DNA repair protein involved in DNA nonhomologous end joining (NHEJ) required for double-strand break (DSB) repair and V(D)J recombination. May serve as a bridge between XRCC4 and the other NHEJ factors located at DNA ends, or may participate in reconfiguration of the end bound NHEJ factors to allow XRCC4 access to the DNA termini. It may act in concert with XRCC6/XRCC5 (Ku) to stimulate XRCC4-mediated joining of blunt ends and several types of mismatched ends that are noncomplementary or partially complementary. Interacts with XRCC4 and the XRCC4-LIG4 complex. Binds DNA in a length-dependent manner. Ubiquitously expressed. Belongs to the XLF family. 2 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.