Figure 2. Core histone post-translational modifications.

The sites of post-translational modifications (PTMs) occur mostly within the histone tail domains (top inset). Specific amino acid PTM sites (acetylation, ADP-ribosylation, biotinylation, citrullination, methylation, phosphorylation, SUMOylation and ubiquitination) that are known to occur on the core histones, H2A, H2B, H3 and H4, are indicated by colored symbols (symbol key in the right inset). Some residues (Lysine, Arginine) can undergo severa,l identical or different, forms of post-translational modification (i.e. methylation or acetylation, dimethylation or trimethylation, etc …), thus increasing the complexity of the histone code. H2AX is an H2A variant with a unique longer C-terminal tail consisting of an evolutionarily conserved motif and a linker of variable sequence and length. The conserved motif contains the C-4 serine that is phosphorylated upon DNA DSB formation (arrow).