DNA repair protein involved in DNA non-homologous end joining (NHEJ); required for double-strand break (DSB) repair and V(D)J recombination. Plays a key role in NHEJ by promoting the ligation of various mismatched and non-cohesive ends. Associates with xrcc4 to form alternating helical filaments that bridge DNA and act like a bandage, holding together the broken DNA until it is repaired. The xrcc4-nhej1/xlf subcomplex binds to the DNA fragments of a DSB in a highly diffusive manner and robustly bridges two independent DNA molecules, holding the broken DNA fragments in close proximity to one other. The mobility of the bridges ensures that the ends remain accessible for further processing by other repair factors. Homodimer. Interacts with xrcc4; the interaction is direct and is mediated via a head-to-head interaction between N-terminal head regions. Associates with the non-homologous end joining (NHEJ) complex which is at least composed of the core proteins xrcc5/Ku80, xrcc6/Ku70, prkdc/DNA-PKcs, nhej1/xlf, lig4 and xrcc4. Localizes to site of double-strand breaks. The coiled-coil region mediates homodimerization. The Leu-lock (Leu-114) site inserts into a hydrophobic pocket in xrcc4. Belongs to the XRCC4-XLF family. XLF subfamily.