ESR1 and neoplasm: In a previous study of our group, we showed that well-established histopathological parameters, i.e., estrogen receptor (ER), progesterone receptor (PR), HER2, and Ki-67 (4-IHC) could define the four common tumor biological subtypes Luminal A, Luminal B, HER2-like, and Basal-like in routine clinical work [13].