The subtypes of BC are luminal A and B (respectively, ER/PR-positive/HER-2-negative and ER-positive, PR-positive or negative, and HER-2-positive); HER-2-enriched; claudin-low (ER/PR-negative, HER-2-positive, and low claudin expression); triple-negative, including the basal-like phenotype (ER/PR/HER-2-negative); normal breast-like (ER/PR-positive or negative and HER-2-negative). This evidence concerns the gene ESR1 and breast cancer.