Although the treatment of breast cancers depends on the biological characteristics and molecular markers of each tumor, such as tumor size, lymph node involvement, distant metastases, histological grade, estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (c-erbB2) status, the response to the appropriate treatment strategy may not always be the same in patient groups with similar criteria, and more different and advanced treatment may be required [1, 2]. This evidence concerns the gene ESR1 and breast cancer.