To examine the long-term effects of FOXO3a on ER function in estrogen-dependent breast cancer cells and to elucidate the molecular mechanisms underlying those effects, we established three MCF7-FO stable cell lines over-expressing FOXO3a (MCF7-FO10, MCF7-FO33, and MCF7-FO41) and three control cell lines (MCF7-C4, MCF7-C5, and MCF7-C12). The gene discussed is ESR1; the disease is breast cancer.