1. MRC Human Immunology Unit, Weatherall Institute of Molecular Medicine, The John Radcliffe, Oxford OX3 9DS, UK
2. University of St Andrews, School of Biology, St Andrews, UK
3. Nuffield Department of Medicine, John Radcliffe Hospital, Oxford OX3 9DU, UK
4. International AIDS Vaccine Initiative, 110 William Street, New York, NY 10038, USA
5. Department of Medical Microbiology, University of Nairobi, Nairobi, Kenya