ADP ribosylating exotoxin produced by Corynebacterium diphtheriae that causes the signs and symptoms of diphtheria; it can be broken into two unequal domains, the smaller, catalytic A domain is the lethal moiety and contains mono ADP ribose transferase which transfers ADP ribose to peptide elongation factor 2 thereby inhibiting protein synthesis; and the larger B domain that is needed for entry into cells.