ESR1 and cancer: Breast cancer profiling of gene expression signatures has defined at least four groups of cancer patients with distinct HER2 and hormone receptor (HR: ER and/or progesterone receptor) expression levels: (1) luminal A: HR+/HER2-, (2) luminal B: HR+/HER2+/-, (3) HER2-overexpressing: HR-/HER2+, and (4) basal-like: HR-/HER2-, generally corresponding to triple negative [30].