In the last two decades, the clinical management of breast cancer patients has been greatly facilitated by the availability of several biomarkers with prognostic and predictive values, such as human epidermal growth factor receptor 2 (HER2/neu) gene amplification, elevated levels of estrogen receptor (ERs) or progesterone receptor (PR) and proliferation-related genes, such as the proliferation index (Ki67), that helped stratify patients for receiving appropriate therapy and predict the likelihood of therapy response [1]. This evidence concerns the gene ESR1 and breast cancer.