1. Retrovirus Center and Virology Section, Department of Experimental Pathology, University of Pisa, I-56127 Pisa
2. Department of Public Health, University of Florence, I-50134 Firenze
3. Department of Pharmaceutical Sciences, University of Salerno, I-89089 Fisciano
4. Laboratory of Peptide and Protein Chemistry and Biology, Department of Pharmaceutical Sciences, University of Florence, I-50019 Sesto Fiorentino, Italy