1 Virology Unit, University of Pisa, I-56127 Pisa, Italy.
2 Virology Unit, University of Pisa, I-56127 Pisa, Italy; Retrovirus Centre and Virology Section, University of Pisa, I-56127 Pisa, Italy.
3 Virology Unit, University of Pisa, I-56127 Pisa, Italy; Retrovirus Centre and Virology Section, University of Pisa, I-56127 Pisa, Italy. Electronic address: mauro.pistello@unipi.it.