MMP9 and non-proliferative diabetic retinopathy: While we found no difference in serum MMP-3, MMP-9, and TIMP-1 concentrations between the DR stages, we observed a pronounced upregulation of MMP-3 in the AH in mild/moderate NPDR and advanced NPDR/PDR, as well as an upregulation of TIMP-1 during the late stage of DR.