CD34 and non-proliferative diabetic retinopathy: In addition, a study evaluating the role of the erythropoietin (EPO) and its receptor (EPOR) in different stages of DR, found that CPCs (CD34+ EPOR+) were reduced in NPDR patients compared to HC while those levels did not significantly vary in PDR patients and DR patients with diabetic nephropathy (DR-DN) [150].