Germline alterations of MRN components give rise to hereditary cancer predisposition syndromes ataxia-telangiectasia like disease (A-TLD) and Nijmegen breakage syndrome (NBS), caused by hippomorphic mutation in the gene encoding Mre11 nuclease and NBS1 (encoding the protein NBN also known as nibrin) (null mutation of Nbs1 causes embryonic lethality). Here, NBN is linked to Nijmegen breakage syndrome.