Chromodomain: The chromodomain is the smallest member, consisting of four curved β-strands and an α-helix. The chromodomains of HP1 and Polycomb were found to recognize histone H3 trimethylated at K9 (H3K9me3) and H3K27me3, respectively, and these proteins were the first examples of readers specific for methyllysine. Chromodomains generally prefer trimethylated lysine, though some have been shown to bind dimethylated species. The aromatic cage of the chromodomain of mouse and fly HP1 contains an aspartate or glutamate residue, accounting for its ability to interact well with H3K9me3 and dimethylated H3K9 (H3K9me2). (1)

Reference
1. Musselman CA, Lalonde ME, Cote J, Kutateladze TG.Perceiving the epigenetic landscape through histone readers. Nat Struct Mol Biol.2012;19(12):1218-27. PMID: 23211769.