MTHFR and diabetic retinopathy: Higher Dnmt1 and hypermethylation of the promoters of both CBS and MTHFR in the retinal microvessels from donors with diabetic retinopathy suggest that decreased CBS and MTHFR, seen in diabetic retinopathy donors, could be due to increased methylated cytosine levels at their promoters, impeding the binding of the transcription factors, and suppressing their gene expressions.