Breast cancer can be divided into four subtypes: luminal A, luminal B, HER-2 positive and triple-negative breast cancer (TNBC), as determined by an immunohistochemical (IHC) analysis of estrogen receptors (ER), progesterone receptors (PR), Ki-67 and epidermal growth factor receptor type 2 (HER-2) (Goldhirsch et al., 2011). The gene discussed is ESR1; the disease is breast cancer.