Component of the MRE11-RAD50-NBN (MRN complex) which plays a critical role in the cellular response to DNA damage and the maintenance of chromosome integrity. The complex is involved in double-strand break (DSB) repair, DNA recombination, maintenance of telomere integrity, cell cycle checkpoint control and meiosis. The complex possesses single-strand endonuclease activity and double-strand-specific 3'-5' exonuclease activity, which are provided by MRE11. RAD50 may be required to bind DNA ends and hold them in close proximity. NBN modulate the DNA damage signal sensing by recruiting PI3/PI4-kinase family members ATM, ATR, and probably DNA-PKcs to the DNA damage sites and activating their functions. It can also recruit MRE11 and RAD50 to the proximity of DSBs by an interaction with the histone H2AX. NBN also functions in telomere length maintenance by generating the 3' overhang which serves as a primer for telomerase dependent telomere elongation. NBN is a major player in the control of intra-S-phase checkpoint and there is some evidence that NBN is involved in G1 and G2 checkpoints. The roles of NBS1/MRN encompass DNA damage sensor, signal transducer, and effector, which enable cells to maintain DNA integrity and genomic stability. Forms a complex with RBBP8 to link DNA double-strand break sensing to resection. Enhances AKT1 phosphorylation possibly by association with the mTORC2 complex (By similarity). Component of the MRN complex composed of two heterodimers RAD50/MRE11 associated with a single NBN (By similarity). As part of the MRN complex, interacts with MCM9; the interaction recruits the complex to DNA repair sites (By similarity). Component of the BASC complex, at least composed of BRCA1, MSH2, MSH6, MLH1, ATM, BLM, RAD50, MRE11 and NBN (By similarity). Interacts with histone H2AX this requires phosphorylation of H2AX on 'Ser-139' (By similarity). Interacts with HJURP, INTS3, KPNA2 and TERF2 (By similarity). Interacts with RBBP8; the interaction links the role of the MRN complex in DNA double-strand break sensing to resection (By similarity). Interacts with SP100; recruits NBN to PML bodies (PubMed:12470659). Interacts with ATF2 (By similarity). Interacts with MTOR, MAPKAP1 isoform 2 and RICTOR; indicative for an association with the mTORC2 complex (By similarity). Interacts with MRNIP (By similarity). Interacts with UFL1; promoting UFL1 recruitment to double-strand breaks following DNA damage (By similarity). Interacts with CYREN (via XLF motif) (PubMed:30017584). Localizes to discrete nuclear foci after treatment with genotoxic agents. Acetylation of 'Lys-5' of histone H2AX (H2AXK5ac) promotes NBN/NBS1 assembly at the sites of DNA damage. High expression in the liver, heart and testis. Low expression in all other tissues analyzed. In the cerebellum the postmitotic Purkinje cells are marked specifically. A low level of expression is observed in all tissues. Highly specific expression was observed in organs with physiologic DNA double strand breakage (DSB), such as testis, thymus and spleen. Enhanced expression is also found at sites of high proliferative activity. These are the subventricular layer of the telencephalon and the diencephalon, the liver, lung, kidney and gut, as well as striated and smooth muscle cells in various organs. The FHA and BRCT domains are likely to have a crucial role for both binding to histone H2AX and for relocalization of MRE11/RAD50 complex to the vicinity of DNA damage. The C-terminal domain contains a MRE11-binding site, and this interaction is required for the nuclear localization of the MRN complex. The EEXXXDDL motif at the C-terminus is required for the interaction with ATM and its recruitment to sites of DNA damage and promote the phosphorylation of ATM substrates, leading to the events of DNA damage response. Phosphorylated by ATM in response of ionizing radiation, and such phosphorylation is responsible intra-S phase checkpoint control and telomere maintenance.