Xeroderma pigmentosum is caused due to a defect in:

● Defective DNA - repair syndromes are a set of syndrome due to a defect in DNA repair genes and resultant DNA instability. ● These are autosomal recessive in inheritance and include: Xeroderma pigmentosum Bloom syndrome Ataxia telangiectasia. ● Whereas HNPCC is associated with inactivation of DNA mismatch repair gene, resulting in increased susceptibility to colon cancer, and cancers of Endometrium, ovary etc., ● Example for Base pair defect is Sickle cell anemia; an example for Translocation defect is Philadelphia chromosome in CML - t (9:22).