ESR1 and breast cancer: Based on genetic expression, BC can be classified into three main subtypes: hormone receptor-positive tumors (ER+ and/or PR+), which have high levels of estrogen and/or progesterone receptors; HER2-positive tumors, which overexpress the ERBB2 oncogenes; and triple-negative tumors (TNBC), which do not have hormone receptors or HER2 enhancement.