DNA methylation is a process by which methyl groups are added to DNA. Methylation modifies the function of the DNA, typically acting to suppress gene transcription. DNA methylation is essential for normal development and is associated with a number of key processes including genomic imprinting, X-chromosome inactivation, suppression of repetitive elements, and carcinogenesis.Two of DNA's four nucleotides, cytosine and adenine, can be methylated.