1. Department of Biochemistry and Molecular Biology, University College London, Gower Street, London WC1E 6BT, U.K.
2. Department of Toxicology, Vrije Universiteit Brussel, Laarbeeklaan 103, B-1090 Brussels, Belgium
3. Department of Biochemistry, Queen Mary and Westfield College, University of London, Mile End Road, London E1 4NS, U.K.