1. CNRS-UMR7001/ENSCP/Gencell S.A., Aventis Pharma, 13 Quai Jules Guesdes, 94403 Vitry-Seine, France, Gencell, Aventis Pharma, 3825 Bay Center Place, Hayward, California, 94545, and Cellular and Molecular Imagery Service, Faculté de Pharmacie, 4 avenue de l'Observatoire, 75270 Paris Cedex 06, France.