The yeast-derived Flp-frt site-specific DNA recombination system was used to achieve pituitary-specific inactivation of the retinoblastoma (Rb) tumor suppressor gene. Whereas mice carrying only frt sites in both alleles of Rb remain tumor free, tumorigenesis ensues when the Flp recombinase is expressed. The rate of tumorigenesis in these mice depends both on the expression level of the Flp recombinase and on the presence of frt sites in one or both Rb alleles. This permitted a more accurate definition of the consecutive steps in pituitary tumorigenesis. Our study illustrates the potential of this approach for studying sporadic cancer in a defined mouse model.