Cells from ATLD or NBS patients, who carry “hypomorphic” mutations in the Mre11 or NBN gene, have been used as an alternative system to analyze the functional properties of the MRN complex in DNA damage signaling and DNA repair, as these cells have either no, or markedly reduced levels of the wild type proteins [33], [34]. This evidence concerns the gene NBN and ataxia-telangiectasia-like disorder.