DNA repair protein involved in DNA non-homologous end joining (NHEJ); required for double-strand break (DSB) repair and V(D)J recombination (PubMed:16439204, PubMed:16439205, PubMed:17717001, PubMed:17317666, PubMed:17470781, PubMed:18644470, PubMed:20558749, PubMed:26100018, PubMed:18158905). Plays a key role in NHEJ by promoting the ligation of various mismatched and non-cohesive ends (PubMed:17717001, PubMed:17470781, PubMed:19056826). Together with PAXX, collaborates with DNA polymerase lambda (POLL) to promote joining of non-cohesive DNA ends (PubMed:30250067, PubMed:25670504). May act in concert with XRCC5-XRCC6 (Ku) to stimulate XRCC4-mediated joining of blunt ends and several types of mismatched ends that are non-complementary or partially complementary (PubMed:16439204, PubMed:16439205, PubMed:17317666, PubMed:17470781). 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 (PubMed:22228831, PubMed:26100018, PubMed:28500754, PubMed:27437582, PubMed:21775435, PubMed:22287571, PubMed:21768349). 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 (PubMed:28500754, PubMed:27437582). The mobility of the bridges ensures that the ends remain accessible for further processing by other repair factors (PubMed:27437582). Binds DNA in a length-dependent manner (PubMed:17317666, PubMed:18158905). Homodimer; mainly exists as a homodimer when not associated with XRCC4 (PubMed:18046455, PubMed:25574025, PubMed:25670504, PubMed:25941166, PubMed:18158905). Interacts with XRCC4; the interaction is direct and is mediated via a head-to-head interaction between N-terminal head regions (PubMed:16439205, PubMed:20558749, PubMed:22228831, PubMed:26100018, PubMed:18158905, PubMed:21936820, PubMed:21775435, PubMed:22287571, PubMed:21768349, PubMed:27437582). Component of the core long-range non-homologous end joining (NHEJ) complex (also named DNA-PK complex) composed of PRKDC, LIG4, XRCC4, XRCC6/Ku70, XRCC5/Ku86 and NHEJ1/XLF (PubMed:16571728, PubMed:17317666, PubMed:33854234). Additional component of the NHEJ complex includes PAXX (PubMed:25574025, PubMed:25941166). Following autophosphorylation, PRKDC dissociates from DNA, leading to formation of the short-range NHEJ complex, composed of LIG4, XRCC4, XRCC6/Ku70, XRCC5/Ku86 and NHEJ1/XLF (PubMed:33854234). 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). Localizes to site of double-strand breaks; recruitment is dependent on XRCC5-XRCC6 (Ku) heterodimer. Ubiquitously expressed. The coiled-coil region mediates homodimerization. The Leu-lock (Leu-115) site inserts into a hydrophobic pocket in XRCC4. Phosphorylated by PRKDC at the C-terminus in response to DNA damage (PubMed:18644470, PubMed:22228831, PubMed:28500754). Phosphorylations by PRKDC at the C-terminus of XRCC4 and NHEJ1/XLF are highly redundant and regulate ability of the XRCC4-NHEJ1/XLF subcomplex to bridge DNA (PubMed:22228831, PubMed:28500754). Phosphorylation does not prevent interaction with XRCC4 but disrupts ability to bridge DNA and promotes detachment from DNA (PubMed:22228831, PubMed:28500754). The disease is caused by variants affecting the gene represented in this entry. A chromosomal aberration involving NHEJ1 is found in a patient with polymicrogyria. Translocation t(2;7)(q35;p22). Was named 'Cernunnos' after the enigmatic Celtic god of hunting, the underworld and fertility. Belongs to the XRCC4-XLF family. XLF subfamily. NHEJ1 mutation db