ESR1 and breast cancer: Expression levels of proteins such as ER (estrogen receptor), Ki67 (a marker of proliferation Ki-67), PR (progesterone receptor), and HER2 (member of the epidermal growth factor receptor family which is involved in the regulation of cell growth, survival, and differentiation via targeting multiple signal transduction pathways) can serve as important biomarkers for breast cancer prognosis and diagnosis (Jafari et al., 2018).