ESR1 and breast carcinoma: According to immunohistochemical staining of estrogen receptor α (ERα), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2), breast cancer is categorized into the following groups; Luminal, subtype A: ERα+/PR+/HER2-; luminal, subtype B: ERα+/PR+/HER2+; HER2 overexpression: ERα-/PR-/HER2+ and triple-negative breast cancer (TNBC): ERα-/PR-/HER2- [2].