H2A.X is a member of the histone H2A family, 1 of 5 families of histone proteins involved in nucleosomal organization of chromatin. H2A.X is synthesised in G1 as well as S phase and is known to be important for recombination between immunoglobulin switch regions. A very early step in the response of mammalian cells to DNA double-strand breaks is the phosphorylation of histone H2A.X at serine 139 at the sites of DNA damage. There is a predicted acetylation at residue 1 and ubiquitination at reside 119. Phosphorylated H2A.X promotes DNA repair and maintains genomic stability.