According to gene expression profiles, breast cancer is usually divided into luminal A-like (ER+/HER2-), luminal B-like (ER+/HER2+ or ER+/HER2-), HER2-enriched (non-luminal, ER-/HER2+), and basal (triple-negative breast cancer, ER-/PR-/HER2-) [4]. The gene discussed is ESR1; the disease is breast cancer.