We have analysed the effect of oxidative guanine lesions on the expression of a transfected reporter gene in mouse embryonic fibroblasts deficient in Cockayne syndrome B protein (Csb) and/or the 8-oxoguanine DNA glycosylase (Ogg1). We used a highly sensitive flow cytometry-based approach and quantitative real-time PCR to measure the changes in gene expression caused by the presence of oxidised guanine residues generated by photosensitisation in the vector DNA. In wild-type cells, small numbers (one or three) of oxidised guanines did not affect gene expression at short times after transfections, whereas progressive reduction of the transgene expression was observed at later time points. Although Ogg1 has a major contribution to the repair of oxidised guanine bases, its absence did not have a strong effect on the gene expression. In contrast, the lack of functional Csb protein caused a pronounced inactivation of the damaged reporter gene. Most strikingly, an additional Ogg1 deficiency significantly attenuated this effect. The results indicate that the processing of oxidative guanine modifications by Ogg1 can mediate host cell inactivation rather than reactivation of the damaged genes and that this effect is strongly enhanced in the absence of Csb.