Chromobox (CBX) genes encode members of the Heterochromatin Protein (HP) family. HP1 was discovered in Drosophila as a dominant suppressor of position-effect variegation and a major component of heterochromatin. The HP1 family is evolutionarily conserved, with members in fungi, plants and animals. Most animal species have several HP1 isoforms; humans have HP alpha, beta and gamme encoded by the genes CBX5, CBX1 and CBX3 respectively. The HP1 amino-terminal chromodomain binds methylated lysine-9 of histone H3, causing transcriptional repression (Lachner et al. 2001). A crystal structure of human HP1 alpha in complex with H3K9(me)3 peptide is available (Amaya et al. 2008). The highly-conserved carboxy-terminal chromoshadow domain enables dimerization and also serves as a docking site for proteins involved in a wide variety of nuclear functions, from transcription to nuclear architecture.