ESR1 and breast carcinoma: Breast cancer can be characterized as four molecular subtypes based on the presence or absence of molecular markers such as estrogen receptor (ER) or progesterone receptor (PR) and epidermal growth factor receptor 2 (ErbB2, also known as HER2): namely, luminal A, luminal B, HER2+, and triple-negative breast cancer (TNBC) (19).