1. The authors are at the Apoptosis, Cancer and Immunity Laboratory associated with the National League Against Cancer, CNRS-UMR1599, Institut Gustave Roussy, 39 rue Calmette Desmoulins, F-94805 Villejuif, France. C. Brenner is also at CNRS-UMR6022, Université de Technologie de Compiègne, BP 20529, F-60205 Compiègne, France.