RAC1 and skin neoplasm: In previous studies with FVB/N wild-type mice in a similar UV-irradiation treatment regimen, first skin tumors appeared after 248 days of irradiation on average.37 In our long-term UV-irradiation experiments, however, we were forced to stop UV-irradiation after 134 days because by then the Rac1-EKO mice developed severe skin erosions.