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Aminopeptidase N （also called CD13） is located in the small-intestinal and renal microvillar membrane, and also in other plasma membranes. In the small intestine aminopeptidase N plays a role in the final digestion of peptides generated from hydrolysis of proteins by gastric and pancreatic proteases. Its function in proximal tubular epithelial cells and other cell types is less clear. The large extracellular carboxyterminal domain contains a pentapeptide consensus sequence characteristic of members of the zinc-binding metalloproteinase superfamily. CD13 is also used by some viruses as a receptor to which these viruses bind to and then enter cells. It is a receptor for human coronavirus 229E, feline coronavirus serotype II （FCoV-II）, TGEV, PEDV, canine coronavirus genotype II （CCoV-II） as well as several Deltacoronaviruses.