The cytosine moiety of cytosine–guanine (CpG) dinucleotides in mammalian DNA can be methylated at carbon 5 to form 5′ methyl-cytosine (figure 1). The methyl group for this chemical modification of the DNA is donated by SAM. This reaction is catalyzed by a family of DNMTs. Of these, DNMT3A and 3B primarily perform de novo methyl transfer, whereas DNMT1 mainly acts as maintenance DNMT with greater affinity for partially methylated (i.e., hemi-methylated) DNA. Methylation of cytosine in CpG-rich regions (i.e., CpG islands) located in or near gene promoters results in gene silencing (Ulrey et al. 2005).
