Histone H2A.X is a variant histone that represents approximately 10% of the total H2A histone proteins in normal human fibroblasts. H2A.X is required for checkpoint-mediated cell cycle arrest and DNA repair following double-stranded DNA breaks. Within minutes following DNA damage, H2A.X is phosphorylated at Ser139 at sites of DNA damage. H2A.X is phosphorylated at Ser139 by DNA-PK in response to cell death receptor activation, c-Jun N-terminal Kinase (JNK1) in response to UV-A irradiation, and p38 MAPK in response to serum starvation. H2A.X is constitutively phosphorylated on Tyr142 in undamaged cells by WSTF (Williams-Beuren syndrome transcription factor).
