A method to identify by function a band on a polyacrylamide gel. Electrophoresis is performed on a mixture of proteins before and after treatment with a strong ligand that is specific to one component, and note is made of the absence or shift in position of a protein due to prior binding to the ligand. A variant is the gel retardation assay, an approach to detecting DNA-binding proteins by their ability to bind to a short radiolabelled DNA fragment and, as a result, decrease its electrophoretic mobility in a gel.