The goal of this proposal is to characterize the genetic and biochemical properties of revertants of Xeroderma pigmentosum, and inherited syndrome that is associated with extreme sunlight sensitivity and a high rate of malignant transformation of sun exposed skin. The principal tool for these studies will be a well characterized immortalized cell line derived from a patient with Xeroderma pigmentosum complementation group A and a series of UV-resistant revertants that we have derived from this cell line. The UV-resistant revertants provide, for the first time, an isogenic set of human cells that differ only in the ability of the cells to repair DNA damage. The unique properties of the revertants will be exploited for these studies.