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Cholera toxin （CTB）belongs to the AB5 –subunit family oftoxins.The native hexameric protein has a molecular mass of ~85 kDa and contains two subunits. It consists of a single A subunit （~27.2 kDa）, responsible for the ADP-ribosylation activity, and five B subunits（~11.6 kDa each）, which are arranged as a pentameric ring with an apparent 5-fold symmetry and are associated with the cell surface receptor binding and subsequent internalization （transmembrane transport）of the enzymatic component.