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