text: In pharmacology,  is often written as , where  is the ligand, equivalent to L, and  is the receptor.  can be expressed in terms of the total amount of receptor and ligand-bound receptor concentrations: .  is equal to the ratio of the dissociation rate of the ligand-receptor complex to its association rate (). Kd is the equilibrium constant for dissociation.  is defined so that , this is also known as the microscopic dissociation constant and is the ligand concentration occupying half of the binding sites. In recent literature, this constant is sometimes referred to as .