Recombinant Anti-Ad fiber knob x Anti-CEA Bispecific Antibody (Diabody-CH3) is a fusion protein of two diabodies connected together by a CH3 region. In a diabody, the two scFvs with short linkers are from anti-Ad fiber knob and anti-CEA antibodies. The VH and VL domains form an inter-chain pairing. The diabody with different specificities is fused to each of the CH3 chains, which results in 2+2 antigen-binding valency and extends the half-life of this BsAb. 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.