Radiation- and carcinogen-induced DNA damage and repair in human cancer-prone genetic diseases is being studied. Attention is presently focused on xeroderma pigmentosum (XP), a disease with ultraviolet sensitivity, ataxia-telangiectasia (AT), a disease which recently has been found to exhibit X-ray sensitivity, and familial malignant melanoma, a disease presently associated with no known defect. Detailed comparisons of the clinical features of these diseases are being made. XP and AT were found to have remarkable similarities, particularly in the neurological abnormalities. Cell fusion studies to demonstrate possible genetic heterogeneity of DNA repair defects among AT strains are in progress. Effects on cell survival and on chromosome abnormalities after treatment with DNA damaging agents are being studied. Correlation of clinical features of XP and AT with in vitro DNA repair defects may lead to further understanding of the link between DNA repair and cancer. Cell survival studies to detect carriers of AT (a population which may also be at an increased risk for environmental carcinogenesis) are in progress.