1. MRC Laboratory of Molecular Biology, Cambridge CB2 2QH, England
2. Christopher Ingold Laboratory, University College London, London WC1H OAJ, England.
3. Fairleigh Dickinson University, Teaneck-Hackensack Campus, Teaneck, NJ 07666.
4. Department of Chemical Physics, Weizmann Institute of Science, 76100 Rehovot, Israel.
5. Départment d'Immunologie Structurale, Institute Pasteur, 75724 Paris, France.
6. Department of Biophysics, University of Leeds, Leeds LS2 9JT, England.