CDKN2A and papilloma: Two out of seven papillomas from p19 Arf+/+p53+/− mice show loss of the remaining p53 allele, while all papillomas examined from p19 Arf+/−p53+/− (n = 8) and p19 Arf−/−p53+/− (n = 8) mice show retention of wild-type p53 (Figure 6B).