MMP9 and diabetes mellitus: The reason for the decrease in 5mC appears to be the concomitant increase in the binding of hydroxymethylase, ten-eleven translocase 2 (Tet2), at the same site of the promoter, and increase in 5 hydroxymethyl cytosine (5hmC), in turn, activates MMP-9 transcription.20 However, the crosstalk between histone methylation and DNA methylation in the regulation of retinal MMP-9 in diabetes remains to be investigated.