1. Department of Genitourinary Medicine, Mortimer Market Centre, Camden PCT, London WC1E 6JB;
2. Department of Histopathology, University College London Hospitals Foundation Trust, London WC1E 6JJ;
3. Centre for Sexual Health and HIV Research, Department of Population Sciences and Primary Care, University College London, Mortimer Market Centre, London WC1E 6JB, UK