KRAS and cancer: In contrast to the combination of the MMTV-Flp with the MMTV-neu oncogene that are not always co-expressed in the same cancer cell-of-origin (Fig. 3), we assumed that MMTV-Flp-induced mammary tumors expressing oncogenic KRAS should also carry a Flp-activated Rosa26CAG-FSF-CreERT2 allele that would subsequently facilitate the tamoxifen (Tam)-controlled activation of Cre recombinase and expression of GFP from the Cre/lox reporter transgene.