Abstract: The present invention relates to uses of mutant proto-oncogenes and oncoproteins expressed by the proto-oncogenes in inhibiting tumor growth and/or inhibiting the transformed phenotype. The preferred oncoprotein is a dominant, interfering mutant of a nuclear E2F transcription factor protein and is preferably a mutant E2F1 transcription factor protein. Methods of treating a target cell are described. Treatment is accomplished by administering to a target cell a dominant interfering mutant of a proto-oncogene in an effective amount. Treatment is also accomplished by administering to a target cell an oncoprotein in an effective amount. Compositions for such use are described as well.