H2AX and Telangiectasia: However, both bear the C-terminal stretch characteristic of metazoan H2A.X containing the serine residue S128/S127, which is phosphorylated by ataxia telangiectasia mutated/ataxia telangiectasia and Rad3-related (ATM/ATR) kinases and is required as a DNA damage checkpoint and for DNA damage repair8.