CDKN2A and papilloma: Thus, loss of p19Arf might reduce or eliminate the need to mutate Ras. All papillomas from p19 Arf+/+ (5/5), p19 Arf+/− (4/4), and p19 Arf−/− (5/5) mice contained the identical A→T transversion at codon 61 of Hras, resulting in an amino acid change from glutamine to leucine and a constitutively activated Ras protein (Quintanilla et al. 1986).