1. Institute of Biochemistry, Faculty of Medicine, University of Giessen, Friedrichstrasse 24, D-35392 Giessen, Germany
2. School of Biological Sciences, University of Wales, Bangor, Gwynedd LL57 2UW, Wales, U.K.
3. London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, U.K.