Despite its heterogeneity, breast cancer can be broadly categorized into four subtypes: luminal A (ER+, and/or PR+, HER2−), luminal B (ER+, PR+, HER2+), HER2 enriched (ER–, PR–, HER2+), and triple-negative breast cancers (TNBC) comprising basal-like (ER–, PR–, HER2–) and claudin-low breast cancers. This evidence concerns the gene ESR1 and breast cancer.