1. Department of Paediatrics, John Radcliffe Hospital, Heading ton, Oxford, 0X3 9DU, U.K.
2. Bath College of Higher Education, Newton Park, Newton St Loe, Bath, BA2 9BN, U.K.
3. Clinical Research Centre, Division of Clinical Cell Biology, Watford Road, Harrow, HA1 3UJ, U.K.