Recent studies have established that breast cancers that are basal-like or “triple-negative” (estrogen receptor–negative (ER–), progesterone receptor–negative (PR–) and human epidermal growth factor receptor 2–negative (HER2–)) respond best to cytotoxic therapies [11],[12] and that HER2-positive (HER2+) tumors respond best to trastuzumab-based regimens [13]. Here, ESR1 is linked to breast cancer.