In summary, we have identified a dual role of Rac1 in HPV-8- and UV-light induced skin carcinogenesis: Rac1, on one hand, transmits growth factor signals which stimulate the growth of papillomas; on the other, its absence facilitates the development of squamous cell carcinomas, probably through the loss of its tissue integrity- maintaining function in the epidermis and hair follicle. The gene discussed is RAC1; the disease is squamous cell carcinoma.