This equation describes the competition of a ligand for two types of receptors. The radioligand has identical affinities for both receptors, but the competitor has a different affinity for each.
Y is binding (total or specific) and X is the logarithm of the concentration of the unlabeled ligand. FRACTION_1 is the fraction of the receptors that have an affinity described by LogEC50_1. The remainder of the receptors have an affinity described by LogEC50_2. If LogEC50_1 is smaller than LogEC50_2, then Fraction_1 is the fraction of high affinity sites. If LogEC50_1 is larger than LogEC50_2, then Fraction_1 is the fraction of low affinity sites.