Breast cancer is a heterogeneous group of diseases that are immunohistochemically categorized by the cancer cell expression of the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor-2 (HER-2) amplification into four main subtypes: luminal A, luminal B, Her-2-positive, and triple-negative [1][2]. The gene discussed is ESR1; the disease is breast cancer.