Research on molecular hallmarks of BC has identified several diagnostic markers including: i) immunohistochemical markers, such as estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2); ii) genetic markers, such as BRCA1, BRCA2, and PIK3CA mutations; iii) immunomarkers, such as programmed death-ligand 1 (PD-L1) and tumor infiltrating lymphocytes; iv) proliferation markers, such as Ki-67. This evidence concerns the gene ESR1 and breast cancer.