospective transition method that does not require restatement of prior periods. The Company is currently in the process of evaluating the impact of the guidance on its consolidated financial statements and is undertaking an assessment of its lease population that it expects to complete during the fourth quarter of 2018. The Company anticipates material adjustments to its balance sheet for the recognition of a lease liability and a right of use asset for its operating leases, which primarily include, but are not limited to, the lease of its corporate headquarters at 200 Sidney Street and additional research space at 215 First Street, both in Cambridge, Massachusetts. The Company does not expect a material impact in its income statement or statement of cash flows.

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments. This standard addresses specific cash flow issues with the objective of reducing existing diversity in practice in how certain cash receipts and cash payments are presented in the statement of cash flows. The standard was effective for the Company on January 1, 2018. The Company adopted this standard as of the required effective date of January 1, 2018. The adoption of this standard did not have any impact on the Company’s financial statements.

In November 2016, the FASB issued ASU 2016-18, Restricted Cash. The new standard requires restricted cash and restricted cash equivalents be included with cash and cash equivalents when reconciling the total beginning and ending amounts for the periods shown on the statement of cash flows. The new standard was effective for the Company on January 1, 2018. The Company adopted the new standard as of the required effective date of January 1, 2018 and will reflect the adoption retrospectively to all periods presented. The Company’s statements of cash flows includes restricted cash with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on such statements. A reconciliation of the cash, cash equivalents, and restricted cash reported within the balance sheet that sum to the total of the same amounts shown in the statement of cash flows is as follows:

	September 30,		December 31,		September 30,		December 31,
	2018		2017		2017		2016
Cash and cash equivalents	$	55,713	$	36,088	$	46,025	$	54,539
Restricted cash		1,513		1,513		1,513		1,400
Total cash, cash equivalents and restricted cash as shown in the statement of cash flows	$	57,226	$	37,601	$	47,538	$	55,939

In June 2018, the FASB issued ASU 2018-07, “Compensation – Stock Compensation (Topic 718).” ASU 2018-07 simplifies the accounting for nonemployee share-based payment transactions. This ASU is effective for public entities for interim and annual reporting periods beginning after December 15, 2018. The Company is currently in the process of evaluating the impact of the guidance on its consolidated financial statements.

In August 2018, the FASB issued ASU No. 2018-13, Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement (“ASU 2018-13”). This standard eliminates, adds and modifies certain disclosure requirements for fair value measurements as part of its disclosure framework project. ASU 2018-13 is effective for annual reporting periods beginning after December 15, 2019 and interim periods within those annual periods and early adoption is permitted. We are currently evaluating the impact of our adoption of ASU 2018-13 on our condensed consolidated financial statements.