Although PE-based immunotoxins have been extensively investigated in various preclinical and clinical settings, and moxetumomab pasudotox, a PE-based immunotoxin targeting CD22, has been approved by the US Food and Drug Administration (FDA) to treat hairy cell leukemia [28], few studies have clearly demonstrated through tumor imaging that immunotoxins could be specifically delivered to tumor tissues, which is crucial for immunotoxin to exert the antitumor functions. The gene discussed is CD22; the disease is neoplasm.