This work was supported with grants SAF2003- 03303 from MCYT PI031596 (CR) and CO3/10 from FIS (IA) (Red de Centros de Cáncer RTICCC). M.P.C. thanks fellowship from IUOPA. V.M. was supported by FICYT (PB 02-085) and MCYT (SAF2003-03303). R.M.S. acknowledges financial support from Programa Ramón y Cajal from MCYT, Spain. F.H. acknowledges a grant BEFI 00/9412 from FIS. G.-S.H. was supported by FICYT (BP03-112).