1. Istituto di Genetica Molecolare, CNR, via Abbiategrasso 207, I-27100 Pavia, Italy
2. Servizio di Virologia, IRCCS Policlinico San Matteo, Pavia, Italy
3. Molecular Medicine Laboratory, International Centre for Genetic Engineering and Biotechnology, Trieste, Italy
4. Max von Pettenkofer-Institut, Lehrstuhl für Virologie, LMU-München, München, Germany
5. Department of Medicine and Surgery, San Paolo Hospital, University of Milano, via A. di Rudinı ` 8, I-20142 Milano, Italy