Supplemental Experimental Procedures Expression Constructs

The following plasmids were used for the generation of retroviruses: pBabe (Morgenstern and Land, 1990), pLPC (the gene of interest is expressed from the CMV promoter and the gene for puromycin resistance is driven by an LTR), pWZL (co-expresses the gene of interest with a selectable marker translated from an internal ribosomal entry site) and pZIPneo (Jat et al., 1986). pBabe-Puro-based vectors encoding H-RasV12, N-RasV12 and K-RasV12 4A and 4B, a pZIPneo derivative containing the SV40 LT-Ag, as well as pLPC-Hygro-based vectors encoding Raf-1 22W (a Raf-1 mutant lacking 305 N-terminal residues) (Stanton et al., 1989), Rac1 N115I (Khosravi-Far et al., 1995), and RalGDS-CAAX (Matsubara et al., 1999), were kindly provided by M. Serrano (CNIO, Madrid). RSV-vJun and pcDNA3-cMyc, were kind gifts from P. Angel (DKFZ, Heidelberg) and M. Eilers (University of Marburg, Germany), respectively, and were subsequently cloned as BamHI-EcoRI fragments into pBabe-Puro. pSGT-Src527 expressing an activated form of chicken c-Src with the mutation Y527F was kindly provided by G. Superti-Furga (CeMM Center for Molecular Medicine of the Austrian Academy of Sciences, Vienna) and cloned as a SalI-SalI fragment into pBabe-Puro. R. Marais (The Institute of Cancer Research, London) kindly provided us with the B-Raf-encodin