Recombinant Anti-Ad fiber knob x Anti-CEA Bispecific Antibody (scDiabody-CH3) is a fusion protein of two scDiabodies connected together by a CH3 region. In single-chain Diabodies, an additional connecting linker is introduced between chains of the anti-Ad fiber knob/anti-CEA diabody. The scDiabody with two specificities is fused to each of the CH3 chains to extend half-life. 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.