ERBB2 and breast cancer: The ability of the MMTV-Flp to activate the Rosa26CAG-FSF-CreERT2 allele and the downstream expression of a GFP-based Cre/lox reporter transgene in a ligand-controlled manner was tested in three experimental models: the normal mammary gland, luminal-type mammary tumors that overexpress ERBB2, and the new KRASG12D-induced mammary cancer model in which tumor cells acquire basal-like and mesenchymal characteristics.