1. Division of Biochemistry and Genetics, Institute of Child Health, University of London, 30 Guilford Street, London WC1N 1EH, U.K.
2. Department of Clinical Neurochemistry, Institute of Neurology, University of London, Queen's Square, London, U.K.
3. Department of Biochemistry, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AS, U.K.