Chromosomal instability can occur when the DNA damage response and repair process fails, resulting in syndromes characterized by growth abnormalities, hematopoietic defects, mutagen sensitivity, and cancer predisposition.

Mutations in ATM, NBS1, MRE11, BLM, WRN, and FANCD2 are responsible for

 ataxia telangiectasia (AT)
 Nijmegen breakage syndrome
 AT-like disorder
 Bloom
 Werner syndrome
 Fanconi anemia group D2

References

 Futaki M, Liu JM. Chromosomal breakage syndromes and the BRCA1 genome surveillance complex. Trends Mol Med. 2001 Dec;7(12):560-5. Trends Mol Med 2002 Jan;8(1):49. PMID: #11733219#