ESR1 and breast carcinoma: Because of its high heterogeneity in molecules and phenotypes, breast cancer is traditionally divided into four clinical subtypes: Luminal A, Luminal B, HER2-positive (Her2+), and triple-negative breast cancer (TNBC), according to the expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) (2, 3).