Affinity-trap polyacrylamide gel electrophoresis (PAGE) has become one of the most popular methods of protein separation. This is not only due to its separation qualities, but also because it can be used in conjunction with a variety of other analytic methods, such as mass spectrometry, and western blotting.  In addition to helping isolate and purify proteins from biological samples, AT-PAGE is anticipated to be helpful in analyses of variations in the expression of particular proteins as well as in investigations of posttranslational modifications of proteins. This method utilizes a two-step approach. First, a protein sample is run through a polyacrylamide gel using electrophoresis. Then, the sample is transferred to a different polyacrylamide gel (the affinity-trap gel) where affinity probes are immobilized. The proteins that do not have affinity for the affinity probes pass through the affinity-trap gel, and proteins with affinity for the probes will be "trapped" by the immobile affinity probes. These trapped proteins are then visualized and identified using mass spectrometry after in-gel digestion.