gammaH2AX

GammaH2AX is the phosphorylated form of the histone variant H2AX at serine 139. It is rapidly generated in response to DNA double-strand breaks (DSBs) and serves as a chromatin platform for assembling DNA damage response factors. The DSB-sensing kinases ATM, and to a lesser extent ATR and DNA-PKcs, phosphorylate H2AX to gammaH2AX within minutes of damage. The modification spreads around the break to megabase-sized regions, creating focal domains that recruit proteins such as MDC1, RNF8, RNF168, 53BP1, BRCA1, and others to coordinate signaling, repair pathway choice, and cell-cycle checkpoints.

GammaH2AX foci can be detected by immunofluorescence microscopy as discrete nuclear spots, or measured as total protein by Western blot or related assays. Because the number and intensity of gammaH2AX foci correlate with the amount of DNA damage, gammaH2AX is widely used as a sensitive biomarker in research and clinical contexts, including radiation biology, cancer therapy, and toxicology studies, as well as in high-throughput screening for DNA-damaging agents or repair inhibitors.

The signal is dynamic: gammaH2AX is dephosphorylated by phosphatases such as PP2A, PP4, and PP6 as repair progresses, and foci dissipate once the damage is resolved. Persistent gammaH2AX can indicate unrepaired breaks, replicative stress, or senescence. While a useful indicator of DSB presence, gammaH2AX is not exclusively specific to a single damage type and can be influenced by chromatin context, replication status, and antibody specificity, requiring careful interpretation alongside other assays.