Diphtheria toxin (535 aa, 58 kDa), is an exotoxin secreted as a proenzyme by Corynebacterium diphtheriae, the pathogen bacterium that causes diphtheria. The toxin gene is encoded by a bacteriophage. The proenzyme undergoes proteolytic nicking and reduction of disulfide bond, giving rise to subunit A (21 kDa) and B (37 kDa). Subunit B (C-terminal) recognizes cellular receptor, HB-EGF receptor, and subunit A (N-terminal) catalyzes the ADP-ribosylation of eukaryotic elongation factor-2 (eEF2), inactivating this protein.