ESR1 and breast cancer: BC has stratified based on the expression of progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and estrogen receptor (ER) into four classes: luminal A (ER+, PR+, HER2-), luminal B (ER-, PR-, HER2±), HER2-positive (ER-, PR-, HER2+), and triple-negative (ER-, PR-, HER2-) 4.