ESR1 and breast cancer: Breast cancer can be classified into four subtypes based on the expression of estrogen receptors (ERs), progesterone receptors, and ErbB2 (HER2) receptors: luminal A (ER+ and HER2-), luminal B (ER+ and HER2+), ErbB2/HER2-positive (ER- and HER2+), and triple-negative (ER- and HER2-) (6, 7, 8).