A biological target is anything within a living organism to which some other entity, like an endogenous ligand or a drug is directed and/or binds. Examples of common classes of biological targets are proteins and nucleic acids. The definition is context-dependent and can refer to the biological target of a pharmacologically active drug compound, the receptor target of a hormone (like insulin), or some other target of an external stimulus. The implication is that a target is «hit» by a signal and its behavior or function is then changed. Biological targets are most commonly proteins such as enzymes, ion channels, and receptors.