Recombinant Anti-HER2 x Anti-HER3 Tandem Fab-SAP immunotoxin (Bispecific Immunotoxin) is designed to be expressed as the Saporin (SAP) fused to a tandem Fab recognizing HER2 and HER3. It contains three chains and a His-tag at the C/N terminus for affinity purification. The two Fd regions are connected by a flexible linker. This bispecific immunotoxin can deliver SAP to antigen-specific cells. Then SAP is a type I ribosome-inactivating protein (RIP) 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.