(a) IHC for p19ARF in Pdx1-Cre; LSL-KrasG12D/KrasWT; p53LSL-R172H/WT tumours showed expression throughout pancreatic tumorigenesis from early acinar-to-ductal metaplasia (ADM) and PanIN lesions to PDAC. (b) IHC for p19ARF exhibited expression in LSL-KrasG12D/KrasWT; p53flox/flox lung adenocarcinomas but not adenomas. (c) IHC for p19ARF in Pdx1-Cre; LSL-KrasG12D/KrasWT (KC) tumours, lacking p53 mutation, displayed expression throughout pancreatic tumorigenesis. These data suggest that p53 mutation in a does not induce p19ARF by feedback upregulation, as has been previously described28. Scale bars, 100 μm (all).