1. Department of Genetics, Research Institute, The Hospital for Sick Children, Toronto, Ontario, M5G 1X8, Canada.
2. Howard Hughes Medical Institute and Departments of Internal Medicine and Human Genetics, University of Michigan, Ann Arbor, MI 48109.
3. Program Resources, Inc., Frederick Cancer Research Facility, Frederick, MD 21701.
4. Departments of Medical Genetics and Medical Biophysics, University of Toronto, Toronto M5S 1A8, Canada.
5. Department of Biochemistry, Research Institute, The Hospital for Sick Children, Toronto, and the Departments of Biochemistry and Clinical Biochemistry, University of Toronto, Toronto M5S 1A8, Canada.