ESR1 and breast cancer: While we did not detect any significant association between mitochondrial ER-α and ER-β and response to endocrine therapies in ER-α (+)/ER-β (+) breast cancer cells (Fig. 4B), we found significant association between sensitivity to tamoxifen and fulvestrant and increased mitochondrial ER-β localization in ER-α (−)/ER-β (+) cells (Fig. 4C).