Figure 5.21 Purification by immunoprecipitation. Monoclonal antibody to the estrogen receptor is added to a preparation of cytosol containing the receptor. The antibody is attached to an insoluble bead. The mixture is then gently stirred for several hours to allow the interaction of the receptor and antibody. The mixture is centrifuged and the supernatant is discarded. The pellet is resuspended in buffer to wash out any trapped cytosolic components. The pellet is again collected and the supernatant discarded. The antibody–