1. M.R.C. Human Genetic Diseases Research Group, Department of Biochemistry, King's College London, Campden Hill Road, London W8 7AH, U.K.
2. Enzyme Section, Department of Clinical Biochemistry, Institute of Child Health (University of London), 30 Guilford Street, London WC1 1EH, U.K.
3. Department of Molecular and Cell Biology, G. D. Searle and Co., St. Louis, MO 63198, U.S.A.
4. Dyson Perrins Laboratory, University of Oxford, South Parks Road, Oxford OXI 3QY, U.K.