Mouse models of cancer have provided novel insights into the timing of p53 loss during tumorigenesis. We have recently identified Fbxw7/Cdc4 as a downstream target of p53 loss that controls genomic instability and tumor development in epithelial tumors. Although p53-deficient mice primarily develop lymphomas and sarcomas, the additional loss of one copy of the Fbxw7 gene drives tumor development in a range of epithelial tissues. These data highlight the importance of genetic instability at the chromosome level in the development of common cancer types, and further illustrate the value of mouse models in identifying causal genetic events in epithelial tumor formation.