The syndromes include Nijmegen breakage syndrome associated with mutations in NBS1, the homologue of yeast XRS2 [11-13]; Bloom's syndrome and Werner syndrome associated with mutations in BLM and WRN , respectively, both related to yeast SGS1 [14,15]; and ataxia telangiectasia associated with mutations in ATM [16], which is related to yeast TEL1 [17]. The gene discussed is ATM; the disease is ataxia telangiectasia.