Cyclic peptides are polypeptide chains which contain a circular sequence of bonds. This can be through a connection between the amino and carboxyl ends of the peptide, for example in cyclosporin ; a connection between the amino end and a side chain, for example in bacitracin ; the carboxyl end and a side chain, for example in colistin ; or two side chains or more complicated arrangements, for example in amanitin . Many cyclic peptides have been discovered in nature and many others have been synthesized in the laboratory. Their length ranges from just two amino acid residues to hundreds. In nature they are frequently antimicrobial or toxic; in medicine they have various applications, for example as antibiotics and immunosuppressive agents.