Variant histone H2A which replaces conventional H2A in a subset of nucleosomes. Nucleosomes wrap and compact DNA into chromatin, limiting DNA accessibility to the cellular machineries which require DNA as a template. Histones thereby play a central role in transcription regulation, DNA repair, DNA replication and chromosomal stability. DNA accessibility is regulated via a complex set of post-translational modifications of histones, also called histone code, and nucleosome remodeling. Required for checkpoint-mediated arrest of cell cycle progression in response to low doses of ionizing radiation and for efficient repair of DNA double strand breaks (DSBs) specifically when modified by C-terminal phosphorylation (By similarity). The nucleosome is a histone octamer containing two molecules each of H2A, H2B, H3 and H4 assembled in one H3-H4 heterotetramer and two H2A-H2B heterodimers. The octamer wraps approximately 147 bp of DNA. Interacts with numerous proteins required for DNA damage signaling and repair when phosphorylated on Ser-139 (By similarity). Expressed mainly in roots. The [ST]-Q motif constitutes a recognition sequence for kinases from the PI3/PI4-kinase family. Phosphorylated to form H2AXS139ph (gamma-H2AX) in response to DNA double strand breaks (DSBs) generated by exogenous genotoxic agents and by stalled replication forks, and may also occur during meiotic recombination events. Phosphorylation can extend up to several thousand nucleosomes from the actual site of the DSB and may mark the surrounding chromatin for recruitment of proteins required for DNA damage signaling and repair. Widespread phosphorylation may also serve to amplify the damage signal or aid repair of persistent lesions. H2AXS139ph in response to ionizing radiation is mediated by ATM while defects in DNA replication induce H2AXS139ph subsequent to activation of ATR. Dephosphorylation of H2AXS139ph by PP2A is required for DNA DSB repair (By similarity). Belongs to the histone H2A family. To ensure consistency between histone entries, we follow the 'Brno' nomenclature for histone modifications, with positions referring to those used in the literature for the 'closest' model organism. Due to slight variations in histone sequences between organisms and to the presence of initiator methionine in UniProtKB/Swiss-Prot sequences, the actual positions of modified amino acids in the sequence generally differ. In this entry the following conventions are used: H2AXS139ph = phosphorylated Ser-139.