The family of
Heterochromatin Protein 1 (HP1) ("Chromobox Homolog", CBX) consists of highly conserved
proteins, which have important functions in the
cell nucleus. These functions include
gene repression by
heterochromatin formation,
transcriptional activation, regulation of binding of cohesion complexes to centromeres, sequesteration of genes to nuclear periphery, transcriptional arrest, maintenance of heterochromatin integrity, gene repression at the single nucleosome level and gene repression by heterochromatization of
euchromatin. HP1
proteins are fundamental units of
heterochromatin packaging that are enriched at the
centromeres and
telomeres of nearly all
Eukaryotic chromosomes with the notable exception of
budding yeast, in which a yeast-specific silencing complex of SIR (silent information regulatory) proteins serve a similar function. Members of the HP1 family are characterized by an N-terminal
chromodomain and a C-terminal
chromoshadow domain, separated by a Hinge region. HP1 is also found at euchromatic sites, where its binding correlates with
gene repression. HP1 was originally discovered by Tharappel C James and
Sarah Elgin in 1986 as a factor in the phenomenon known as
position effect variegation in
Drosophila melanogaster.