1. Departments of Biochemistry & Molecular Biology and Genetics, University of Leeds, Leeds LS2 9JT, U.K.
2. Beatson Institute for Cancer Research, CRC Beatson Laboratories, Switchback Road, Bearsden, Glasgow G61 1BD, U.K.
3. Department of Biochemistry and Applied Molecular Biology, UMIST, PO Box 88, Manchester M60 1QD, U.K.