gammaH2AX S139

H2AX is a member of the H2A histone family that complex with DNA and other histones to form the repeating nucleosome units characteristic of eukaryotic chromatin. Nucleosomes consist of approximately 147 base pairs of DNA wrapped around an octamer of histones composed of two each of the four histone proteins: H2A, H2B, H3 and H4. After induction of DNA damage such as double-strand breaks by irradiation, genotoxic stresses, replication errors or gene recombination, PI3K-like kinases (e.g., ataxia telangiectasia mutated (ATM), ataxia telangiectasia Rad-3-related (ATR), and DNA-dependent protein kinase (DNA-PK) are activated to phosphorylate serine 139 in H2AX. This early phosphorylation event plays a critical role in recruiting proteins involved in DNA repair.