O6-Methylguanine-DNA transmethylase converts O6-methylguanine thymine base pairs to guanine thymine base pairs in DNA.

O6-Methylguanine lesions in natural and synthetic DNAs were studied as substrates for the O6-methylguanine-DNA methyltransferase in vitro. The results indicate that O6-methylguanine is repaired by this protein when base paired to T as well as to C in double-stranded DNA. The results also indicate that O6-methylguanine is less subject to repair than previously reported when it is in single-stranded DNA and suggest that O6-methylguanine may not be repaired when it is at the 3' terminus.