Related Party Transactions	12 Months Ended
Dec. 31, 2016
Related Party Transactions [Abstract]
Related Party Transactions	12. Related Party Transactions The Trustees of the University of Pennsylvania Penn received an equity interest in the Company as consideration for the intellectual property license granted to the Company in February 2009, and prior to September 30, 2015, the Company considered Penn to be a related party. See Note 6 for further information, including costs incurred by the Company, regarding its license from Penn. In addition to the license agreement, Penn also provides manufacturing and research and development services to the Company. During the nine months ended September 30, 2015, and the year ended December 31, 2014, the Company incurred $0.1 million and $0.1 million, respectively, in services from Penn for the manufacturing of reagents for sale by the Company as well as $5.1 million and $1.3 million, respectively, in research and development services provided by Penn. As a result of various factors, including the dilution to the Company’s stockholders resulting from the sale and issuance of Series C Preferred Stock and Series D Preferred Stock, and the closing of the Company’s IPO on September 22, 2015, the Company no longer considers Penn to be a related party subsequent to the period ended September 30, 2015. GlaxoSmithKline LLC GSK received an equity interest in the Company as consideration for the intellectual property license granted to the Company in March 2009, and prior to September 30, 2015, the Company considered GSK to be a related party. See Note 6 for further information, including costs incurred by the Company, regarding its license from GSK. As a result of various factors, including the dilution to the Company’s stockholders resulting from the sale and issuance of Series C Preferred Stock and Series D Preferred Stock, and the closing of the Company’s IPO on September 22, 2015, the Company no longer considers GSK to be a related party subsequent to the period ended September 30, 2015. Dimension Therapeutics, Inc. The Company and a number of its members, directors and management received common stock of Dimension as consideration for a license granted to Dimension in October 2013. Additionally, at the date of the license, three of the Company’s board members also served on the board of directors of Dimension. As a result, prior to September 30, 2015, the Company considered Dimension to be a related party. As of September 30, 2015, the Company no longer had any overlapping board members with Dimension. Additionally, the Company’s directors and management do not have significant influence over Dimension. Accordingly, the Company no longer considers Dimension to be a related party subsequent to the period ended September 30, 2015. In October 2013, the Company granted an exclusive, sublicensable, worldwide commercial license to Dimension for preclinical and clinical research and development and commercialization of drug therapies using the Company’s licensed patents for the treatment of hemophilia A and hemophilia B, as well as a one year option to obtain exclusive licenses for the commercialization of two other diseases to be elected by Dimension in the future. The agreement requires on-going annual maintenance fees of less than $0.1 million, for each indication elected by Dimension, beginning in October 2014. The agreement also requires Dimension to pay royalties on net sales, if any, to the Company at an amount intended to approximate the royalties that will be due by the Company to Penn and GSK on such sales. In connection with the license agreement granted to Dimension, the Company entered into an arrangement with Penn and Dimension in which the Company helped coordinate and manage research and development activities performed by Penn on behalf of Dimension. Under the arrangement, Dimension reimbursed the Company at an amount equal to costs incurred and paid to Penn, and the Company retains rights to certain intellectual property discovered under the contracted research and development. Due to the uncertainty of any future intellectual property rights that may be discovered and retained by the Company, and because such intellectual property would have no future alternative use due to the stage of development of the drug therapies under development, the Company has not recognized any benefit from the arrangement as consideration paid by Dimension to the Company as a result of the license agreement. Management has evaluated the facts and circumstances of the arrangements with regards to ASC 605-45 Revenue Recognition-Principal Agent Considerations and determined that the proceeds received from Dimension should be recorded on a net basis. Accordingly, proceeds received from Dimension under the arrangement were recorded as a reduction of research and development expense in the statements of operations and comprehensive loss. For the year ended December 31, 2014, the Company recorded research and development expense to Penn, and related reimbursements from Dimension of $6.2 million, for a net cost of $0 for the period. The arrangement expired on December 31, 2014, and during the year ended December 31, 2015, the final payments under this arrangement were received by the Company and paid to Penn. In September 2014, Dimension elected its third indication under the license agreement, and the license was amended to extend the term of the option to elect the fourth and final disease indication for an additional six months. In consideration for the extension of the option, Dimension paid an extension fee of $0.2 million. In January 2015, Dimension elected its fourth and final indication under the license. In March 2015, the Company entered into an option and license agreement with Dimension that granted Dimension options to commercial exclusive licenses for four new disease indications to be elected by Dimension in the future. If elected, each option carries an option fee of $1.0 million payable to the Company upon exercise, and annual maintenance fees of $0.1 million. Additionally, for each option exercised, Dimension is obligated to pay the Company up to $9.0 million upon achievement of various substantive milestones, as well as mid to upper-single digit percentage royalties on net sales of licensed products and mid-single digit to low-double digit percentage sublicense fees, if any. In May 2015, Dimension exercised its first option under the agreement and paid $1.0 million to the Company. In August 2015, Dimension exercised its second option under the agreement and paid $1.0 million to the Company. In December 2015, Dimension exercised its third option under the agreement and paid $1.0 million to the Company. During the nine months ended September 30, 2015, and the year ended December 31, 2014, the Company recognized license revenue of $2.0 million and $0.2 million, respectively, from commercial licenses granted to Dimension, which is included in license revenue from related party in the statements of operations and comprehensive loss. Revenue earned and recognized from Dimension subsequent to September 30, 2015 is no longer included in revenue from related party in the statements of operations and comprehensive loss. During the year ended December 31, 2014, the Company received $0.2 million from Dimension for the purchase of materials owned by the Company and used in the Company’s manufacturing process for research and development and clinical trials. The $0.2 million was recognized as a gain on disposal of the material in the periods in which the material was delivered to Dimension. During the nine months ended September 30, 2015, and the year ended December 31, 2014, the Company recognized gains of less than $0.1 million in each period for the delivery of the material to Dimension. As of December 31, 2016, the entire gain of $0.2 million had been recognized. FoxKiser The Company was party to a services agreement, as amended from time to time, with FoxKiser, an affiliate of certain stockholders of the Company and an affiliate of one current and one former member of the Board of Directors, which was terminated in January 2015. Under the agreement, the Company paid a fixed monthly fee plus an additional support fee, as determined by FoxKiser on a monthly basis, as consideration for office facilities, equipment, supplies, general management, accounting, financial management, human resources, legal, secretarial, regulatory compliance and other services provided to the Company. The amounts outstanding to FoxKiser under the services agreement in excess of 30 days from their due date accrued interest at 1.5 percent per month, compounding monthly. The Company allocated the service fees from FoxKiser between research and development and general and administrative expense. For the years ended December 31, 2015 and 2014, the Company incurred costs under the arrangement of $0.2 million and $1.3 million, respectively, allocated to research and development expense, and $0.2 million and $1.7 million, respectively, allocated to general and administrative expense. In July 2014, the Company entered into an agreement with FoxKiser whereby all principal and interest owed under the services agreement, with a maximum amount of $2.0 million, may be settled at the option of FoxKiser, in whole or in part, (i) upon the next issuance of preferred equity securities by the Company, in shares of such preferred equity securities issued at a price per share equal to the issuance price of such shares, or (ii) if an issuance of preferred equity securities has not taken place by December 31, 2014, in shares of common equity securities of the Company at a price per share equal to the fair value of such common equity securities as determined by the Board of Directors. The debt is deemed fully settled upon conversion into equity securities of the Company. In July 2014, the Company received $1.8 million in exchange for a promissory note issued to FoxKiser, and in September 2014, the Company received $0.6 million in exchange for a second promissory note issued to FoxKiser. Both promissory notes accrued interest at the Short-Term Applicable Federal Rate, compounding annually, and were payable on demand by FoxKiser at the earlier of December 31, 2014 or the next issuance of preferred equity securities by the Company. Both promissory notes may be settled at the option of FoxKiser, in whole or in part, (i) upon the next issuance of preferred equity securities by the Company, in shares of such preferred equity securities issued at a price per share equal to the issuance price of such shares, or (ii) if an issuance of preferred equity securities has not taken place by December 31, 2014, in shares of common equity securities of the Company at a price per share equal to the fair value of such common equity securities as determined by the Board of Directors. The debt is deemed fully settled upon conversion into equity securities of the Company. The promissory notes with FoxKiser accrued interest at below-market rates. Accordingly, the Company imputed interest on the promissory notes and recorded a discount equal to the difference between the face value of the promissory notes and the present value of the notes at an estimated market rate of 15 percent. The aggregate discount of $0.1 million on the promissory notes was amortized using the effective interest method through December 31, 2014, at which date the notes became payable upon demand by FoxKiser. The discount was recorded as additional paid-in capital from FoxKiser due to the related party nature of the borrowing arrangements. Amortization of the discount was recorded as interest expense in the statements of operations and comprehensive loss. In January 2015, in conjunction with the Company’s issuance of Series C Preferred Stock, FoxKiser elected its share settlement options and converted $1.4 million of the amount due under the services agreement and $2.4 million of principal and interest due under the promissory notes, for a total of $3.8 million, into 585,578 shares of Series C Preferred Stock at a per share price of $6.477. The promissory notes were settled in full upon the conversion, and in January 2015, the Company terminated the services agreement with FoxKiser and paid the remainder of the outstanding service fees in full. Interest expense to FoxKiser under the services agreement and promissory notes, inclusive of imputed interest, for the years ended December 31, 2015 and 2014 was less than $0.1 million and $0.3 million, respectively. In January 2015, the Company entered into an operating lease with FoxKiser for office space in Washington, D.C. The lease agreement, which had a month-to-month term, required monthly payments of less than $0.1 million. The lease was terminated in April 2015. Total rent expense to FoxKiser during the term of the lease was $0.1 million. The Company entered into a Professional Services Agreement with FoxKiser effective as of January 1, 2016, pursuant to which the Company incurs a fixed monthly fee in consideration for certain strategic planning, development and regulatory services to be provided by FoxKiser. The agreement expired in December 2016, and effective as of January 1, 2017, the Company entered into a new Professional Services Agreement with FoxKiser, which has a term of one year and is terminable by either party, at any time, upon sixty days’ prior written notice to the other party. Costs incurred by the Company under the Professional Service Agreements with FoxKiser for the year ended December 31, 2016 were $1.0 million and have been recorded as research and development expenses in the statements of operations and comprehensive loss. Chief Scientific Advisor In September 2014, the Company entered into an advisory agreement with its Chief Scientific Advisor, who is also the Chairman of the Company’s Scientific Advisory Board. The agreement was amended in May 2015 and expires in March 2017. During the years ended December 31, 2016, 2015 and 2014, the Company incurred advisory fees of $0.3 million, $0.2 million and less than $0.1 million, respectively, to the Chief Scientific Advisor under the agreement. Additionally, since the inception of the agreement, the Company has granted options to purchase 211,600 shares of common stock to the Chief Scientific Advisor as compensation for services to be provided to the Company, which vest partially upon the completion of future service conditions and partially upon the achievement of specified performance conditions as set forth in the award agreements.

12. Related Party Transactions

The Trustees of the University of Pennsylvania

Penn received an equity interest in the Company as consideration for the intellectual property license granted to the Company in February 2009, and prior to September 30, 2015, the Company considered Penn to be a related party. See Note 6 for further information, including costs incurred by the Company, regarding its license from Penn.

In addition to the license agreement, Penn also provides manufacturing and research and development services to the Company. During the nine months ended September 30, 2015, and the year ended December 31, 2014, the Company incurred $0.1 million and $0.1 million, respectively, in services from Penn for the manufacturing of reagents for sale by the Company as well as $5.1 million and $1.3 million, respectively, in research and development services provided by Penn.

As a result of various factors, including the dilution to the Company’s stockholders resulting from the sale and issuance of Series C Preferred Stock and Series D Preferred Stock, and the closing of the Company’s IPO on September 22, 2015, the Company no longer considers Penn to be a related party subsequent to the period ended September 30, 2015.

GlaxoSmithKline LLC

GSK received an equity interest in the Company as consideration for the intellectual property license granted to the Company in March 2009, and prior to September 30, 2015, the Company considered GSK to be a related party. See Note 6 for further information, including costs incurred by the Company, regarding its license from GSK.

As a result of various factors, including the dilution to the Company’s stockholders resulting from the sale and issuance of Series C Preferred Stock and Series D Preferred Stock, and the closing of the Company’s IPO on September 22, 2015, the Company no longer considers GSK to be a related party subsequent to the period ended September 30, 2015.

Dimension Therapeutics, Inc.

The Company and a number of its members, directors and management received common stock of Dimension as consideration for a license granted to Dimension in October 2013. Additionally, at the date of the license, three of the Company’s board members also served on the board of directors of Dimension. As a result, prior to September 30, 2015, the Company considered Dimension to be a related party.

As of September 30, 2015, the Company no longer had any overlapping board members with Dimension. Additionally, the Company’s directors and management do not have significant influence over Dimension. Accordingly, the Company no longer considers Dimension to be a related party subsequent to the period ended September 30, 2015.

In October 2013, the Company granted an exclusive, sublicensable, worldwide commercial license to Dimension for preclinical and clinical research and development and commercialization of drug therapies using the Company’s licensed patents for the treatment of hemophilia A and hemophilia B, as well as a one year option to obtain exclusive licenses for the commercialization of two other diseases to be elected by Dimension in the future. The agreement requires on-going annual maintenance fees of less than $0.1 million, for each indication elected by Dimension, beginning in October 2014. The agreement also requires Dimension to pay royalties on net sales, if any, to the Company at an amount intended to approximate the royalties that will be due by the Company to Penn and GSK on such sales.

In connection with the license agreement granted to Dimension, the Company entered into an arrangement with Penn and Dimension in which the Company helped coordinate and manage research and development activities performed by Penn on behalf of Dimension. Under the arrangement, Dimension reimbursed the Company at an amount equal to costs incurred and paid to Penn, and the Company retains rights to certain intellectual property discovered under the contracted research and development. Due to the uncertainty of any future intellectual property rights that may be discovered and retained by the Company, and because such intellectual property would have no future alternative use due to the stage of development of the drug therapies under development, the Company has not recognized any benefit from the arrangement as consideration paid by Dimension to the Company as a result of the license agreement. Management has evaluated the facts and circumstances of the arrangements with regards to ASC 605-45 Revenue Recognition-Principal Agent Considerations and determined that the proceeds received from Dimension should be recorded on a net basis. Accordingly, proceeds received from Dimension under the arrangement were recorded as a reduction of research and development expense in the statements of operations and comprehensive loss. For the year ended December 31, 2014, the Company recorded research and development expense to Penn, and related reimbursements from Dimension of $6.2 million, for a net cost of $0 for the period. The arrangement expired on December 31, 2014, and during the year ended December 31, 2015, the final payments under this arrangement were received by the Company and paid to Penn.

In September 2014, Dimension elected its third indication under the license agreement, and the license was amended to extend the term of the option to elect the fourth and final disease indication for an additional six months. In consideration for the extension of the option, Dimension paid an extension fee of $0.2 million. In January 2015, Dimension elected its fourth and final indication under the license.

In March 2015, the Company entered into an option and license agreement with Dimension that granted Dimension options to commercial exclusive licenses for four new disease indications to be elected by Dimension in the future. If elected, each option carries an option fee of $1.0 million payable to the Company upon exercise, and annual maintenance fees of $0.1 million. Additionally, for each option exercised, Dimension is obligated to pay the Company up to $9.0 million upon achievement of various substantive milestones, as well as mid to upper-single digit percentage royalties on net sales of licensed products and mid-single digit to low-double digit percentage sublicense fees, if any. In May 2015, Dimension exercised its first option under the agreement and paid $1.0 million to the Company. In August 2015, Dimension exercised its second option under the agreement and paid $1.0 million to the Company. In December 2015, Dimension exercised its third option under the agreement and paid $1.0 million to the Company.

During the nine months ended September 30, 2015, and the year ended December 31, 2014, the Company recognized license revenue of $2.0 million and $0.2 million, respectively, from commercial licenses granted to Dimension, which is included in license revenue from related party in the statements of operations and comprehensive loss. Revenue earned and recognized from Dimension subsequent to September 30, 2015 is no longer included in revenue from related party in the statements of operations and comprehensive loss.

During the year ended December 31, 2014, the Company received $0.2 million from Dimension for the purchase of materials owned by the Company and used in the Company’s manufacturing process for research and development and clinical trials. The $0.2 million was recognized as a gain on disposal of the material in the periods in which the material was delivered to Dimension. During the nine months ended September 30, 2015, and the year ended December 31, 2014, the Company recognized gains of less than $0.1 million in each period for the delivery of the material to Dimension. As of December 31, 2016, the entire gain of $0.2 million had been recognized.

FoxKiser

The Company was party to a services agreement, as amended from time to time, with FoxKiser, an affiliate of certain stockholders of the Company and an affiliate of one current and one former member of the Board of Directors, which was terminated in January 2015. Under the agreement, the Company paid a fixed monthly fee plus an additional support fee, as determined by FoxKiser on a monthly basis, as consideration for office facilities, equipment, supplies, general management, accounting, financial management, human resources, legal, secretarial, regulatory compliance and other services provided to the Company. The amounts outstanding to FoxKiser under the services agreement in excess of 30 days from their due date accrued interest at 1.5 percent per month, compounding monthly. The Company allocated the service fees from FoxKiser between research and development and general and administrative expense. For the years ended December 31, 2015 and 2014, the Company incurred costs under the arrangement of $0.2 million and $1.3 million, respectively, allocated to research and development expense, and $0.2 million and $1.7 million, respectively, allocated to general and administrative expense.

In July 2014, the Company entered into an agreement with FoxKiser whereby all principal and interest owed under the services agreement, with a maximum amount of $2.0 million, may be settled at the option of FoxKiser, in whole or in part, (i) upon the next issuance of preferred equity securities by the Company, in shares of such preferred equity securities issued at a price per share equal to the issuance price of such shares, or (ii) if an issuance of preferred equity securities has not taken place by December 31, 2014, in shares of common equity securities of the Company at a price per share equal to the fair value of such common equity securities as determined by the Board of Directors. The debt is deemed fully settled upon conversion into equity securities of the Company.

In July 2014, the Company received $1.8 million in exchange for a promissory note issued to FoxKiser, and in September 2014, the Company received $0.6 million in exchange for a second promissory note issued to FoxKiser. Both promissory notes accrued interest at the Short-Term Applicable Federal Rate, compounding annually, and were payable on demand by FoxKiser at the earlier of December 31, 2014 or the next issuance of preferred equity securities by the Company. Both promissory notes may be settled at the option of FoxKiser, in whole or in part, (i) upon the next issuance of preferred equity securities by the Company, in shares of such preferred equity securities issued at a price per share equal to the issuance price of such shares, or (ii) if an issuance of preferred equity securities has not taken place by December 31, 2014, in shares of common equity securities of the Company at a price per share equal to the fair value of such common equity securities as determined by the Board of Directors. The debt is deemed fully settled upon conversion into equity securities of the Company.

The promissory notes with FoxKiser accrued interest at below-market rates. Accordingly, the Company imputed interest on the promissory notes and recorded a discount equal to the difference between the face value of the promissory notes and the present value of the notes at an estimated market rate of 15 percent. The aggregate discount of $0.1 million on the promissory notes was amortized using the effective interest method through December 31, 2014, at which date the notes became payable upon demand by FoxKiser. The discount was recorded as additional paid-in capital from FoxKiser due to the related party nature of the borrowing arrangements. Amortization of the discount was recorded as interest expense in the statements of operations and comprehensive loss.

In January 2015, in conjunction with the Company’s issuance of Series C Preferred Stock, FoxKiser elected its share settlement options and converted $1.4 million of the amount due under the services agreement and $2.4 million of principal and interest due under the promissory notes, for a total of $3.8 million, into 585,578 shares of Series C Preferred Stock at a per share price of $6.477. The promissory notes were settled in full upon the conversion, and in January 2015, the Company terminated the services agreement with FoxKiser and paid the remainder of the outstanding service fees in full.

Interest expense to FoxKiser under the services agreement and promissory notes, inclusive of imputed interest, for the years ended December 31, 2015 and 2014 was less than $0.1 million and $0.3 million, respectively.

In January 2015, the Company entered into an operating lease with FoxKiser for office space in Washington, D.C. The lease agreement, which had a month-to-month term, required monthly payments of less than $0.1 million. The lease was terminated in April 2015. Total rent expense to FoxKiser during the term of the lease was $0.1 million.

The Company entered into a Professional Services Agreement with FoxKiser effective as of January 1, 2016, pursuant to which the Company incurs a fixed monthly fee in consideration for certain strategic planning, development and regulatory services to be provided by FoxKiser. The agreement expired in December 2016, and effective as of January 1, 2017, the Company entered into a new Professional Services Agreement with FoxKiser, which has a term of one year and is terminable by either party, at any time, upon sixty days’ prior written notice to the other party. Costs incurred by the Company under the Professional Service Agreements with FoxKiser for the year ended December 31, 2016 were $1.0 million and have been recorded as research and development expenses in the statements of operations and comprehensive loss.

Chief Scientific Advisor

In September 2014, the Company entered into an advisory agreement with its Chief Scientific Advisor, who is also the Chairman of the Company’s Scientific Advisory Board. The agreement was amended in May 2015 and expires in March 2017. During the years ended December 31, 2016, 2015 and 2014, the Company incurred advisory fees of $0.3 million, $0.2 million and less than $0.1 million, respectively, to the Chief Scientific Advisor under the agreement. Additionally, since the inception of the agreement, the Company has granted options to purchase 211,600 shares of common stock to the Chief Scientific Advisor as compensation for services to be provided to the Company, which vest partially upon the completion of future service conditions and partially upon the achievement of specified performance conditions as set forth in the award agreements.