1. From the Departments of Health Care and Epidemiology (S.W., S.B.S., M.T.S.) and Medical Genetics (P.W., M.H., S.A., J.T., MB., M.R.H.), University of British Columbia, Vancouver. Address reprint requests to Dr. Hayden at the Department of Medical Genetics, University of British Columbia, Rm. 416, 2125 E. Mall, Vancouver, BC V6T 1Z4, Canada.