1. Twin Research and Genetic Epidemiology Unit, St Thomas' Hospital, Lambeth Palace Road, London SE1 7EH, U.K.
2. Georgia Prevention Institute, Medical College of Georgia, Augusta, GA 30912, U.S.A.
3. Department of Chemical Pathology, St Thomas' Hospital, Lambeth Palace Road, London SE1 7EH, U.K.
4. Molecular Endocrinology Group, Nuffield Department of Medicine, University of Oxford, John Radcliffe Hospital, Oxford OX3 9DU, U.K.