Oncogene- or virus-controlled expression systems

1. Field of the Invention Nucleic acid constructs for expressing an effector gene, with the nucleic acid construct comprising a promoter I (component a) which controls the expression of a transcription factor gene (component b), a transcription factor gene (component b), a promoter II (component c) to which the gene product of the transcription factor gene binds and which controls the expression of an effector gene (component d), and effector gene (component d), wherein the activity of the gene product of the transcription factor gene depends on one or more cellular regulatory proteins which bind to this gene product and affect its activity, and isolated cells containing the nucleic acid constructs, can be used for preparing a drug for treating diseases and in methods of treating diseases.