CBS and diabetic retinopathy: Compared to nondiabetic donors, DNA at the promoters of both CBS and MTHFR was hypermethylated and DNMT1 was activated in the retinal microvessels from donors with diabetic retinopathy as observed by the 2-fold increase in 5mC levels at CBS promoter and ~ 2.5-fold increase at MTHFR promoter, and ~ 60% increase in DNMT1 gene transcripts (Fig. 5a-c).