DNA glycosylase that excises thymine from T/G mismatches and uracil from U/G mismatches. Can also process T/GO and U/GO, but not A/G, T/C and U/C. Has weak AP lyase activity. 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.