To test whether mitochondrial localization of ER-α and ER-β affects response to endocrine therapy response in breast cancer cells, we have chosen 10 ER-α (+)/ER-β (+) and 10 ER-α (−)/ER-β (+) and we analyzed the association of mitochondrial localization of ER-α and ER-β and EC50 values of cells for tamoxifen, anastrozole, and fulvestrant. Here, ESR1 is linked to breast cancer.