At the turn of the 21st century, Perou39 and Sorlie40 reported four significant groups of breast cancer subtypes using RNA expression profiling: luminal A (ER‐positive, HER‐2 negative), luminal B (ER‐positive, HER‐2 positive), HER2‐overexpressing (ER and PR negative, HER‐2 positive), and basal‐like. The gene discussed is ESR1; the disease is breast cancer.