In 7 of 37 patients with cutaneous melanoma, mutations in the N-ras gene were found. The primary tumors of these seven patients were exclusively localized on body sites continuously exposed to sunlight. Moreover, the ras mutations were all at or near dipyrimidine sites known to be targets of UV damage. Two primary tumors were biclonal with respect to ras mutation. An active role for UV irradiation in induction of the mutations is suggested.In animals, a wide range of xenobiotic agents are capable of inducing mutations in the r-as oncogene family (2). Little is known, however, about the involvement of mutagenic agents in the induction of ras mutations in humans. For human cutaneous melanoma, there is epidemiological evidence suggesting a role of UV irradiation in occurrence of the tumor (18).N-ras activated in melanoma. To investigate the role of UV irradiation in the activation of ras genes, we studied 63 tumor samples from 37 cutaneous melanoma patients (Table 1) for mutations in H-, K-, or N-r-as. These samples included 10 primary tumors, 40 metastases, and 13 cell lines, all of the non-lentigo maligna type. To analyze mutational activation of ras genes, polymerase chain reaction-amplified DNA samples were prepared aR described previously (17) either from DNA obtained from fresh tumor specimens and cell lines or from Formalin-fixed and paraffin-embedded material (16). All samples contained at least 50% tumor cells. In certain cases (e.g., primary tumors of patients 4 and 10), parts of paraffin sections that contained more than 90% tumor cells were chosen. The amplified DNA was spotted onto nylon membranes and hybridized to probes 20 nucleotides long as described elsewhere (21) (Fig.