TandAb (Tandem diabody) is a type of bispecific antibody fragments. TandAb is a tetravalent bispecific molecule, 2+2 antigen-binding valency, consisting solely of Fv domains. TandAb is expressed as a monomeric subunit (single chain Diabody, scDb) with four variable domains from two parental antibodies. In the subunit, the linkers are too short to form intra-chain pairing, so the 2 polypeptides dimerize in a head-to-tail fashion. TandAbs have been studied as BiTEs (bispecific T cell engagers) or BiKEs (bispecific NK engagers) in tumor immunotherapy.