MBD4 (MED1; methyl-CpG-binding domain protein 4) cleaves thymine mispaired with guanine at CpG islands, which is a consequence of the oxidative deamination of 5-methylcytosine (Petronzelli et al. 2000). The MBD4 catalytic site is located at the C-terminus (Wu et al. 2003). MBD4 may be involved in the maintenance of genomic stability and active DNA demethylation.