In lung cancer for instance, EGFR mutations and KRAS mutations are mutually exclusive and found each in about 25% of non-small cell lung cancers.4 In melanoma, about half of the cases present a typical mutation in the BRAF gene.5 In the exocrine pancreas, the large majority of carcinomas contain a KRAS mutation.6 It seems thus that, for a specific tissue, the number of mutational paths a normal cell can follow to become a tumor cell is rather limited. This evidence concerns the gene KRAS and carcinoma.