The quality control system in the endoplasmic reticulum of eukaryotic cells 
ensures that newly synthesized proteins that fail to fold into the correct 
conformation or unassembled orphan subunits of oligomeric proteins are rapidly 
eliminated by proteolytic degradation. This entails the export of proteins from 
the endoplasmic reticulum to the cytosol followed by their destruction by the 
cytosolic ubiquitin/proteasome pathway. While this mechanism effectively 
prevents the cellular accumulation of non-functional or unwanted endogenous 
proteins, it renders the cell vulnerable to certain viruses and toxins that are 
able to subvert this degradative mechanism for their own advantage.