1. Department of Virology, Faculty of Medicine, Imperial College London, St Mary's Campus, Norfolk Place, London W2 1PG, UK
2. LICR, Experimental Medicine Unit, Université Catholique de Louvain, Brussels, Belgium
3. Department of Biochemistry & Molecular Biology, New Jersey Medical School, Newark, USA
4. Department of Genetics, Harvard Medical School, Department of Molecular Biology, Massachusetts General Hospital, Boston, MA 02114, USA