ESR1 and breast carcinoma: Breast cancer (BC) is a highly heterogeneous neoplasm with distinct molecular subtypes which can be classified as Luminal A or B, human epidermal growth factor receptor 2 (HER2) positive, or triple negative breast cancer (TNBC) based on the immunohistochemical expression of hormone receptors, such as the expression of estrogen receptors (ER +), progesterone receptors (PR +), human epidermal growth factor receptors (HER2 +), grade, and proliferation index (Ki-67) [3–5].