Division of Virology, Department of Pathology, University of Cambridge, Addenbrooke's Hospital, Hills Road, Cambridge, CB2 0QQ, UK. Division of Virology, Department of Pathology, University of Cambridge, Addenbrooke's Hospital, Hills Road, Cambridge, CB2 0QQ, UK. aj.te.velthuis@princeton.edu. Lewis Thomas Laboratory, Department of Molecular Biology, Princeton University, Washington Road, Princeton, 08544 NJ, USA. aj.te.velthuis@princeton.edu.