The chemical integrity and proper functioning of DNA is threatened by numerous chemical and physical agents that cause a wide spectrum of DNA lesions. When unrepaired, DNA injury interferes with vital, cellular functions such as DNA replication and transcription and give rise to mutations leading to genetic defects, carcinogenesis and cell death. The contribution of DNA repair systems in preventing cancer is apparent from the high rate of tumorigenesis found in many repair syndromes. A classical example is the excision repair disorder xeroderma pigmentosum (XP) in which patients exhibit hypersensitivity to sun (UV) light and predisposition to skin cancer. Genetic analysis of cultured cells from XP patients has revealed the presence of at least 7 complementation groups, all showing a deficiency in the excision of UV-induced lesions in the DNA. To identify the genes and characterize the genetic defects in these complementation groups, cloning of human DNA repair genes has been attempted by a number of investigators. Recently, the first human DNA repair genes have been cloned including at least two genes involved in XP. Comparison of the coding sequences of these genes with sequences of cloned (repair) genes of lower organisms (e.g. E. coli and yeast) provides information on their function. This leads to understanding of the relationship between molecular defect at the level of the gene and the gene product and the clinical manifestation of the disease in different XP patients and complementation groups.