Histone H2A.X also known as X variant Histone H2A, H2AX is a member of the Histone H2A family involved in the nucleosomal organization of chromatin, differing from other H2A variants by a highly conserved C-terminal motif. Histone H2A.X plays a crucial role in the cellular response to DNA damage, particularly in the recognition and repair of double-stranded breaks. Upon exposure to ionizing radiation or other genotoxic stress, H2A.X is rapidly phosphorylated at Ser139, leading to the formation of distinct nuclear foci that serve as recruitment sites for DNA repair proteins, including Rad50, Rad51, and BRCA1. This phosphorylation event is vital for initiating the DNA damage response, as γ-H2A.X marks the sites of damage and facilitates the assembly of repair complexes, thereby maintaining genomic stability.