CD13 also known as APN, ANPEP is located in the small-intestinal and renal microvillar membrane, and also in other plasma membranes. CD13 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.  CD13 functions as a zinc-binding metalloprotease, playing a crucial role in the digestion of peptides and the inactivation of regulatory peptides like enkephalins, which are important for pain modulation and stress response. CD13 is significant in cancer biology, as CD13 may enhance the invasive capacity and metastatic behavior of cancer cells, making CD13 a potential target for therapeutic intervention.