DNA glycosylase that excises thymine from T/G mismatches and uracil from U/G mismatches (PubMed:8895589, PubMed:11788726). Acts as a repair enzyme able to counteract the mutagenic effect of spontaneous hydrolytic deamination of DNA 5-methylcytosine (5-meC) residues that leads to the formation of T/G mismatches (PubMed:8895589). May also repair U/G mismatches arising from hydrolytic deamination of DNA cytosine residues (PubMed:8895589). G/G, A/G, T/C and U/C are minor substrates (PubMed:8895589, PubMed:11788726). Hydrolyzes mismatched double-stranded DNA and polynucleotides, releasing free thymine. Binds 1 [4Fe-4S] cluster. The cluster has a structural role. Belongs to the Nth/MutY family.