Histones are chromosome proteins that form spools around which DNA winds, enabling the compaction necessary to fit the large genomes of eukaryotes inside cell nuclei. Five histone types are known (H1, H2A, H2B, H3, and H4). Two each of type H2A, H2B, H3 and H4 assemble to form one nucleosome, together with DNA. H1 is needed for histone-DNA-complexes to form a 30-nm fiber, which packs the DNA even more tightly. In general, genes that are active have less bound histone, while inactive genes are highly associated with histones during interphase.
Histones can be chemically modified by enzymes primarily on their N-terminal tails, but also in their globular domains. Such modifications include methylation, acetylation, phosphorylation, ubiquitination, and ADP-ribosylation. These modifications can regulate gene expression in an epigenetic manner.