Peptides short amino acid chains or epitopes or blocking antagonists. The shortest peptides are dipeptides, consisting of 2 amino acids joined by a single peptide bond, followed by tripeptides, tetra peptides, ... till polypeptides that are long, continuous, and unbranched synthetic peptide chains. These biological oligomers and polymers can be Solid-phase peptide synthesis (SPPS), or in continue produced for custom peptide synthesis projects. The High-efficiency solid phase peptide synthesis (HE-SPPS) is give very low production costs.The receptors are ligand binding factors of type 1, 2 or 3 and protein-molecules that receive chemical-signals from outside a cell. When such chemical-signals couple or bind to a receptor, they cause some form of cellular/tissue-response, e.g. a change in the electrical-activity of a cell. In this sense, am olfactory receptor is a protein-molecule that recognizes and responds to endogenous-chemical signals, chemokinesor cytokines e.g. an acetylcholine-receptor recognizes and responds to its endogenous-ligand, acetylcholine. However, sometimes in pharmacology, the term is also used to include other proteins that are drug-targets, such as enzymes, transporters and ion-channels.