DNA methylation is a chemical modification in which a methyl group is added to DNA, most often to cytosine bases in CpG dinucleotides. It is a key epigenetic mechanism that regulates gene activity without changing the underlying genetic code. In many organisms, high methylation in gene promoters is associated with gene silencing, while low methylation tends to permit gene expression.

Research shows that DNA methylation patterns are crucial during development. They help control processes such as cell differentiation, X chromosome inactivation and genomic imprinting, where only one parental copy of a gene is expressed. Methylation patterns are established by DNA methyltransferase enzymes and can be maintained through cell division, providing a form of molecular memory.

Studies have revealed characteristic methylation changes in cancer. Tumor cells often exhibit global hypomethylation, which can promote genomic instability, alongside focal hypermethylation at tumor suppressor gene promoters, which switches these genes off. These patterns can be used as biomarkers for diagnosis, prognosis and treatment monitoring.

Environmental factors including diet, toxins, stress and aging influence DNA methylation. This has led to intense interest in the potential reversibility of methylation changes and their role in linking environment to disease risk. Drugs that inhibit DNA methyltransferases are already used to treat some blood cancers, and researchers are exploring more targeted approaches to edit methylation at specific genomic sites. Overall, DNA methylation research is illuminating how cells interpret the genome and how epigenetic misregulation contributes to human disease.