According to their radiosensitive cellular phenotype, LIG4-deficient patients belong to the group of human radiosensitivity syndromes which include ataxia telangiectasia (AT), Nijmegen breakage syndrome (NBS), Rad 50 and Mre 11 deficiency, thrombocytopenia absent radii syndrome (TAR), Artemis syndrome, and Cernunnos-XLF syndrome [1,5-8]. Here, LIG4 is linked to ataxia telangiectasia.