Their evaluation is important to classify BC into categories, that have different impact on treatment choice and prognosis [luminal A (ER+ and/or PR+, HER2−, low Ki67); luminal B (ER+ and/or PR+, HER2+/−, high Ki67); HER2-enriched (ER−, PR−, HER2+); and triple-negative (TN) (ER−, PR−, HER2−)]. This evidence concerns the gene ESR1 and breast cancer.