The tumour suppressor gene, p53, has proved to be one of the genes most often modified in human cancers. These alterations consist mainly of point mutations located in the evolutionarily conserved sequences which render the protein inactive for its normal biological functions. In fact the p53 gene presents nearly 300 potential mutation sites whose analysis should enable the correlation of specific mutation spectra with different causal agents in cancer development. In this study we have analysed the mutation spectrum of the p53 gene in skin tumours from normal individuals and repair-deficient xeroderma pigmentosum (XP) patients in comparison with mutations found in internal cancers. Point mutations are mainly GC-->AT transitions in skin tumours (74% in non-XP, 87% in XP), and also to a lesser extent in internal tumours (47%) where, however, they are mainly located at CpG (63%) sequences probably due to the deamination of the unstable 5-MeC. Moreover, mutations are targeted at py-py sequences in over 90% of skin tumours whereas the distribution of mutations in internal malignancies is proportional to the frequency of py-py sites (61%) and other sequences (39%) at mutable sites. Indeed, in XP skin tumours 100% of the mutations are targeted at py-py sequences and 55% of these are tandem CC-->TT transitions considered as a signature of UV-induced lesions. In skin tumours from normal individuals, 14% of the p53 mutations are double mutations and as in XP skin tumours all these are CC-->TT transitions. In contrast, internal tumours rarely contain tandem mutations (0.8%), and of these only 2/14 were CC-->TT transitions. Finally, nearly all (95%) of the mutations in XP are located on the non-transcribed strand while internal or non-XP skin tumours do not show this strand bias. Hence, the mutation spectrum analysed in XP skin tumours also demonstrates for the first time the existence of preferential repair in humans. In conclusion, the specificity of UV-induced p53 mutation spectra in skin tumours shows that this gene is a particularly appropriate candidate for the correlation of mutation spectra with specific damaging agents.