ESR1 and breast cancer: Here we examined the expression of miR-200a, miR-200b, and miR-200c and their regulation by estradiol (E2) and 4-hydroxytamoxifen (4-OHT), an active TAM metabolite, in a panel of ERα-positive breast cancer cell lines derived from MCF-7 endocrine-sensitive cells representing a cellular model of progression towards endocrine/TAM-resistance.