The clinical management of breast cancer relies on parameters such as age, tumor size, and lymph node status, as well as on histopathological biomarkers including histological grade, expression of the hormone receptors estrogen receptor α (ER) and progesterone receptor (PR), and the presence or absence of amplification and concomitant overexpression of the human epidermal growth factor receptor 2 (HER2/ERBB2/neu, hereafter called HER2). This evidence concerns the gene ESR1 and breast cancer.