The invention provides a single chain, polypeptide fusion protein, comprising: a non-cytotoxic protease, or a fragment thereof, which protease or protease fragment is capable of cleaving a protein of the exocytic fusion apparatus of a target cell; a Targeting Moiety that is capable of binding to a Binding Site on the target cell, which Binding Site is capable of undergoing endocytosis to be incorporated into an endosome within the target cell; a protease cleavage site at which site the fusion protein is cleavable by a protease, wherein the protease cleavage site is located between the non-cytotoxic protease or fragment thereof and the Targeting Moiety; and a translocation domain that is capable of translocating the protease or protease fragment from within an endosome, across the endosomal membrane and into the cytosol of the target cell.