In contemporary clinical management of patients with breast cancer, prognostications and therapeutic decisions are based on the assessment of clinicopathological factors as well as on the expression status of biomarkers with established clinical validity (i.e., human epidermal growth factor receptor 2 [HER2]; estrogen receptor [ER]; progesterone receptor [PgR]; and Ki67, a marker of cell proliferation) [1, 2]. This evidence concerns the gene ESR1 and breast cancer.