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Xeroderma pigmentosum (XP) is an autosomal recessive photosensitive disorder with an extremely high incidence of UV-related skin cancers associated with impaired ability to repair UV-induced DNA damage. There are seven nucleotide excision repair (NER) complementation groups (A through G) and an NER proficient form (XP variant). XPA, B, D and G patients may also develop XP neurological disease. The laboratory diagnosis of XP can be performed by autoradiography. Recently, the isolation and characterization of the genes responsible for XP have made it possible to use molecular biological techniques to diagnose XP patients, for carrier detection and for prenatal diagnosis, especially in Japanese XPA patients. These techniques include polymerase chain reaction (PCR) and plasmid host cell reactivation assays with cloned XP genes. DNA damage is not repaired by the NER system equally throughout the genome. There are two DNA repair pathways: 1) transcription-coupled repair, and 2) global genome repair. Many factors involved in these pathways are related to the pathogenesis of XP and a related photosensitive disease, Cockayne syndrome. Clinical management consists of early diagnosis followed by a rigorous program of sun protection including avoidance of unnecessary UV exposure, wearing UV blocking clothing, and use of sunblocks on the skin. Although there is no cure for XP, the efficacy of oral retinoids for the prevention of new skin cancers, local injection of interferon, and the external use of a prokaryotic DNA repair enzyme have been reported.