A sequence variant of histone H2A called H2AX is one of the key components of 
chromatin involved in DNA damage response induced by different genotoxic 
stresses. Phosphorylated H2AX (γH2AX) is rapidly concentrated in chromatin 
domains around DNA double-strand breaks (DSBs) after the action of ionizing 
radiation or chemical agents and at stalled replication forks during replication 
stress. γH2AX foci could be easily detected in cell nuclei using 
immunofluorescence microscopy that allows to use γH2AX as a quantitative marker 
of DSBs in various applications. H2AX is phosphorylated in situ by ATM, ATR, and 
DNA-PK kinases that have distinct roles in different pathways of DSB repair. The 
γH2AX serves as a docking site for the accumulation of DNA repair proteins, and 
after rejoining of DSBs, it is released from chromatin. The molecular mechanism 
of γH2AX dephosphorylation is not clear. It is complicated and requires the 
activity of different proteins including phosphatases and chromatin-remodeling 
complexes. In this review, we summarize recently published data concerning the 
mechanisms and kinetics of γH2AX loss in normal cells and tissues as well as in 
those deficient in ATM, DNA-PK, and DSB repair proteins activity. The results of 
the latest scientific research of the low-dose irradiation phenomenon are 
presented including the bystander effect and the adaptive response estimated by 
γH2AX detection in cells and tissues.