1. the Growth Factor and Cell Differentiation Laboratory (A.B.), University of Bordeaux I, 33405 Talence, France
2. Department of Cell Biology (S.K., G.P), and the Kaplan Cancer Center, New York University Medical Center, New York, New York 10016;
3. the Raymond and Beverly Sackler Foundation Laboratory (D.B.R.), and the Kaplan Cancer Center, New York University Medical Center, New York, New York 10016;