The human skin cancer-prone disease xeroderma pigmentosum variant (XPV) results from a mutation in RAD30, which encodes the novel lesion bypass DNA polymerase η. XPV cells are characterized by delayed completion of DNA replication and increased mutagenesis following UV irradiation. In cell-free extracts of XPV lymphoblasts, functional DNA polymerase η is required for the complete replication of a doublestranded plasmid containing either a single (6-4) photoproduct or a cyclobutane pyrimidine dimer (CPD), the major mutagenic UV-induced lesion. In cultured XPV cells, replication arrest activates downstream signalling pathways, leading to hyperphosphorylation of the 34-kDa subunit of the trimeric single-stranded DNAbinding protein, RPA (replication protein A). Many of the RAD30 mutations identified in XPV cells result in truncation and inactivation of DNA polymerase η. To examine whether polymorphisms in the RAD30 gene that result in altered polymerase η function, rather than enzyme inactivation, might contribute to individual susceptibility to skin cancer, methods to screen for sequence changes in the RAD30 gene in human genomic DNA have been developed.

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