The term might also include a set of linker histone (H1) variants, which lack a distinct canonical isoform.
Complex multicellular organisms typically have a large number of histone variants providing a variety of different functions.
[1] According to this nomenclature, letter suffixes or prefixes are mainly used to denote structurally distinct monophyletic clades of a histone family (e.g. H2A.Z, H2B.W, subH2B).
Number suffixes are assumed to be species-specific (e.g. H1.1), but are encouraged to be used consistently between species where unique orthologies are clear.
[3][4] Histone H3.3 has been found to play an important role in maintaining genome integrity during the mammalian development.
[1] In Drosophila there are H4 replacement genes that are constitutively expressed throughout the cell cycle that encode proteins that are identical in sequence to the major H4.
[2][7][8] H2A.X is the most common H2A variant, with the defining sequence motif ‘SQ(E/D)Φ’ (where Φ-represents a hydrophobic residue, usually Tyr in mammals).
It becomes phosphorylated during the DNA damage response, chromatin remodeling, and X-chromosome inactivation in somatic cells.
H2A.X and canonical H2A have diverged several times in phylogenetic history, but each H2A.X version is characterized by similar structure and function, suggesting it may represent the ancestral state.
Recently discovered variant H2B.E is involved in the regulation of olfactory neuron function in mice.