The common feature of the recognition is that the methylated lysine residue is coordinated via a conserved aromatic cage around the moiety. Chromodomain was first identified as methyllysine binding motif in Drosophila melanogaster heterochromatin protein-1 and Polycomb as regulators of chromatin structure that are involved in epigenetic repression. The structures of the HP1 chromodomain in complex with a methyl-Lys 9 histone H3 peptide and the Polycomb chromodomain in complex with a methyl-Lys 27 histone H3 peptide reveal the molecular mechanism of chromodomain binding to methylated histone H3. Many other chromodomain-containing proteins, such as CHD1, Eaf3, MSL3, MPP8 and so on, were also reported to recognize methylated histone tails. Most chromodomain-containing proteins participate in the formation of large multiprotein complexes to facilitate their recruitment to target loci, resulting in chromatin remodeling and transcription repression. The M-phase phosphoprotein 8, which was firstly identified to coimmunoprecipitate with the RanBPM-comprised large protein complex, was shown to associate with methylated H3K9 both in vivo and in vitro. The binding of MPP8 to methylated H3K9 recruited the H3K9 methyltransferases GLP and ESET, as well as DNA methyltransferase 3A to the promoter of the E-cadherin gene, a key regulator of tumor cell growth and epithelial-to-mesenchymal transition. The recruitment of those enzymes and enzyme complexes, which regulated the H3K9 and DNA methylation at the promoter of Ecadherin gene, respectively, repressed the tumor suppressor gene expression and, in turn, played an important role in epithelial-tomesenchymal transition and metastasis. Here, we reported the crystal structures of human MPP8 chromodomain both in free form and in complex with the trimethylated histone H3 lysine 9 peptide. Consistent with the high sequence homology of MPP8 with Polycomb and HP1 chromodomains, the complex structure of hMPP8-H3K9me3 uncovers the detailed molecular mechanism of recruitment of MPP8 chromodomain by HK9me3 as well as its unexpected homodimerization. In this way, our study sheds lights on the roles of MPP8 in regulating gene expression. To unveil the molecular architecture of the chromodomain of hMPP8, hMPP8 chromodomain was recombinantly expressed and crystallized. The crystals of the free-hMPP8 and hMPP8-H3K9me3 complex both diffracted to 2.05 A˚ resolution and the structures were solved using molecular replacement. The quality of the X-ray diffraction data and the structure BMN673 refinement parameters are shown in Table 1. In the free form, the hMPP8 chromodomain consists of a twisted anti-parallel b-sheet formed by three b-strands, and a helix located at the C-terminal end packing against one edge of the b-sheet next to b2. In the asymmetric unit of the crystal, two hMPP8 chromodomain monomers form a dimer through the interaction between the b2 strand from each monomer. The b2 strand from one subunit runs anti-parallel to the b2′ strand from the neighboring one.