 XRCC4 dimer binds to a second dimer on an adjacent DNA strand to create a tetramer for DNA bridging early on in NHEJ.Prior to ligation, Lig IV binds to the C-terminal stalk of XRCC4 at the site of the break and displaces the second XRCC4 dimer.The BRCT2 domain of Lig IV hydrogen bonds with XRCC4 at this domain through multiple residues and introduces a kink in the two alpha helical tails.