MMP9 and diabetes mellitus: Interestingly, Zhong et al. found that the promoter methylation level (H3K9me2) of Matrix Metalloproteinase-9 (MMP-9) in retinal microvascular endothelial cells of diabetes mice decreased, while the acetylation level (H3K9Ac) increased [99,112,113,114], resulting in high expression of MMP-9, damaging mitochondria and inducing apoptosis of retinal microvascular cells [112,115].