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, resulting in a change in its behavior or function. 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.  Biological targets are most commonly proteins such as enzymes, ion channels, and receptors.