ESR1 and breast cancer: BC can be stratified though the expression of molecular markers, such as estrogen receptors (ER), progesterone receptors (RP), and human epidermal growth factor receptor 2 (HER2) overexpression or amplification, classifying the disease into four major molecular subtypes: Luminal A (LA; ER/PR + HER2-), Luminal-B HER2-positive (LB; ER/PR + HER2+), HER2-enriched (HER2; ER-PR-HER2+) and triple-negative (TN; ER-PR-HER2-) [7,8].