An ADP-ribosylating polypeptide 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) transferases 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.