The time course of cholesterol α-oxide formation in the skin of hairless mice receiving chronic suberythemic levels of ultraviolet radiation was determined. The concentration of this carcinogen increased and reached a peak level at 10 weeks, after which squamous cell carcinomas appeared. The facts that an increase in formation of this carcinogen precedes the appearance of tumors and that this increase is apparently related to the dose of ultraviolet received suggest that this photoproduct, of natural origin, is involved in the etiology of ultraviolet-induced carcinogenesis.

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Supported in part by the Morrison Trust of San Antonio, Texas, USPHS Grant CA 13464-01 from the National Cancer Institute, and the American Cancer Society.

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