Immunotoxins, characterized by tumor-specific cytotoxicity and high potency, have been developed as one of the new promising treatment modalities for primary malignant brain tumors (1 –3 ). Fusion toxins bind to the cell-surface receptor specific for them, are internalized by receptor-mediated endocytosis, and inhibit protein synthesis, causing apoptotic cell death (4 , 5 . The cytotoxic activity of the immunotoxin can be influenced by many factors, such as binding to the receptor, intracellular routing, degradation in the lysosome by enzymes, and immunogenicity (3 . The first step in the mechanism of action of the immunotoxin is binding to its receptor.