||H2AX is one of several genes coding for histone H2A. In humans and other eukaryotes, the DNA is wrapped around histone-groups, consisting of core histones H2A, H2B, H3 and H4. Thus, the H2AX contributes to the histone-formation and therefore the structure of DNA. H2AX becomes phosphorylated on serine 139, then called gamma-H2AX, as a reaction on DNA Double-strand breaks (DSB). The kinases of the PI3-family (Ataxia telangiectasia mutated, ATR and DNA-PKcs) are responsible for this phosphorylation, especially ATM. The modification can happen accidentally during replication fork collapse or in the response to ionizing radiation but also during controlled physiological processes such as V(D)J recombination. Gamma-H2AX is a sensitive target for looking at DSBs in cells. The role of the phosphorylated form of the histone in DNA repair is under discussion but it is known that because of the modification the DNA becomes less condensed. Delivering space for the recruitment of proteins necessary during repair of DSBs.