1. Division of Retrovirology, NIBSC, Blanche Lane, South Mimms, Potters Bar, Herts EN6 3QG, UK1
2. School of Biological Sciences, University of Surrey, Guildford, Surrey GU2 7XH, UK2
3. Division of Virology, University College of London Medical School, Windeyer Building, 46 Cleveland Street, London W1P 6DB, UK3
4. School of Animal and Microbial Sciences, University of Reading, Whiteknights, PO Box 228, Reading, Berks RG6 2AJ, UK4