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Epigenetics refers to the chemical modifications to DNA and histones that control gene expression without changing the DNA base sequence, and these changes are usually inherited after cell division. Typical examples of epigenetic changes include DNA methylation and acetylation and methylation of histones. Also, while the general term for the information of the DNA base sequence is called the “genome,” the general term for the epigenetic information that modifies this genome is called the “epigenome.” DNA methylation is the only physiological modification in the genome of vertebrates. DNA methylation is a reaction in which a methyl group is added to the 5-position carbon atom of cytosine (C) in a 2-base sequence (CpG) arranged from the 5′ side of DNA in the order of cytosine (C), guanine (G), and is catalyzed by DNA methyltransferase (DNMT). Many gene promoter regions have clusters of CpG sequences called CpG islands (Fig. 18.1). In the promoter regions of genes that generally show active expression, DNA methylation is not observed, and histones are acetylated. On the other hand, when the ninth lysine residue (H3K9) of histone H3 is methylated, it recruits DNMT and histone deacetylase (HDAC), DNA is methylated, and histones are deacetylated. These modifications cause the chromatin structure to aggregate and gene expression to be inactivated (Fig. 18.1). Thus, changes in the epigenome play a role like a switch that controls the ON and OFF of gene expression.