1. Department of Biochemistry and Molecular Biology, University of Leeds, Leeds LS2 9JT, U.K.
2. Department of Biochemistry and Applied Molecular Biology, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 IQD, U.K.
3. Department of Biochemistry and Chemistry, Royal Free Hospital School of Medicine, Rowland Hill Street, London NW3 2PF, U.K.
4. Hannah Research Institute, Ayr KA6 5HL, U.K.