1. Mutagenesis Laboratory, Imperial Cancer Research Fund, Clare Hall Laboratories, Blanche Lane, South Mimms, Hertfordshire, EN6 3LD, U.K., Biomolecular Modeling, and Molecular Structure and Function Laboratories, Imperial Cancer Research Fund, 44 Lincoln's Inn Fields, London, WC2A 3PX, and Division of Physical Biochemistry, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA.