ESR1 and cancer: Molecular markers, such as estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), proliferation marker Ki67, and multigene signatures, which may comprise a set of multiple genes associated with cancer development and progression, are increasingly being used due to their clinical value demonstrated in various randomized clinical trials, and more recently, BRCA1/2 genes, cyclin D1, vascular endothelial growth factor (VEGF), and topoisomerase II (TOPOII) have been extensively studied, and their prognostic value has been established [14,15].