Wild-type histones and variant histones. Sites of Lys residue methylation (CH₃) and Ser residue phosphorylation Ⓟ are indicated. HFD is the histone-fold domain, a structural motif common to all core histones. H3.3 and CENPA are H3 variants; H2AX, H2AZ, and macroH2A are H2A variants. The canonical (wild-type) histone subunits (H3, H2A) are shown above their variant versions. Significant regions of nonsimilarity between a canonical histone and its variant(s) are indicated by different colors. The top three illustrations show wild-type H3 and its two variants, H3.3 and CENPA; the next four show wild-type H2A and its variants, H2AX, H2AZ, and macroH2AZ. There are no known variants of H2B and H4; their wild-type versions are shown.