All cells of a given organism contain nearly identical genetic information, yet 
tissues display unique gene expression profiles. This specificity is in part due 
to transcriptional control by epigenetic mechanisms that involve 
post-translational modifications of histones. These modifications affect the 
folding of the chromatin fiber and serve as binding sites for non-histone 
chromosomal proteins. Here we discuss functions of the Heterochromatin Protein 1 
(HP1) family of proteins that recognize H3K9me, an epigenetic mark generated by 
the histone methyltransferases SU(VAR)3-9 and orthologues. Loss of HP1 proteins 
causes chromosome segregation defects and lethality in some organisms; a 
reduction in levels of HP1 family members is associated with cancer progression 
in humans. These consequences are likely due to the role of HP1 in centromere 
stability, telomere capping and the regulation of euchromatic and 
heterochromatic gene expression.