The addition of methyl groups to constituent bases of DNA. In both prokaryotes and eukaryotes certain bases of the DNA generally occur in a methylated form, and in eukaryotes it has a major role in epigenetic control of DNA transcription. The enzyme DNA methyltransferase catalyses the addition of a methyl group (−CH₃) to certain cytosine residues, particularly ones adjacent to guanine residues, which are abundant in the so-called CpG islands frequently associated with the promoter regions of genes. The pattern of such methylated cytosine residues in the DNA of an individual is called the methylome. These 5′-methylcytosine residues tend to bind repressor proteins, thereby inactivating the gene. Such changes in expression patterns are heritable even though no changes in DNA sequence have occurred. However, many genes become demethylated in sperm, eggs, and early developing embryos. Only later, as cells become specialized for certain functions, does DNA methylation become widespread again as a means of silencing redundant genes. In bacteria this methylation protects the cell’s DNA from attack by its own restriction enzymes, which cleave foreign unmethylated DNA and thereby help to eliminate viral DNA from the bacterial chromosome. Methylation is also important in helping DNA repair enzymes to distinguish the parent strand from the progeny strand when repairing mismatched bases in newly replicated DNA. See chromatin remodelling.