ESR1 and breast carcinoma: Triple-negative breast cancer (TNBC), an aggressive subtype of breast cancer with a poor prognosis,3 is characterized by tumors that do not express estrogen receptors (ERs) or progesterone receptors (PRs), nor display an overexpression of human epidermal growth factor receptor 2 (HER2).4 Therapies targeted against HER2-positive breast cancers, such as trastuzumab (Herceptin),5 and those targeted against ER-positive breast cancers, such as tamoxifen,6 have no therapeutic benefit to individuals with the TNBC subtype.