Prepropeptides often produce multiple copies of the same peptide or many different peptides. Proteins are polymer chains made of amino acids linked together by peptide bonds. MHC molecules in complex with peptide epitopes are essentially ligands for TCRs. It is a cyclic fungal peptide, composed of 11 amino acids. Viral proteins inhibit TAP preventing the MHC from picking up a viral antigen peptide. The protein is activated by cleaving off the inhibitory peptide. A simplified strategy is often employed to purify antibodies generated against peptide antigens. It comprises 244 individually folded protein domains connected by unstructured peptide sequences. Bradykinin is a 9-amino acid peptide chain. The enteroendocrine cells also secrete glucose dependent insulinotropic peptide. Renin is a circulating enzyme that acts on a circulating peptide, angiotensinogen. Bremelanotide is metabolized via hydrolysis of its peptide bonds. One of the bottlenecks in peptide/protein-based therapy is their toxicity. PEGylation is a big step forward regarding administration of peptide and protein drugs. They form as side products in peptide synthesis.