1. Department of Physiology, Charing Cross and Westminster Medical School, University of London, St Dunstan's Road, London W6 8RP, U.K.,
2. Division of Biochemistry,
3. Department of Anatomy and Developmental Biology, St George's Hospital Medical School, University of London, Cranmer Terrace, London SW17 0RE, U.K.,
4. Office of Naval Research Europe, 223 Old Marylebone Road, London NW1 5TH, U.K.