Although gene expression tests, e.g. Oncotype DX, Mammaprint and Prosigna, are frequently used for BC, classification of surrogate subtyping based on tumor grade and immunohistochemical analysis of the estrogen (ER) and progesterone (PR) receptors, proliferation factor Ki67, and human epidermal growth factor receptor − 2 (HER2) still remains the routine method for tumor classification. Here, ESR1 is linked to breast cancer.