1. Center for AIDS Research, Program in Molecular Medicine, Department of Molecular Genetics and Microbiology, University of Massachusetts Medical School, Biotech II, 373, Plantation Street, Worcester. MA 01605, USA1
2. The Wohl Virion Center, Department of Immunology and Molecular Pathology, The Windeyer Institute for Medical Sciences, University College London, 46 Cleveland Street, London W1P 6DB, UK2