Xeroderma pigmentosum (XP) is an autosomal recessive human disease, clinically characterized by high incidence of skin cancers on sun-exposed areas. XP cells are hypersensitive to killing by ultraviolet light (UV), because they have a defect in DNA excision repair of UV-induced DNA damages. Genetic complementation analysis by cell fusion has identified 9 genetic complementation groups, designated groups A through H and a variant. However, the genetic basis of the physiological defect of XP has not yet been characterized. Recently, XP genes and human DNA repair genes have been molecularly cloned by DNA transfection methods. Molecular biological analysis of these genes should be a clue to elucidating the molecular mechanism of DNA repair in human. Moreover, an in vivo microinjection system and an in vitro system for study of DNA repair synthesis promoted by human cell extract have been developed and they can be utilized as assays during the purification of protein factors that complement repair defective XP cells. A nuclear factor that binds to DNA lesion has been identified and it was defective in group E XP cells. Yeast homolog of this nuclear factor appears to be a photolyase.