1. Department of Pharmacology, University of Oxford, Mansfield Road, Oxford OXI 3QT, U.K.
2. Institute of Molecular Medicine, John Radcliffe Hospital, Oxford OX3 9DU, U.K.
3. ICRF Clare Hall Laboratories, Blanche Lane, Potters Bar, Herts. EN6 3LD, U.K.
4. MRC Human Biochemical Genetics Unit (UCL), The Galton Laboratory, Wolfson House, 4 Stephenson Way, London NW1 2HE, U.K.