MMP9 and diabetic retinopathy: Epigenetic modifications play an important role in regulation of many molecular mechanisms associated with the development of diabetic retinopathy; diabetes induces histone modifications in nuclear DNA (nDNA)-encoded genes, including matrix metalloproteinase-9 (MMP-9)15 and manganese superoxide dismutase (Sod2),7 contributing to increased oxidative stress in the retina.