Recombinant Anti-HER2 x Anti-HER3 Tandem Fab-SAR immunotoxin (Bispecific Immunotoxin) is designed to be expressed as the α-Sarcin (SAR) 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 SAR to antigen-specific cells. Then SAR is a ribonuclease which can inhibit protein synthesis and kill the target cells. The bispecific immunotoxin is designed for the research of Breast cancer; Gastric cancer; Prostate cacncer; Ovarian cancer; Solid tumors therapy.