ESR1 and breast cancer: To further investigate this, two E2-responsive ERα-positive breast cancer cell lines (MCF-7 and T47D) and two E2-independent ERα-negative breast cancer cell lines (MDA-MB-231 and MDA-MB-468) were withdrawn from steroids for five days followed by culture for 72 hours using steroid-depleted media (control), 1nM E2 or 1nM E2 with tamoxifen.