SINCE the initial observations on defective DNA repair in fibroblasts cultured from patients with xeroderma pigmentosum (XP)1,2, a large number of cases have been collected and studied in laboratories around the world. The results of these studies indicate a surprising degree of complexity in the DNA repair defect(s) in this disease. At an early stage it became apparent that not all cell strains are totally defective in the excision repair of ultraviolet radiation damage in vivo, there being varying degrees of "leakiness"3-5. Cell fusion studies have since indicated that a number of distinct genetic complementation groups exist, suggesting a multigenic basis for the disease6-9. In addition, defects in photoreactivation10 and in post-replication repair11 of ultraviolet radiation damage in some XP strains have been reported.
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