ESR1 and breast cancer: As such, breast cancer treatments can be categorized as: (a) estrogen receptor (ER)-positive and thus, treatable by endocrine therapy; (b) HER2 overexpressed/amplified, treatable with trastuzumab or other HER2-targeted therapies; or (c) triple-negative (TN), with no current specific therapeutic biomarker, typically treated with cytotoxic chemotherapy.