Methylation of the DNA is an important epigenetic (i.e., not associated with alteration in the primary structure of the DNA) phenomenon, which plays important roles in regulation of gene expression, maintenance of genome integrity, and genomic imprinting. Although not only other nucleotides, but also proteins and lipids can be methylated, in the context of the present discussion, “methylation” designates only that of cytosine residues that are located 5′ to guanines (CpG cytosines). A methyl residue is added to the 5 position of the pyrimidine ring of cytosine (5-methylcytosine) in the course of DNA replication, a process mediated by DNA(cytosine-5)-methyltransferases.