DNA ligase IV along with Xrcc4 functions in DNA non-homologous end joining. This process is required to mend double-strand breaks. Upon ligase binding to an Xrcc4 dimer, the helical tails unwind leading to a flat interaction surface [1].

1: Crystal structure of an Xrcc4-DNA ligase IV complex. Sibanda BL, Critchlow SE, Begun J, Pei XY, Jackson SP, Blundell TL, Pellegrini L; Nat Struct Biol. 2001;8:1015-1019. PMID:11702069