Due to increased binding of Tet2 at the MMP-9 promoter, the levels of 5hmC are increased and hypomethylation of the promoter results in its transcriptional activation.22 Novel results presented here show that once the retinal Tet is activated in diabetes, it does not benefit from the good glycemic control for a duration that follows the hyperglycemia, and the MMP-9 promoter continues to be hypomethylated with MMP-9 activation, further compromising mitochondrial homeostasis. This evidence concerns the gene MMP9 and diabetes mellitus.