The identification of cloning of tumor suppressor genes has mostly relied on familial human cancer predisposition syndromes and reverse genetics. Recent advances in our abilities to manipulate endogenous genes in the mouse by applying gene targeting techniques to ES cells has allowed animal models to be generated which have the same mutations as the human syndromes. Mice lacking one or both alleles of known tumor suppressor genes have been generated to evaluate the normal function of these genes. These animals have proven to be highly susceptible to tumor development, indicating that these mice are a potent in vivo assay system for tumor suppressor genes. The initiation of gonadal tumor development in mice lacking both copies of the alpha inhibin gene proved this assay and showed that inhibin is a novel tumor suppressor. Future identification of novel tumor suppressor genes will undoubtedly also include murine ES cell/gene targeting strategies for the identification and analysis of the function of these gene products in negative growth control.