Erythropoietin, also known as EPO, is a member of the EPO/TPO family and encodes a secreted, glycosylated cytokine hormone composed of four alpha helical bundles. The protein is found in the plasma and regulates red cell production by promoting erythroid differentiation and initiating hemoglobin synthesis. Overexpression of EPO is associated with several pathophysiological conditions, such as polycythemias vera, which is caused by the EPO -independent growth of erythrocytic progenitors from abnormal stem cells. Recombinant human EPO protein, fused to His-tag at N-terminus, was expressed in E.coli.