['p53 Is known to be a tumor-suppressor gene and has been suggested to play an important role for multistep carcinogenesis of cutaneous squamous cell carcinoma (SCC). To evaluate the role of ultraviolet light (UV) in p53 mutation in squamous cell carcinogenesis, paraffin-embedded sections of SCCs were immunohistochemically stained with the CM-1 antibody for p53 protein. Positive staining suggests mutation of the p53 gene since the mutant p53 protein gains prolonged half-life to be detectable by this method. The specimen included SCCs induced by UV (SCCs on sun-exposed areas, SCCs on patients with xeroderma pigmentosum), roentgen rays, scar, and miscellaneous causes. In addition, solar keratoses that are precancerous lesions of UV-related SCCs were also analyzed.', 'Fourteen (54%) of 26 UV-related SCCs were positive, whereas five (19%) of 26 UV-unrelated SCCs were positive. Among the UV-related SCCs, five (45%) of 11 well-differentiated SCCs and nine (60%) of 15 moderately to poorly differentiated SCCs were positive. Eleven (48%) of 23 solar keratoses were positive for p53.'] Is expression of p53 protein an early event in ultraviolet light-induced cutaneous squamous cell carcinogenesis?