NBS1 (Nijmegen breakage syndrome 1, p95 protein of the MRE11/RAD50 complex) is a 95 kD nuclear protein that contains a FHA domain and BRCT domain. NBS1 is involved in DNA double-strand break repair, and DNA damage-induced checkpoint activation. NBS1 mutations are associated with Nijmegen breakage syndrome, characterized by microcephaly, growth retardation, immunodeficiency, and cancer predisposition. This protein is activated by irradiation, upregulated by c-Myc, and modified by phosphorylation. NBS1 is a component of the double-strand break repair complex consisting of DNL IV/XRCC4, Ku, MRE11, and RAD50. NBS1 has also been shown to associate with FANCD2. The Poly6138 antibody recognizes the C-terminal region of human and mouse NBS1 and has been shown to be useful for Western blotting.