Members of the heterochromatin protein 1 (HP1) class of non-histone chromosomal proteins are components of heterochromatin and are involved in the epigenetic regulation of the genome. HP1 proteins are modular and consist of two sequence-related domains called the chromodomain (CD) and the chromo shadow domain (CSD). In order to investigate the role of the murine HP1-like protein M31 in heterochromatin formation and gene silencing, recombinant CSD was overexpressed in Escherichia coli and crystallized using the hanging-drop vapour-diffusion method with PEG 4000 as precipitant. Diffraction data to 2.9 A were collected from a native crystal belonging to space group C222(1), with unit-cell parameters a = 60.0, b = 95.6, c = 91.7 A, alpha = beta = gamma = 90 degrees.