Breast cancer is clinically divided into three main subtypes: luminal estrogen receptor (ER)-positive and progesterone receptor (PR)-positive, which are further subdivided into luminal A and B; human epidermal growth factor receptor 2 (HER2)-positive; and 15% triple-negative breast cancer (TNBC), which accounts for 25% of all breast cancer-related deaths [1,2]. The gene discussed is ESR1; the disease is breast cancer.