1. M.R.C. Human Genetic Diseases Research Group, Department of Biochemistry, Queen Elizabeth College (University of London), Campden Hill Road, London W8 7AH, U.K.
2. Department of Clinical Veterinary Medicine, University of Cambridge, Madingley Road, Cambridge CB3 0ES, U.K.
3. Department of Biochemistry, Imperial College of Science and Technology (University of London), London SW7 2AS, U.K.