RAC1 and papilloma: Interestingly, only 3 out of 20 (15%) K14 HPV-8 transgenic mice with epidermis specific expression of only one Rac1 allele (K14 HPV-8/Rac1-EKOhet) developed skin papillomas starting from P126, suggesting a dose effect of Rac1 expression on papilloma formation (Figure 2A right panel and 2B).