ESR1 and breast cancer: Based on immunohistochemical analyses of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor-2 (HER2), breast cancer has been classified into luminal A-like (ER-positive (ER+) and/or PR-positive (PR+), HER2-negative (HER2–)), luminal B-like (ER+ and/or PR+, HER2-positive (HER2+)), HER2-enriched (ER-negative (ER–)/PR-negative (PR–)/HER2+), and triple-negative breast cancer (TNBC; ER–/PR–/HER2–) [2–10].