(A) The two-state model presents an equilibrium between two states, and , with the relative population defined by the equilibrium constant, , and their binding to an allosteric ligand, . For the inactive state, the binding equilibrium constant is given by , and for the active state, by . Due to the complete circle of equilibrium, the equilibrium constant between and is automatically deduced as with the previous three mass equations. Also, the forward reaction with implies a population shift due to the allosteric binding event. In this schematic allostery description, the conformation selection scheme emphasizes that the microscopic path of dominates the equilibrium process in contrast to the induced-fit scheme which implies the path prevails. (B) A typical sigmoid response-concentration curve in the allosteric two-state model. If we accept the assumption that a measured biological response is proportional to the fraction of receptors in the activated state, as defined in the ATSM, manipulation of the three equilibrium equations in ATSM (Figure 2A) deduces the response, , as a function of ligand concentration with three independent parameters, , , and . The sigmoid response-concentration curve of ATSM is established by three quantities, the basal activity as , , the maximum activity , , and the activity at the middle point of the transition, which corresponds to ligand concentration at .