Recombinant Anti-CEA x Anti-DTPA Bispecific Antibody (TandAb) is designed to be expressed as a monomeric subunit (single chain Diabody, scDb), with four variable domains from two parental antibodies. Then the two peptide chains can dimerize in a head-to-tail fashion. 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.