1. Pharmacology and Immunology Unit, CEA, DSV/DRM, CEA/Saclay, 91191 Gif-sur-Yvette Cedex, France, Internal Medicine Unit, University Hospital of Bicêtre, Kremlin-Bicêtre, France, Neurovirology Unit, CEA, DSV/DRM, CEA/Fontenay, 92265 Fontenay aux Roses Cedex, France, Organique Chemistry Unit, Institut Pasteur, Paris, France, and Bioorganique Synthesis Unit, University Louis Pasteur, Strasbourg, France