Reactive cellular catabolites can cause DNA damage by methylation of the O-6-guanine. O-6-methylguanine can pair ambiguously with both C and T, and can cause transition mutations. Active reversal of such damage is achieved by the action of 6-O-methylguanine DNA methyltransferase (MGMT). MGMT removes the methyl group from the guanine, causing MGMT to itself become methylated. Because the enzyme is not regenerated at the end of the reaction, this DNA repair reaction is energetically expensive approach because an entire protein molecule is sacrificed for lesion that is corrected.