Numerous cancer-prone strains of mice have been created by the introduction of candidate tumor-promoting genes into fertilized eggs. Each transgenic strain is predisposed to develop specific types of tumors, but they usually arise stochastically because of the need for spontaneous mutation of genes that collaborate with the introduced oncogene. These mice are providing insights into the effects of individual oncogenes on cellular proliferation, differentiation, and viability, as well as on oncogene cooperativity. Their predisposed state imposes sensitivity to viral and chemical carcinogenesis, and the mice should prove valuable in tests of potential carcinogens, therapies, and preventive measures.