Several protein toxins, such as the potent plant toxin ricin, enter mamma-lian cells by endocytosis and undergo retrograde transport via the Golgi complex to reach the endoplasmic reticulum (ER). In this compartment the catalytic moieties exploit the ER-associated degradation (ERAD) pathway to reach their cytosolic targets. Bacterial toxins such as cholera toxin or Pseudomonasexotoxin A carry KDEL or KDEL-like C-terminal tetrapep-tides for efficient delivery to the ER.