One way that multivalent binding could affect CT function would be by enhancing the binding avidity for cell membranes containing GM 1 . In this and in our previous studies of the CT binding site mutants, we observe a loss of avidity for binding GM 1 when the GM 1 binding pockets are mutated. In the current studies, the loss of apparent avidity for the one-binding-site species is 10-fold, but the toxin still binds to GM 1 applied to the well at nanomolar concentrations, suggesting this may not fully explain the strong loss of toxin function.