The hereditary renal carcinoma (RC) in the rat, originally reported by R. Eker in 1954, is an excellent example of a Mendelian dominantly inherited predisposition to a specific cancer in an experimental animal. Recently, we have identified a germline mutation of the sclerosis (Tsc2) gene in the Eker rat (Nature Genetics 9, 70-74, 1995), suggested to be a novel tumor suppressor gene, fitting Knudson's two-hit hypothesis. In this study, the effect of wild-type Tsc2 gene expression in Eker RC cells was tested using a tetracycline-responsive promoter system. Transfection and expression of a exogenous Tsc2 gene affected both cell morphology and growth rate. This demonstration of suppression of the neoplastic phenotype provides direct evidence for an essential role of the Tsc2 gene in tumorigenesis.