Gene Therapy Program, Department of Medicine, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104. Department of Microbiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104. Abramson Family Cancer Research Institute, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104. Department of Pediatrics, University of Massachusetts Medical School, Worcester, MA 01655. Gene Therapy Program, Department of Medicine, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104; wilsonjm@upenn.edu.