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Xeroderma pigmentosum-variant (XP-V) patients have sun sensitivity and increased skin cancer risk. Their cells have normal nucleotide excision repair, but have defects in the POLH gene encoding an error-prone polymerase, DNA polymeraseη (polη). To survey the molecular basis of XP-V worldwide, we measured polη protein in skin fibroblasts from putative XP-V patients (aged 8–66 years) from 10 families in North America, Turkey, Israel, Germany, and Korea. Polη was undetectable in cells from patients in eight families, whereas two showed faint bands. DNA sequencing identified 10 different POLH mutations. There were two splicing, one nonsense, five frameshift (3 deletion and 2 insertion), and two missense mutations. Nine of these mutations involved the catalytic domain. Although affected siblings had similar clinical features, the relation between the clinical features and the mutations was not clear. POLH mRNA levels were normal or reduced by 50% in three cell strains with undetectable levels of polη protein, indicating that nonsense-mediated message decay was limited. We found a wide spectrum of mutations in the POLH gene among XP-V patients in different countries, suggesting that many of these mutations arose independently.Journal of Investigative Dermatology (2008) 128, 2055–2068; doi:10.1038/jid.2008.48; published online 27 March 2008 [ABSTRACT FROM AUTHOR]