Histones are a family of nuclear proteins that compacts DNA in eukaryotic cells, and are, therefore, therefore structural components of chromosomes. Except for histone H1, histones are molecules with low molecular weight, comprised of 102-135 amino acids. Five subtypes of histones are known: H1, H2A, H2B, H3 and H4. The latter four histones (H2A, H2B, H3, H4) do form octamereric complexes containing two molecules of each histone, thus forming granule-like units known as nucleosomes, around which DNA segments of approximately 146 base pairs winds almost twice. In highly condensed chromosomes, nucleosomes all line up and are tightly held together by histone H1, the largest histone of the family. These repeated units of complexed histones (nucleosomes) and wrapped-around DNA segments constitute the chromatin fiber that can be compared to a pearl necklace.

The family of genes encoding histones are highly...