To dissect the role of Dll1 in autochthonous mammary tumors, we crossed Dll1 conditional-knockout (Dll1cKO) mice in which K14-Cre recombinase efficiently knocks out Dll1 in mammary epithelial cells25,26 to MMTV-PyMT to generate PyMT-Dll1cKO animals (Fig. 1a). This evidence concerns the gene DLL1 and breast cancer.