Studies of cancer predisposition have largely concentrated on the role of high-penetrance susceptibility genes. Less than 10% of the total human tumor burden, however, is accounted for by mutations in these genes. More genetic variation in cancer risk is likely to be due to commoner but lower penetrance alleles. In man, such modifier genes will be difficult to find since they do not segregate as single Mendelian traits. The mouse offers a powerful system for studying polygenic traits such as cancer and has been widely used for this purpose. Novel approaches that might accelerate the identification of these low-penetrance cancer susceptibility genes by using mouse models will be discussed.