1. Department of Virology, Biomedical Primate Research Centre, Rijswijk 2280 GH, Netherlands.
2. St. George's Hospital Medical School, Division of Oncology, Department of Cellular and Molecular Medicine, Cranmer Terrace, London SW17 0RE, UK.
3. Wohl Virion Centre, Division of Infection and Immunity, University College, London W1T 4JF, UK.