Many DNA repair defects are associated with chromosomal instability [73] most commonly observed in FA, AT, Bloom syndrome, and Nijmegen breakage syndrome (NBS), but also in ATLD, ICF and NER syndromes [74], DNA ligase I (LIG1) deficiency [75], and DNA recombinase repair defects [76, 77]. The gene discussed is LIG1; the disease is ataxia telangiectasia.