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Related Party Transactions	6 Months Ended
Related Party Transactions	Jun. 30, 2017
Related Party Transactions [Abstract]
Related Party Transactions	10. Related Party Transactions In September 2012, the Company began receiving consulting and management services pursuant to an unwritten agreement with Third Rock Ventures, which is one of the Company’s largest shareholders. Kevin Starr, a director of the Company, is a partner of Third Rock Ventures. Charles Homcy, a former director who resigned from the Board of Directors in March 2017, is a venture partner of Third Rock Ventures. The consulting fees paid to Third Rock Ventures were incurred by the Company in the ordinary course of business, and were $23,000 and $32,000 for the three and six months ended June 30, 2017, respectively, and $10,000 and $24,000 for the three and six months ended June 30, 2016, respectively. As of June 30, 2017 and December 31, 2016, the Company had an outstanding liability to Third Rock Ventures of $4,000 and $9,000, respectively.