1. NIMR, The Ridgeway, London NW7 1AA, U.K., EMBL, Meyerhofstrasse 1, D-69017 Heidelberg, Germany, University College London, London WC1E 6BT, U.K., Medical School, University of Ioannina, Ioannina 65110, Greece, and Institut für Organische Chemie, Universität Tübingen, D-72076 Tübingen, Germany