The DNA repair protein O6-alkylguanine-DNA alkyltransferase (ATase; also known as AGT, AGAT and MGMT) reverses O6-alkylation DNA damage by transferring O6-alkyl adducts to an active site cysteine irreversibly, without inducing DNA strand breaks. ATases are specific for repair of guanines with O6-alkyl adducts, however human ATase is not limited to O6-methylguanine, repairing many other adducts at the O6-position of guanine as well. ATase is widely distributed among species. Most ATases have N- and C-terminal domains. The C-terminal domain contains the conserved active-site cysteine motif (PCHR), the O6-alkylguanine binding channel, and the helix-turn-helix (HTH) DNA-binding motif. The active site is located near the recognition helix of the HTH motif. While the C-terminal domain of ATase contains residues that are necessary for DNA binding and alkyl transfer, the function of the N-terminal domain is still unknown. Removal of the N-terminal domain abolishes the activity of the C-terminal domain, suggesting an important structural role for the N-terminal domain in orienting the C-terminal domain for proper catalysis. Some ATase C-terminal domain homologs are either single-domain proteins that lack an N-terminal domain, or have a tryptophan substituted in place of the acceptor cysteine (i.e. the motif PCHR is replaced by PWHR). ATase null mutant mice are viable, fertile, and have a normal lifespan.