The present study was conducted to examine the inhibitory effects of 4-OH-TAM on growth and proliferation of breast cancer cells, to identify novel 4-OH-TAM-induced gene signatures in both ER+- and ER--breast cancer cell lines as well as in ER+- and ER--breast tumor tissues and their corresponding tumor-adjacent tissues, aiming to elucidate the mechanisms underlying the anti-cancer effects of TAM. This evidence concerns the gene ESR1 and breast cancer.