Provided are conjugates useful in cancer or infectious disease therapy. The conjugate is a drug or modified toxin which is a native toxin devoid of a functioning receptor binding domain and a protein which reacts with a substance associated with a targeted cell or pathogen. The targeted substance internalizes the conjugate into the cell cytoplasm, and the kills the cell. The protein prior to conjugation has at least one mercapto group which becomes a site for conjugation to the toxin or drug. Also provided are methods of therapy, methods for producing the conjugate and pharmaceuticals compositions of the conjugates.