Molecular events that cause tumor formation enhance a number of HOX genes, called switch genes, coding for RNApolymeraseII transcription factors. Thus, in tumor cells, RNApolymeraseII is more active than in other somatic cells. Amanita phalloides contains amanitin which inhibits RNApolymeraseII. Partial inhibition with amanitin influences tumor cell - but not normal cell - activity. To widen the treatment spectrum, dilutions of Amanita phalloides, containing amanitin, are applied to a patient with mammary duct cancer. For monitoring tumormarkers, different doses of amanitin are applied. The former duplication time of tumor growth represented three months; however within a period of 18 months the patient can be stabilized without further growth of the tumor. There are also no severe symptoms, no liver damage and no continuous erythrocyte deprivation. This new principle of tumor therapy shows high potential to provide a medical treatment.