1. Department of Biochemistry, Imperial College of Science and Technology, South Kensington, London SW7 2AZ, U.K.
2. Department of Clinical Pharmacology, Royal Postgraduate Medical School, Hammersmith Hospital, DuCane Road, London W12 OHS, U.K.
3. Imperial Cancer Research Fund, Clare Hall Laboratories, South Mimms, Herts. EN6 3LD, U.K.
4. MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, U.K.