1. From the de Belle Laboratory for Biochemical Genetics, Montreal Children's Hospital Research Institute, and the Department of Pediatrics (Faculty of Medicine) and Biology (Faculty of Science), McGill University, Montreal, PQ, and the Department of Biochemistry, College of Agriculture and Life Sciences, University of Wisconsin, Madison, WI (address reprint requests to Dr. Scriver at Montreal Children's Hospital, 2300 Tupper St., Montreal, PQ, Canada H3H 1P3).