Mismatch-specific DNA N-glycosylase involved in DNA repair. Has thymine glycosylase activity and is specific for G:T mismatches within methylated and unmethylated CpG sites. Can also remove uracil or 5-fluorouracil in G:U mismatches. Has no lyase activity. Was first identified as methyl-CpG-binding protein. {ECO:0000269|PubMed:10097147, ECO:0000269|PubMed:10930409}. This is the function of Ensembl gene identifier ENSG00000129071 (MBD4, methyl-CpG binding domain 4, DNA glycosylase).