Spark Therapeutics will be the study sponsor; prior vector administration sites or referral/follow up sites will participate as study sites. The long-term follow-up (LTFU) study will consist primarily of endpoints including annual history, physical examination, blood tests, urinalysis, and periodic liver ultrasound. Adverse event reporting will focus on the development of oncologic, hematologic, neurologic and auto-immune events following gene transfer as specified in the November 2006 FDA Center for Biologics Evaluation and Research (CBER) guidance document entitled, "Gene Therapy Clinical Trials - Observing Subjects for Delayed Adverse Events."1 The LTFU study seeks to provide longitudinal information, and should adverse events occur, more timely discovery and treatment.