1. Department of Molecular and Experimental Medicine, The Scripps Research Institute, La Jolla, California 92037
2. Fondazione ‘Istituto Pasteur-Cenci Bolognetti,' Dipartimento di Biotechnologie Cellulari ed Ematologi, Università La Sapienza, and I.N. Malattie Infettive, I.R.C.C.S. “L. Spallanzani,” Rome, Italy