To determine the role of Zeb1 in breast cancer initiation and progression, we crossed the floxed Zeb1 allele homozygously into PyMT mice to generate PyMT;Zeb1cKO (MMTV-Cre;PyMT;Zeb1fl/fl) mice (Fig. 1a)27. Here, ZEB1 is linked to breast carcinoma.