dna methylation is the process in which a methyl group is added to either a cytosine or adenine. this process causes the activation or inactivation of gene expression, thereby resulting in gene regulation in eukaryotic cells. dna methylation process is also known to be involved in certain types of cancer formation. in order for this chemical modification to occur, it is necessary that the target base flips out of the dna double helix to allow the methyltransferases to catalyze the reaction.