Defects of DNA repair underlie genetic syndromes. Chromosomal aberrations and mutations might cause specific inborn defects. There are several syndromes with characteristic clinical features, which appear to be caused by chromosome instability which is a consequence of DNA repair defects. This article describe syndromes where hereditary mutations are the reason of chromosomal instability and cause serious clinical results: ataxia-telangiectasia, Nijmegen breakage syndrome, Bloom syndrome, Fanconi's anemia, ICF syndrome, Roberts syndrome, dominantly inherited--PCD, Werner syndrome, xeroderma pigmentosum, Cockayne syndrome, trichothiodystrophy (TTD) and Rothmund-Thomson syndrome (RTS).