Breast cancer is a heterogeneous disorder characterized by the expressions of different receptors, like the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2, or receptor tyrosine kinase erbB2 (ERBB2)), as well as the proliferation marker Ki-67 [19,20,21]. The gene discussed is ESR1; the disease is breast cancer.