To determine the consequence of p53 loss on tumour initiation in pancreatic cancer, we took advantage of the fact that G2-X recombination and labelling could occur during pancreatic development (as Pdx1-Cre is expressed as early as E8.5 (ref. 12)) before transformation, resulting in PanINs comprising cells of uniform colour. Here, TP53 is linked to familial pancreatic carcinoma.