ESR1 and breast cancer: The most aggressive forms of breast cancer are triple negative breast cancer (TNBC), defined histologically by the absence of estrogen receptor (ER), progesterone receptor (PR) and epidermal growth factor 2 (HER2), and a subset of TNBC referred to as basal-like breast cancer, characterized by CK5/6 and/or epidermal growth factor receptor (EGFR) expression [1–3].