The Hill equation as given in the article is not wrong, but can be completed as follows:

Ybar=K.[X]^n/(1+K.[X]^n)=[X]^n/(K*+[X]^n)=[X]^n/(Kd^n+[X]^n)

where n is the "Hill coefficient", [X] denotes ligand concentration, K denotes an apparent association constant (used in the original form of the equation), K* is an apparent dissociation constant and Kd a microscopic dissociation constant (used in modern forms of the equation and equivalent to an EC50).

The previous erratum saying that K* is equivalent to an EC50 is wrong. We thank Prof. Ronald K. Gary, Ph.D. from the University of Nevada, Las Vegas, for pointing this out to us.