ESR1 and breast cancer: Doctors subtype breast cancers for different treatments based on mutations2; estrogen receptor (ER), progesterone receptor, and human epidermal growth factor receptor 2 (HER2) protein expression status6; and more specific molecular subtyping, such as the PAM50 classification that groups tumors based on a panel of gene expression (ie, as luminal A, luminal B, HER2‐enriched, basal‐like, or normal‐like tumors).8