Four subtypes of breast cancer have been classified as: luminal A [estrogen receptor (ER)+ and/or progesterone receptor (PR)+ and human epidermal growth factor receptor-2 (Her2)-], luminal B (ER+ and/or PR+ and Her2+), Her2-enriched (ER−, PR−, and Her2+) and basal-like triple-negative breast cancer. Here, ESR1 is linked to breast cancer.