Molecular biology has contributed a concept, novel in pharmacology, in which the receptor is an independent variable. Site-directed mutagenesis of ligand-gated ion channels is now commonplace. The mutant receptor is usually characterized by the Hill parameters that describe concentration-response curves from transfected, voltage-clamped cells. In this article, Charles Spivak describes how to convert parameters for realistic models of channel activation into Hill parameters. Correlations among the Hill parameters that the models enforce can be useful in tentatively assigning a physiological function to the mutation site.