The clinically relevant classification of BC is based on its histopathological appearance, tumor grade, lymph node involvement, and immunohistochemical properties including the presence of the estrogen receptor (ER), the progesterone receptor (PR), human epidermal growth factor receptor 2 (Her2; ErbB2, c-erbB2), and Ki-67 [1, 2]. The gene discussed is ESR1; the disease is breast cancer.