This equation describes the competition of a ligand for receptor binding. It is identical to the sigmoid dose-response curve with HILLSLOPE = -1.0.

The variable LogEC50 is the concentration of the competitor required to compete for half the specific binding. We use the term EC50 to be consistent with the equations for the other sigmoid curves. The term IC50 is used more frequently ("E" stands for effective; "I" stands for inhibitory).

Usually the Y values are total binding. If you enter specific binding instead, fix BOTTOM to have a constant value of zero. If you enter percent specific binding, also set TOP to be a constant equal to 100.