Induced cell proliferation and the initiation of skin tumour formation in mice by ultraviolet light

A single short exposure of the skin of mice to ultraviolet light did not produce any tumours, but a significant number of tumours developed when the exposure was followed by repeated applications of croton oil. Ultraviolet radiation therefore behaves as an initiating agent, resembling this action of some other carcinogens for the skin of mice. Treatment of the skin of the mice with acetic acid, xylene or croton oil 20 hours before irradiation increased the number of tumours. It is considered that cells induced to proliferate are more susceptible to the carcinogenic effect of ultraviolet radiation than their normal counterparts.