It is possible that the attenuated toxicity seen with holotoxins with reduced numbers of native GM 1 BS could be enhanced by the presence of one or more CTB-G33D polypeptides that confer increased stability to the holotoxins (and thus might decrease the delivery of the CT-A1 subunit to the cytosol). We raise this idea because we observed that insertion of even a single CTB-G33D monomer into the pentameric B subunit rendered the holotoxin partially resistant to dissociation by SDS, and increasing the number of CTB-G33D monomers resulted in holotoxins that were completely resistant to dissociation by SDS. Conversely, there is no a priori reason to assume that the presence of one or more CTB-G33D polypeptides would affect chaperone-mediated release of CT-A1 from the nicked and reduced holotoxin in the ER, which appears to facilitate CT-A1 retrotranslocation from the ER to the cytosol (29) . Further experiments will be required to test these possibilities.