The tendency to develop breast cancer brain metastases (BCBMs) differs by the BC subtype, varying from almost half with triple-negative breast cancer (TNBC) (HER2− ER− PR−), one-third with HER2+ (human epidermal growth factor receptor 2-positive, and around one-tenth with luminal subclass (ER+ (estrogen positive) or PR+ (progesterone positive)) breast cancer. The gene discussed is ESR1; the disease is breast cancer.