Gene methylation is a modification of DNA that is associated with the changes in the patterns of gene expression. The process is related to the attachment of the methyl group to cytosine resulting in the formation of 5-methylcytosine. This methylation regulates gene expression as this modification may reduce or increases the rate transcription. As a result, if two persons have different levels of DNA methylation, the level of specific gene expression may vary. As a result, these people will have different phenotypes due to the variations in the expression of a gene that regulates the phenotype features.