Histone modification represents an important mechanism of epigenetic regulation. The methylation of Lysine9 in histone H3 results in the specific binding of the HP1 (heterochromatin protein 1) factor and subsequent changes in chromatin structure and gene regulation. In an Advanced Online Publication from Nature, Peter Nielsen and colleagues at the University of Cambridge, UK, present the three-dimensional structure of the chromodomain from HP1 bound to Lysine9-methylated histone H3 (Nature 2002, DOI 10.1038/nature722).