Breast cancer is classified according to estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) expression status into hormone receptor HR+/HER2– (ER+ or PR+/HER2–), HR+/HER2+ (ER+ or PR+/HER2+), HR–/HER2+ (ER–/PR–/HER2+), or triple-negative breast cancer (TNBC; ER–/PR–/HER2–); the HR+/HER2– subtype accounts for 25–40% of breast cancers (1, 2). This evidence concerns the gene ESR1 and breast cancer.