From the Division of Genetics, Children's Hospital, and the Department of Pediatrics, Harvard Medical School, Boston (E.P.H., L.M.K.), the Howard Hughes Medical Institute, Boston (L.M.K.), the Department of Neurology, Hospital of the University of Pennsylvania, Philadelphia (K.H.F.), the Neurology Service, Massachusetts General Hospital, Boston (R.H.B.), the Muscular Dystrophy Group Laboratories, Newcastle General Hospital, Newcastle-upon-Tyne, Great Britain (M.J., J.B.H.), the Division of Neurology (R.M., M.B.) and the Department of Genetics (R.W.), Washington University, St. Louis, the Department of Physiology and Cellular Biophysics, College of Physicians and Surgeons of Columbia University, New York (J.D.L.), the Departments of Neurology (L.S.), Pathology (W.K.), and Orthopaedic Surgery (F.S.), Children's Hospital, Boston, and the Robert J. Kleberg Jr. Center for Human Genetics, Baylor College of Medicine, Texas Medical Center, Houston (J.C., C.T.C.). Address reprint requests to Dr. Kunkel at the Howard Hughes Medical Institute Research Laboratories, Children's Hospital, 300 Longwood Ave., Boston, MA 02115.