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Radon increases the risk of lung cancer in smoking and non-smoking underground miners. To investigate the mutational spectrum associated with exposure to high levels of radon, we sequenced exons 5-9 of the p53 tumour suppressor gene and codons 12-13 of the Ki- rasprotooncogene in 19 lung cancers from uranium miners exposed to radon and tobacco smoke. Mutations were not found in Ki- ras,but 9 p53 mutations, including 2 deletions, were found in 7 patients by direct DNA sequencing after polymerase chain reaction amplification of DNA from formalin-fixed, paraffin-embedded tissue. In tumours from 5 patients, the mutation produced an aminoacid change and an increased nuclear content of p53 protein. The tumours with either a stop codon or frame-shift deletion in the p53 gene were negative by immunohistochemistry. None of the mutations were G:C to T:A transversions in the coding strand of the p53 gene, which are the most frequent base substitutions associated with tobacco smoking, and none were found at the hotspot codons described in lung cancer. The observed differences from the usual lung cancer mutational spectrum may reflect the genotoxic effects of radon.