1. Laboratory of Virology, Istituto Superiore di Sanità, Rome, Italy and Istituto Tecnologie Biomediche, CNR, Rome, Italy
2. Department of Biology, University of Rome 3, Rome, Italy
3. Laboratory of Virology, Istituto Superiore di Sanità, Rome, Italy
4. Laboratory of Virology, Istituto Superiore di Sanità, Rome, Italy and Department of Biology, University of Rome 3, Rome, Italy