1. From the Division of Molecular Medicine (S.B.S., A.R.T., C.L.W.), the Department of Medicine, Institute of Human Nutrition (S.B.S., C.D.L., R.L.L., A.R.T.), and the Division of Molecular Genetics (C.D.L., R.L.L.), Department of Pediatrics, Columbia University, New York; and the Laboratory of Biochemical Genetics and Metabolism (J.L.B.), The Rockefeller University, New York, NY.