This work was supported by grants of the Spanish Ministry of Science and Technology (BMC2002-03599 to C.M.), the Plan Regional de Investigación del Principado de Asturias (GEMEDO1-05 to J.A.S.), and the NIH (CA 91901 to J.R.). M.P. was the recipient of a pre-doctoral fellowship of the Spanish Ministry of Science and Technology. We thank Obra Social Cajastur for financial support to F.L.
MCT / BMC2002-03599 GEMEDO1-05 NCI / NIH / CA 91901