Which chemical DNA modification is commonly associated with gene silencing in mammals?

DNA methylation is a chemical DNA modification commonly associated with gene silencing in mammals.

In mammals, methyl groups are often added to the fifth carbon of cytosine, producing 5-methylcytosine. This occurs especially at cytosine–guanine sites called CpG sites. Methylation near gene promoters can reduce transcription by interfering with regulatory proteins or by recruiting proteins that compact chromatin.

DNA methylation patterns are copied during cell division but can also be changed during development and in response to cellular signals. They are part of epigenetic regulation because they influence gene activity without changing the underlying DNA sequence.

The relationship is not universal: methylation can have different effects depending on its location, and some gene regions are actively regulated by methylation rather than simply switched off. Abnormal methylation patterns are observed in many cancers. DNA methylation is distinct from histone modification, although both can affect chromatin and gene expression.