ESR1 and breast neoplasm: (1) luminal A (ER and/or PR-positive, HER2-negative); (2) luminal B (ER-positive and/or PR-positive, HER2-positive); (3) HER2 overexpressing (ER-negative, PR-negative, HER2-positive); (4) basal-like (ER-negative, PR-negative, HER2-negative, cytokeratin 5/6-positive, and/or HER2-positive); and (5) unclass­ified breast tumors [7].