Genomic DNA from many organisms has modified nucleotides. In the mammalian genome, the modified base is predominately 5-methylcytosine (5-mC), which is involved in gene expression regulation, including selective inactivation of one X chromosome in females of mammalian species (1,2). Symmetrical CpG DNA methylation is heritable, but also reversible. This change of methylation status results in an enormous number of combinations of epigenetic states that can regulate gene expression. In mammalian cells, DNA methylation mainly occurs in CpG dinucleotides and is carried out by two methyltransferase enzymatic activities, namely, maintenance methylation and de novo methylation. The maintenance methyltransferase, DNMT1, is involved in the DNA methylation after every cellular DNA replication cycle. DNMT3a and DNMT3b are the de novo methyltransferases that are active in early development.