 Two XRCC4 proteins are post-translationally modified for recognition and localization to Ku70/80 (5).The two XRCC4 proteins dimerize together and bind to Ku70/80 at the ends of the DNA strands to promote ligation.XRCC4 then forms a strong complex with DNA ligase IV, LigIV, which is enhanced by Cernunnos XRCC4-like factor, Cer-XLF.