1 National Centre for Human Retrovirology, St Mary's Hospital, Imperial College Healthcare NHS Trust, London, UK; Section of Virology, Imperial College London, London W2 1PG, UK.
2 National Centre for Human Retrovirology, St Mary's Hospital, Imperial College Healthcare NHS Trust, London, UK; Section of Virology, Imperial College London, London W2 1PG, UK. Electronic address: g.p.taylor@imperial.ac.uk.