ESR1 and breast cancer: Breast cancers are classified based on the expression of estrogen receptors (ERs), progesterone receptors (PRs), and human epithelial growth factor receptor 2 (HER2) into three categories: hormone receptor-positive (ER+, PR+, HER2−), HER2−rich (ER−, PR−, HER2+), and triple-negative (ER−, PR−, HER2−) [3,4].