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:27601299, PubMed:27798842, PubMed:27601633, PubMed:27830975, PubMed:28051062, PubMed:29077092). 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:28051062). Involved in NHEJ by promoting the ligation of blunt-ended DNA ends (By similarity). Together with NHEJ1/XLF, collaborates with DNA polymerase lambda (POLL) to promote joining of non-cohesive DNA ends (By similarity). Constitutes a non-essential component of classical NHEJ: has a complementary but distinct function with NHEJ1/XLF in DNA repair (PubMed:27601299, PubMed:27798842, PubMed:27830975, PubMed:28051062). Homodimer (By similarity). Interacts with the DNA-bound XRCC5/Ku80 and XRCC6/Ku70 heterodimer (Ku complex); the interaction is direct (By similarity). 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 (By similarity). Interacts with POLL (DNA polymerase lambda); promoting POLL recruitment to double-strand breaks (DSBs) and stimulation of the end-filling activity of POLL (By similarity). Predominantly localizes to the nucleus. Accumulates at sites of DNA damage generated by laser microirradiation. The N-terminus (residues 1-115) 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). No visible phenotype in normal conditions (PubMed:27798842, PubMed:28051062, PubMed:29511619, PubMed:29511621). Mice are viable, grow normally, are fertile and have normal lymphocyte development (PubMed:27798842, PubMed:29511619, PubMed:29511621). They however show mild radiosensitivity (PubMed:29511619, PubMed:29511621). Mice lacking both Paxx and Nhej1/Xlf show embryonic lethality caused by severe defects in classical non-homologous end joining (NHEJ) (PubMed:27601299, PubMed:27798842, PubMed:27601633, PubMed:27830975, PubMed:28051062, PubMed:29077092). Belongs to the XRCC4-XLF family. PAXX subfamily.