1. Laboratory of Experimental Virology, Department of Medical Microbiology, Center for Infection and Immunity Amsterdam (CINIMA), Academic Medical Center of the University of Amsterdam, 1105 AZ Amsterdam, The Netherlands.
2. Department of Cell Biology and Histology, Center for Immunology Amsterdam (CIA), Academic Medical Center of the University of Amsterdam, 1105 AZ Amsterdam, The Netherlands.
3. Division of Retrovirology, National Institute for Biological Standards and Control, Potters Bar, Hertfordshire EN6 3QG, United Kingdom.