Xeroderma pigmentosum (XP) is a rare hereditary human disorder clinically associated with severe sun sensitivity and predisposition to skin cancer. Some XP patients also show clinical characteristics of Cockayne syndrome (CS), a disorder associated with defective preferential repair of DNA lesions in tran-scriptionally active genes. Cells from the two XP-patients who belong to complementation group D and exhibit additional clinical symptoms of CS are strikingly more sensitive to the cytotoxic effects of UV-light than cells from classical XP-D patients. To explain the severe UV-sensitivity it was suggested that XP-D-CS cells have a defect in preferential repair of UV-induced 6'4 photoproducts (6'4PP) in active genes. We investigated the capacity of XP-D and XP-D-CS cells to repair UV-induced DNA lesions in the active adenosine deaminase gene (ADA) and in the inactive 754 gene by determining (i) the removal of specific lesions, i.e. cyclobutane pyrimidine dimers (CPD) and 6'4PP, or (ii) the formation of BrdUrd-labeled repair patches. No differences in repair capacity were observed between XP-D and XP-D-CS cells. In both cell types repair of CPD was completely absent whereas 6'4PP were inefficiently removed from the ADA gene and the 754 gene with similar kinetics. However, whereas XP-D cells were able to restore UV-inhibited RNA synthesis after a UV-dose of 2 J/m2, RNA synthesis in XP-D-CS cells remained repressed up to 24 h after irradiation. Our results are inconsistent with the hypothesis that differences in the capacity to perform preferential repair of UV-induced photolesions in active genes between XP-D and XP-D-CS cells are the cause of the extreme UV-sensitivity of XP-D-CS cells. Rather, the enhanced sensitivity of XP-D-CS cells may be associated with a defect in transcription regulation superimposed on the repair defect.