1. Laboratory for Clinical and Molecular Virology, University of Edinburgh, Summerhall, Edinburgh EH9 1QH, UK1
2. Department of Infectious and Tropical Diseases, London School of Hygiene & Tropical Medicine, Keppel Street, London WC1E 7HT, UK2
3. Leukaemia Research Fund Virus Centre, University of Glasgow, Bearsden, Glasgow G61 1QH, UK3