Two different approaches have been undertaken to develop targeted biomolecules for therapeutics. The first was the construction of immunotoxins consisting of monoclonal antibodies chemically linked through a disulfide bond to a plant or bacterial toxin or radionuclide. Instability of the chemical conjugation of some of the earlier immunotoxins led to the concept of using protein engineering and recombinant DNA to assemble fusion genes combining the sequences for the enzymatically active and translocation domains of a toxin with those of a specific targeting ligand. From the outset, the prospect of using recombinant DNA methods to assemble the structural genes encoding bacterial toxin growth factor fusion toxins, or fusion proteins, offered significant advantages over chemical conjugation in the assembly of chimeric proteins. Most importantly, the fusion junction, or point at which the substitute receptor binding domain was linked to the toxin fragment, could be precisely determined. Expression of the fusion gene in recombinant Escherichia coli would then result in the synthesis of a single homogeneous gene product rather than the mixture of isomeric forms which result from the chemical conjugation process used in the generation of immunotoxins, thereby leading to a theoretically more uniform agent for clinical studies.