Histone proteins (green), illustration. Histones are a self-assembly protein framework, that enables DNA (deoxyribonucleic acid) to organise itself into a compact structure, called chromatin. By dynamically changing its degrees of compaction and three-dimensional features, it accommodates its regulatory effects, roles, and functions. Histone proteins assemble DNA into sub-structural units called nucleosomes. Histone proteins can be distinguished into the four core histones. H2A, H2B, H3, and H4. The interaction between the histone proteins and DNA is mediated by the amino-terminal (N-terminal) tail of the histone proteins.