Heterochromatin Protein 1 (HP1) was first 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 but not prokaryotes, and there are multiple members 
within the same species. The amino-terminal chromodomain binds methylated lysine 
9 of histone H3, causing transcriptional repression. 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. In addition to heterochromatin packaging, 
it is becoming increasingly clear that HP1 proteins have diverse roles in the 
nucleus, including the regulation of euchromatic genes. HP1 proteins are 
amenable to posttranslational modifications that probably regulate these 
distinct functions, thereby creating a subcode within the context of the 
'histone code' of histone posttranslational modifications.