Peptides are compounds formed by multiple amino acids linked by peptide bonds and can be prepared via recombinant DNA expression, biological extraction, or chemical synthesis. Peptides represent a unique class of drugs with certain characteristics of small molecule drugs as well as properties of large proteins and other biologics. Most intrinsic signaling molecules are peptide-based, such as enzymes, hormones, and immune response mediators. Over 7,000 natural peptides have been identified.