Biallelic defects in NBN gene result in Nijmegen Breakage Syndrome (NBS; OMIM:251,260), while homozygous defects in MSH6, PMS2 or MLH1 genes are molecular cause of Constitutional Mismatch Repair Deficiency Syndrome (CMRDS; OMIM:276,300), hereditary disorder associated with increased risk of cancers including medulloblastoma [25]. This evidence concerns the gene MLH1 and Nijmegen breakage syndrome.