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, sequestration of genes to the nuclear periphery, transcriptional arrest, maintenance of heterochromatin integrity, gene repression at the single nucleosome level, gene repression by heterochromatization of euchromatin, and DNA repair. 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 some euchromatic sites, where its binding can correlate with either gene repression or gene activation. 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. Paralogs and orthologs Three different paralogs of HP1 are found in Drosophila melanogaster, HP1a, HP1b and HP1c. Subsequently orthologs of HP1 were also discovered in S. pombe (Swi6), Xenopus (Xhp1α and Xhp1γ), Chicken (CHCB1, CHCB2 and CHCB3), Tetrahymena (Pdd1p) and many other metazoans. In mammals, there are three paralogs: HP1α, HP1β and HP1γ. In Arabidopsis thaliana (a plant), there is one structural homolog: Like Heterochromatin Protein 1 (LHP1), also known as Terminal Flower 2 (TFL2). HP1β in mammals HP1β interacts with the histone methyltransferase (HMTase) Suv(3-9)h1 and is a component of both pericentric and telomeric heterochromatin. HP1β is a dosage-dependent modifier of pericentric heterochromatin-induced silencing and silencing is thought to involve a dynamic association of the HP1β chromodomain with the tri-methylated histone H3 K9me3. The binding of the K9me3-modified H3 N-terminal tail by the chromodomain is a defining feature of HP1 proteins. Interacting proteins HP1 interacts with numerous other proteins/molecules (in addition to H3K9me3) with different cellular functions in different organisms. Some of these HP1 interacting partners are: histone H1, histone H3, histone H4, histone methyltransferase, DNA methyltransferase, methyl CpG binding protein MeCP2, and the origin recognition complex protein ORC2. Binding affinity and cooperativity HP1 has a versatile structure with three main components; a chromodomain, a chromoshadow domain, and a hinge domain. The chromodomain is responsible for the