The chromodomain of the HP1 family of proteins recognizes histone tails with 
specifically methylated lysines. Here, we present structural, energetic, and 
mutational analyses of the complex between the Drosophila HP1 chromodomain and 
the histone H3 tail with a methyllysine at residue 9, a modification associated 
with epigenetic silencing. The histone tail inserts as a beta strand, completing 
the beta-sandwich architecture of the chromodomain. The methylammonium group is 
caged by three aromatic side chains, whereas adjacent residues form discerning 
contacts with one face of the chromodomain. Comparison of dimethyl- and 
trimethyllysine-containing complexes suggests a role for cation-pi and van der 
Waals interactions, with trimethylation slightly improving the binding affinity.