The task of separating complex mixtures of proteins into individual components was revolutionized by the development of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Modifications of this technique employing gradient gels significantly improved the resolution of proteins and polypeptides based on their molecular weights. Two-dimensional (2-D) gels that combine initial isoelectric focusing in a tube gel with SDS page electrophoresis on a slab gel produce extremely high resolution of proteins. In this method, each protein migrates to a position determined both by its isoelectric point and molecular weight. If the mixture of proteins is complex, however, it can still be difficult to separate all proteins such that they occupy unique positions on the gel, because many proteins have similar isoelectric points and molecular weights.