Department of Ophthalmology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania; Center for Cellular and Molecular Therapeutics, Inc., Children's Hospital of Philadelphia, Philadelphia, Pennsylvania. Electronic address: amaguire@pennmedicine.upenn.edu. Department of Ophthalmology and Visual Sciences, University of Iowa Institute for Vision Research, University of Iowa, Iowa City, Iowa. Center for Cellular and Molecular Therapeutics, Inc., Children's Hospital of Philadelphia, Philadelphia, Pennsylvania; Spark Therapeutics, Inc., Philadelphia, Pennsylvania. Statistics Collaborative, Inc., Washington, DC. Westat Biostatistics and Data Management Core, Children's Hospital of Philadelphia, Philadelphia, Pennsylvania. Center for Cellular and Molecular Therapeutics, Inc., Children's Hospital of Philadelphia, Philadelphia, Pennsylvania. University of Iowa Health Care, Iowa City, Iowa. Multidisciplinary Department of Medical, Surgical and Dental Sciences, University of Campania "Luigi Vanvitelli," Naples, Italy. Division of Ophthalmology and Center for Cellular and Molecular Therapeutics, Inc., Children's Hospital of Philadelphia, Philadelphia, Pennsylvania; Department of Ophthalmology and Center for Medical Genetics Ghent, Ghent University Hospital, Ghent, Belgium. Department of Ophthalmology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania; Center for Cellular and Molecular Therapeutics, Inc., Children's Hospital of Philadelphia, Philadelphia, Pennsylvania.