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). The [ST]-Q motif constitutes a recognition sequence for kinases from the PI3/PI4-kinase family. Phosphorylated. Phosphorylation of Ser-139 (H2AX139ph) occurs in response to DNA double strand breaks (DSBs) generated by exogenous genotoxic agents, by stalled replication forks and by meiotic recombination events. Phosphorylation is dependent on the DNA damage checkpoint kinases ATR and ATM, spreads on either side of a detected DSB site 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. Dephosphorylation of Ser-139 is required for DNA DSB repair. Phosphorylation at Tyr-142 (H2AXY142ph) by baz1b/wstf determines the relative recruitment of either DNA repair or pro-apoptotic factors. Phosphorylation at Tyr-142 (H2AXY142ph) favors the recruitment of pro-apoptosis factors. In contrast, dephosphorylation of Tyr-142 by EYA proteins (eya1, eya2, eya3 or eya4) favors the recruitment of MDC1-containing DNA repair complexes to the tail of phosphorylated Ser-139 (H2AX139ph). Monoubiquitination of Lys-120 (H2AXK119ub) by ring1 and rnf2/ring2 complex gives a specific tag for epigenetic transcriptional repression. Following DNA double-strand breaks (DSBs), it is ubiquitinated through 'Lys-63' linkage of ubiquitin moieties by the E2 ligase ube2n and the E3 ligases rnf8 and rnf168, leading to the recruitment of repair proteins to sites of DNA damage. Ubiquitination at Lys-14 and Lys-16 (H2AK13Ub and H2AK15Ub, respectively) in response to DNA damage is initiated by rnf168 that mediates monoubiquitination at these 2 sites, and 'Lys-63'-linked ubiquitin are then conjugated to monoubiquitin; rnf8 is able to extend 'Lys-63'-linked ubiquitin chains in vitro. H2AK119Ub and ionizing radiation-induced 'Lys-63'-linked ubiquitination (H2AK13Ub and H2AK15Ub) are distinct events (By similarity). Belongs to the histone H2A family.