a model used to explain the allosteric form of cooperativity; in this model, an oligomeric protein can exist in two conformational states in the absence of the ligand; these states are in equilibrium, and the one that is predominant has a weaker affinity for the ligand (which binds to the protein in a rapid equilibrium fashion). SYN: concerted model. [Jacques L. Monod, Jeffries Wyman, Jean-Pierre Changeux]