ESR1 and breast cancer: Estrogen receptor alpha (ERα)-positive MCF-7 cells and ERα-negative SKBR3 breast cancer cells were determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium (MTT) assay to analyze the vitality of cell proliferation, and through immunoblotting analysis and DNA fragment gap staining, apoptosis was analyzed.