The experimental introduction of foreign DNA into the germ line of a mouse is a powerful tool used to identify genetic elements required for appropriate tissue expression, to determine the developmental specificity of gene expression and to determine the consequences of inappropriate expression of cellular oncogenes. Moreover, by fusing a tissue-specific regulatory region of a particular gene to transforming genes, it is possible to direct expression of the transforming gene to specific cell types, thereby producing lines of mice that develop tumors in specific organs. Transgenic mice may then serve as useful models to study the multistage development of certain diseases such as cancer and may also be used to test various types of therapies for treatment of these diseases.