1. MRC Virology Unit, Institute of Virology, University of Glasgow, Church Street, Glasgow G11 5JR, United Kingdom
2. Institute of Infection, Immunity and Inflammation and Division of Microbiology, University of Nottingham, Queen's Medical Centre, Nottingham NG7 2UH, United Kingdom
3. Laboratoire de Vectorologie Rétrovirale et Thérapie Génique, INSERM U412, IFR128 BioSciences Lyon-Gerland, Ecole Normale Supérieure de Lyon, 69364 Lyon Cedex 07, France