A key step in the intoxication process is the transport of the toxin from the cell surface to the ER. How GM 1 confers the specificity for lipid trafficking has not yet been determined. Pentameric binding of GM 1 by the B subunit of CT may itself induce membrane curvature and induce invagination to begin the entry process, as is seen with simian virus 40 (SV40) (12) and Shiga toxin (13) (14) (15) . Interestingly, invasion by SV40 occurs only with GM 1 that has native long-chain acyl groups. Clustering of GM 1 may enable the toxin to associate with lipid rafts that serve as platforms for trafficking of CT through the retrograde pathway or parts of it. Lipid rafts are viewed as highly dynamic microdomains that may self-assemble in membranes from sphingomyelin, cholesterol, glycolipids, and proteins that favor a lipid-ordered microenvironment (reviewed in references 16 to 18). Some functions attributed to lipid rafts may require interactions with protein components or scaffolds to stabilize them, extend their lifetimes, or facilitate their coalescence into larger physiologically significant structures. By binding to and cross-linking five GM 1 molecules, CTB might serve as such a protein scaffold and promote the function of lipid rafts in toxin trafficking. Conversion of PM sphingomyelin to ceramide or acute depletion of membrane cholesterol both prevent endocytosis of CT (19, 20) , consistent with a role for lipid rafts in CT trafficking. A requirement for the lipid raftassociated proteins flotillin 1 and flotillin 2 in a zebra fish model of intoxication by cholera toxin (21) also supports the key role of lipid rafts in trafficking of CT.