The best-known CIS include Bloom syndrome (BS) which appears due to mutations in BLM helicase [5, 6] and results in increased sister chromatid exchanges [7], Ataxia Telangiectasia (AT) that shows particular clonal chromosome rearrangements as a consequence of mutations in the checkpoint kinase ATM gene[8–11], and Fanconi anemia (FA) [12] whose phenotype results from mutations in any of the genes that conform the FA/BRCA pathway [13–19] and consists of chromatidic breaks, iso-chromatidic breaks and radial exchange figures among chromosomes. The gene discussed is BLM; the disease is ataxia telangiectasia.