The steps outlined above should provide a framework for developing and evaluating a scheme for isolating a particular membrane system of interest. Further characterization often focuses on the specific research interests of an investigator. Most subsequent techniques will probably involve biochemical analyses of protein and lipid components and biophysical analyses of membrane properties. The techniques to be used, such as SDS gel electrophoresis, enzyme kinetics, receptor binding, protein purification, etc. are not unique to membrane systems and will not be described in detail. However, it is likely that the functional relevance and success of subsequent analyses will be aided if the starting membranes are as homogeneous and well-characterized as possible. Even if some of the fractionation steps are bypassed, such as might occur if a specific membrane protein is detergent-extracted, purified, and reconstituted directly from a crude homogenate, a full understanding of membrane protein function will probably require complementary studies on intact cells and isolated membranes.