From the Stem Cell Fate Laboratory (C.D., E.L., S.I., O.G., G.M.), Institute of Genetics and Biophysics “A. Buzzati-Traverso,” Consiglio Nazionale delle Ricerche, Naples, Italy; Institute of Genetics and Biophysics “A. Buzzati-Traverso” (C.D., E.L., S.I., O.G., A.M.L., G.L.L., G.M.), Consiglio Nazionale delle Ricerche, Naples, Italy; Vesalius Research Center (M.A., P.C.), VIB, Leuven, Belgium; and Vesalius Research Center (P.C.), Katholieke Universiteit Leuven, Belgium. (At 2009-07-31)