Only 16 XPG-defective patients with 20 different mutations have been described. The current hypothesis is that missense mutations impair repair (XP symptoms) whereas truncating mutations impair repair and transcription (XP and CS symptoms). We identified three cell lines of XPG-defective patients (XP40GO, XP72MA, XP165MA). Patients' fibroblasts showed a reduced post-UVC cell survival. The reduced repair capability, assessed by host cell reactivation, could be complemented by XPG cDNA. XPG mRNA expression of XP165MA, XP72MA, and XP40GO was 83, 97, and 82.5% compared to normal fibroblasts. XP165MA was homozygous for a p.G805R mutation, XP72MA and XP40GO were both compound heterozygous (p.W814S and p.E727X, p.L778P and p.Q150X, respectively). Allele-specific complementation analysis of these 5 mutations revealed that p.L778P and p.W814S retained considerable residual repair activity. In line with the severe XP/CS phenotypes of XP72MA and XP165MA, even the missense mutations failed to interact with the transcription factor IIH subunits XPD and to some extend cdk7 in co-immunoprecipitation assays. Immunofluorescence techniques revealed that the mutations destabilized early recruitment of XP proteins to localized photodamage and delayed their redistribution in vivo. Thus, we identified 3 XPG missense mutations in the I-region of XPG that impaired repair and transcription and resulted in severe XP/CS.Journal of Investigative Dermatology accepted article preview online, 31 January 2013;. doi:10.1038/jid.2013.54.