MGMT, just like its E.coli homolog Ada, is an O-6-methylguanine transferase (Lindahl et al. 1983, Moore et al. 1994) that removes the methyl group from the guanine and transfers it to the cysteine residue at position 145 on the protein itself. MGMT thus methylated is not regenerated, as the S-methylcysteine is very stable. This is an energetically expensive approach to DNA repair as one entire protein molecule is sacrificed per lesion that is corrected in this manner (Rasimas et al. 2004, Tubbs et al. 2007, Mitra and Kaina 1993).