1. National AIDS Center
2. Department of Drug Research and Evaluation, Istituto Superiore di Sanità, Viale Regina Elena 299, 00161 Rome, Italy
3. Istituto Pasteur-Fondazione Cenci Bolognetti, Dipartimento di Studi Farmaceutici, P.le Aldo Moro 5, 00185 Rome, Italy
4. Laboratory of Molecular Pharmacology, National Institutes of Health, Bethesda, Maryland 20892