Different mouse models have been generated to understand the molecular mechanisms involved in human tumorigenesis. Some of these models are produced by gene-targeting procedures, whereas others use the normal mouse as an experimental animal. The pathogenetic relevance of these mouse models has generated optimism for the development of effective targeted therapies, advances in rational drug design and has provided valuable information about human cancer disease. By combining genetic analysis and molecule screening techniques, the mouse is emerging as a powerful system for identifying novel cancer genes and for cancer drug discovery. However, there are both parallelisms and discrepancies between mouse and human carcinogenesis important for elucidating the human disease process.