ESR1 and breast carcinoma: Based on the immunohistochemistry biomarkers, including estrogen receptor (ER), progesterone receptor (PR), and HER2, breast cancer is classified into the following subgroups; luminal A (ER+, PR±, HER2−), luminal B (ER+, PR±, HER2+), HER2 (ER−, PR−, HER2 overexpression) and basal-like triple-negative (ER−, PR−, HER2−) breast cancer [20,21].