Subsequently, Carey et al. reported that breast cancers can be divided into the five intrinsic subtypes by a protein expression analysis of estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor type 2 (HER2), epidermal growth factor receptor (EGFR) and cytokeratin 5 or 6 (CK5/6), using immunohistochemistry (IHC) [2]. The gene discussed is ESR1; the disease is breast carcinoma.