Mutations in genes encoding Mre11 and NBS1 result in two chromosomal instability syndromes, ataxia–telangiectasia-like disorder (ATLD; Stewart et al. 1999) and Nijmegen breakage syndrome (NBS; Varon et al. 1998), where both exhibit features that are characteristic for A–T. Here, MRE11 is linked to Nijmegen breakage syndrome.