Breast cancer can be classified based on gene expression and histology including the expression of estrogen receptor (ER), progesterone receptor (PgR), and human epidermal growth factor receptor 2 (HER2) into several subtypes, characterized as luminal, normal-like, HER2-overexpressing, and triple negative breast cancer (TNBC) [20]. The gene discussed is ESR1; the disease is breast cancer.