Non-essential DNA repair protein involved in DNA non-homologous end joining (NHEJ); participates in double-strand break (DSB) repair and V(D)J recombination (PubMed:25574025, PubMed:25670504, PubMed:25941166, PubMed:27705800). May act as a scaffold required for accumulation of the Ku heterodimer, composed of XRCC5/Ku80 and XRCC6/Ku70, at double-strand break sites and promote the assembly and/or stability of the NHEJ machinery (PubMed:25574025, PubMed:25670504, PubMed:25941166). Involved in NHEJ by promoting the ligation of blunt-ended DNA ends (PubMed:27703001). Together with NHEJ1/XLF, collaborates with DNA polymerase lambda (POLL) to promote joining of non-cohesive DNA ends (PubMed:30250067, PubMed:25670504). Constitutes a non-essential component of classical NHEJ: has a complementary but distinct function with NHEJ1/XLF in DNA repair (PubMed:27705800). Able to restrict infection by herpesvirus 1 (HSV-1) via an unknown mechanism (PubMed:29144403). Homodimer (PubMed:25574025). Interacts with the DNA-bound XRCC5/Ku80 and XRCC6/Ku70 heterodimer (Ku complex); the interaction is direct (PubMed:27601299, PubMed:27705800, PubMed:25574025). Associated component of the non-homologous end joining (NHEJ) complex, composed of the core proteins PRKDC, LIG4, XRCC4, XRCC6/Ku70, XRCC5/Ku86 and NHEJ1/XLF (PubMed:25670504, PubMed:25941166). Interacts with POLL (DNA polymerase lambda); promoting POLL recruitment to double-strand breaks (DSBs) and stimulation of the end-filling activity of POLL (PubMed:30250067). Predominantly localizes to the nucleus. Accumulates at sites of DNA damage generated by laser microirradiation. (Microbial infection) Upon infection by herpesvirus 1 (HSV-1), it is partially translocated into the cytoplasm in an HSV-1-dependent manner. The N-terminus (residues 1-113) forms a head domain that is structurally related to those of XRCC4, XLF/NHEJ1, and SASS6. Phosphorylation may inhibit interaction with the DNA-bound XRCC5/Ku80 and XRCC6/Ku70 heterodimer (Ku complex). Belongs to the XRCC4-XLF family. PAXX subfamily.