Although loss of Rac1 from epidermal keratinocytes decreased epidermal hyper-proliferation, keratinocyte apoptosis was not affected.32 Inhibition of Rac1 activity by a pharmacological inhibitor, NSC23766, in epidermal keratinocytes also reduced skin papilloma formation in a chemical skin carcinogenesis model.29 We have shown recently that Rac1 activity is, on one hand, required for the development of skin papillomas in human papilloma virus type 8 transgenic mice. This evidence concerns the gene RAC1 and skin papilloma.