CD22 and hairy cell leukemia: 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.