What distinguishes DNA methylation in vertebrate genomes is the fact that not all CGs within a common sequence are methylated in any given cell type.95 Different CGs are methylated in different cell types, generating cell type-specific patterns of methylation. Thus, the DNA methylation pattern confers a cell-specific identity upon the genome. Such variation is presumably related to cell-specific patterns of gene expression. Since DNA methylation is part of the chemical structure of the DNA itself, it remains long after all other proteins and epigenomic markers are degraded and thus it has extremely important diagnostic potential.96,97 It was originally believed that the DNA methylation pattern is established during development and is then maintained faithfully through life by the maintenance DNA methyltransf erase.95,98 The DNA methylation reaction was believed to be irreversible and that the only way methyl residues were lost was through replication in the absence of DNA methyltransferase, resulting in the loss of the cytosine methylation in the daughter cell,93,94 a mechanism that is not applicable to postmitotic cells such as neurons. However, recent findings together with the data reviewed
