1. Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB2 1QW, U.K.
2. Unit of Paediatric Surgery, Institute of Child Health and Great Ormond Street Hospital for Children NHS Trust, University of London, 30 Guilford Street, London WC1N 1EH, U.K.
3. Department of Biochemistry and Molecular Biology, Hannah Research Institute, Ayr KA6 5HL, Scotland, U.K.