A number of causes have been identified to account for ER-α inactivation, such as homozygous deletion, loss of heterozygosity, or ER gene mutation.14, 15, 16, 17, 18, 19 Here, we further demonstrated that ectopic ZEB1 represses ER-α expression in breast cancer cells and thus recapitulates a loss of response to antiestrogen treatment in vitro and in vivo. This evidence concerns the gene ZEB1 and breast cancer.