DNA damages caused by cellular metabolites and environmental agents induce mutations, that may predispose to cancer. Nucleotide excision repair (NER) is a major cellular defence mechanism acting on a variety of DNA lesions. Here, we show that spontaneous mutant frequencies at the Hprt gene increased 30-fold in T-lymphocytes of 1 year old Xpc−/− mice, possessing only functional transcription-coupled repair (TCR). Hprt mutant frequencies in Xpa−/− and Csb−/− mice that both have a defect in this NER subpathway, remained low during ageing. In contrast to current models, the elevated mutation rate in Xpc−/− mice does not lead to an increased tumour incidence or premature ageing.