ESR1 and breast cancer: According to the expression status of these receptors, breast cancers are classified as luminal A (ER+ or PR+/HER2-), luminal B (ER+ or PR+/HER2+), HER2 (ER-/PR-/HER2+) or triple-negative breast cancer (TNBC, ER-/PR-/HER2-) [1,2].