Transcriptional regulator that binds CpG islands in promoters where the DNA is methylated at position 5 of cytosine within CpG dinucleotides. In addition, binds specifically methylated m(5)CpNpN but not m(5)CpNpG (N is A, T or C). Plays probably a role in gene silencing. Homodimer and heterodimer with MBD6. Interacts with DDM1 via its MBD domain. Associated with heterochromatin, and particularly at perinucleolar chromocenters. Mostly expressed in flowers, and, to a lower extent, in seedlings, buds, stems and mature seeds, but barely in roots, exclusively in root meristem cells at tips (at protein level). The methyl-CpG-binding domain (MBD) functions both in binding to methylated DNA and in protein interactions.