Recombinant Anti-HER2 x Anti-HER3 Tandem scFv-PAP immunotoxin (Bispecific Immunotoxin) is designed to be expressed as the pokeweed antiviral protein (PAP) fused to a tandem scFv recognizing HER2 and HER3. It contains a His-tag at the C/N terminus for affinity purification. This bispecific immunotoxin can deliver PAP to antigen-specific cells. Then PAP is a ribosome inhibitory single chain plant toxin and can kill the target cells. The bispecific immunotoxin is designed for the research of Breast cancer; Gastric cancer; Prostate cacncer; Ovarian cancer; Solid tumors therapy.