Polycomb group (PcG) protein that binds to the Polycomb response elements (PREs) found in the regulatory regions of many genes. PcG proteins act by forming multiprotein complexes, which are required to maintain the transcriptionally repressive state of homeotic genes throughout development. PcG proteins are not required to initiate repression, but to maintain it during later stages of development. They probably act via the methylation of histones, rendering chromatin heritably changed in its expressibility. Necessary but not sufficient to recruit a functional PcG repressive complex that represses target genes, suggesting that the recruitment of the distinct PRC1 complex is also required to allow a subsequent repression (By similarity). Interacts with pho as a component of the pho-repressive complex (PhoRC). MBT repeats have unique discriminatory binding activity for methylated Lys residues in H3 and H4; the MBT repeats bind mono- and dimethylated H3K9Me1, H3K9Me2, H4K20Me1 and H4K20Me2 but fail to interact with these residues if they are unmodified or trimethylated.