Recombinant Anti-Ad fiber knob x Anti-CD40 Bispecific Antibody (Single-chain Diabody) is designed to be expressed as one polypeptide chain with four variable domains from two parental antibodies. The middle linker is long enough to form scFv dimers between the two diabody chains. This BsAb can retarget of adenoviral vectors to tumour cells. It can be used in cancer gene therapy to transfer a therapeutic gene into tumour cells aiming at selective and efficient cell killing.