DNA methylation is a chemical modification of DNA that can be inherited and subsequently removed without altering the original DNA sequence. Therefore, it is part of the epigenetic code and is the most clearly characterized epigenetic mechanism. DNA methylation results in the addition of a methyl group to DNA - for example, on carbon 5 of the cytosine pyrimidine ring - which involves a reduction in gene expression. In adult somatic tissues, DNA methylation usually occurs in the context of CpG dinucleotides. Non-cpg methylation is prevalent in embryonic stem cells.
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