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Histone proteins are a family of highly conserved proteins found in eukaryotic cells that package and order DNA into structural units called nucleosomes. They are the main protein components of chromatin, the complex of DNA and protein that makes up chromosomes. There are five main types of histones: H1, H2A, H2B, H3, and H4. These histones are responsible for the first level of DNA compaction, wrapping DNA around themselves to form nucleosomes, which resemble beads on a string. This structural organization is crucial for fitting the vast amount of DNA within the nucleus and also plays a significant role in gene regulation.

The core histones (H2A, H2B, H3, and H4) assemble into an octamer, forming the central part of the nucleosome. Approximately 147 base pairs of DNA wrap around this histone octamer. Histone H1, often referred to as the linker histone, binds to the DNA where it enters and exits the nucleosome, further compacting the DNA and stabilizing the chromatin structure.

Histones are subject to various post-translational modifications, such as acetylation, methylation, phosphorylation, and ubiquitination. These modifications occur primarily on the N-terminal tails of the histones and can alter the charge and structure of chromatin, thereby influencing the accessibility of DNA to transcription factors and other proteins. This dynamic regulation of chromatin structure, often referred to as the "histone code," plays a critical role in controlling gene expression, DNA replication, and DNA repair. Changes in histone modifications are associated with various cellular processes and diseases, including cancer.