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Previous data have shown that FOXM1 expression is elevated in breast cancer tissues and is strongly correlated with the expression pattern of ERα in breast cancer cells. The expression of ERα is a good prognostic factor in breast cancer, as about two-thirds of these ERα-positive patients respond to treatment with antiestrogens. However, approximately one-half of the patients that initially respond to hormonal therapy develop resistance. Since FOXM1 is critical for the progression of the cell cycle, we investigated the regulation of FOXM1 by ERα and its role in endocrine sensitivity and resistance in breast cancer cells. We firstly observed by quantitative RT-PCR a strong and significant positive correlation between ERα and FOXM1 mRNA expression in breast cancer patient samples. We showed that FOXM1 protein and mRNA expression was regulated by ER ligands. We also demonstrated that ectopic conditional expression of ERα, in the presence of estrogens, leads to induction of FOXM1 expression in ER-negative U2OS cells. Using reporter gene assays, we demonstrated that ERα activates FOXM1 transcription through an estrogen-response element site. The direct binding of ERα to the FOXM1 promoter was confirmed in vitro by mobility shift and DNA pull-down assays and in vivo by chromatin immunoprecipitation analysis. Importantly, silencing of FOXM1 by RNA interference abolishes the estrogen-mediated MCF-7 cell proliferation. Conversely, ectopic expression of a constitutively active FOXM1 can abrogate the cell cycle arrest mediated by the antiestrogen tamoxifen. Taken together, the results clearly demonstrate FOXM1 as a key mediator of the mitogenic functions of ERα and estrogen in breast cancer cells. Our findings that antiestrogens repress FOXM1 expression in endocrine-sensitive but not endocrine-resistant breast carcinoma cell lines and that ectopic expression of an active FOXM1 can abrogate the anti-proliferative effects of tamoxifen also suggest that deregulation of FOXM1 may also contribute to antiestrogen insensitivity.