1. Department
of Biochemistry, University of Cambridge, 80 Tennis Court Road, Cambridge CB2 1GA, U.K.
2. Biomolecular Interactions Consultancy, 3 Danesbury Park, Hertford SG14 3HX, U.K.
3. MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge Biomedical Campus, Cambridge CB2 0QH, U.K.