Diphtheria toxin mutant CRM197 is an inhibitor of protein synthesis that induces cellular toxicity.

Diphtheria toxin (DT) ADP-rybosylates elongation factor-2 on a modified histidine residue called diphthamide, leading to a block in protein synthesis. CRM197 is a nontoxic DT mutant commonly used as a carrier for conjugate vaccines, and may have a potential as an anti-tumor agent. We now report that CRM197 expression is indeed toxic to cells, and inhibits protein synthesis. These results should be considered for future vaccine studies, and to investigate the mechanism of CRM197-mediated anti-tumor effect.