This study describes a tumor progression model for ductal pancreatic cancer in mice overexpressing TGF-α. Activation of Ras and Erk causes induction of cyclin D1-Cdk4 without increase of cyclin E or PCNA in ductal lesions. Thus, TGF-α is able to promote progression throughout G₁, but not S phase. Crossbreeding with p53 null mice accelerates tumor development in TGF-α transgenic mice dramatically. In tumors developing in these mice, biallelic deletion ofInk4a/Arf or LOH of the Smad4 locus is found suggesting that loci in addition to p53 are involved in antitumor activities. We conclude that these genetic events are critical for pancreatic tumor formation in mice. This model recapitulates pathomorphological features and genetic alterations of the human disease.