Recombinant Anti-CEA x Anti-HSG 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 be used in pretargeting system of PET, radioimaging or radiotherapy. It can mediate targeting of radioactivity to tumor cells. It is designed for the research of Breast cancer; Ovary cancer; Colon cancer; Lung cancer imaging and therapy.