The crystal structure of the ligand binding domain of ERR-α in complex with a ligand that forms a reversible thioether bond to Cys325 of ERR-α, methods to measure dissociation rates for ligands that form reversible covalent bonds, and methods to design ligands that form reversible covalent bonds for use as modulators of ERR-α activity are disclosed. The crystal structure and methods provide a novel molecular mechanism for modulation of the activity of ERR-α and provide the basis for rational drug design to obtain potent specific ligands for use as modulators of the activity of this new drug target.