Molecular binding is an interaction between molecules that results in a stable physical association between those molecules. Cooperative binding occurs in binding systems that are constituted by more than one type (species) of molecule (say molecules A and B) and in which one of the partners is not mono-valent; i.e., it binds more than one molecule of the other molecular species. For example, one molecule of type A can bind 6 molecules of type B (in such cases, B is usually referred to as the ""ligand""). Binding in this type of system can be considered ""cooperative"" if the binding of B to one site on A is affected by the binding of B to other site(s) on A. In other words, the binding of B molecules to the different sites on A do not constitute mutually independent events. This can be due, for instance, to an affinity for the ligand that depends on the amount of ligand bound. Cooperativity can be positive or negative. Cooperative binding is observed in many biopolymers, including proteins and nucleic acids. Cooperative binding has been shown to be the mechanism underlying a large range of biochemical and physiological processes.