AGT repair of guanine O6-methyl and guanine O6-chloroethyl lesions. Panel A, direct reversal of a guanine O6-methyl lesion by AGT. Panel B, progression of a guanine O6-chloroethyl lesion via the formation of a cyclic intermediate, N1,O6-ethanoguanine, then to a interstrand DNA G-C cross-link and AGT reparability of these lesions. Note that repair of the cyclic intermediate by AGT yields AGT tethered to guanine N-1 and that AGT does not produce repair of the DNA cross-link, because the ethylene linkage between the two DNA strands does not involve guanine O-6.