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Fig. 7. Comparison of the kinase activities associated with CDK2 and CDK4 in control and differentiating A.1 and 1.3 EC cells. EC cells were cultured for 48 h with and without 10-6 M RA, and then proteins were extracted. Equal amounts of total protein were entered into each CDK2- and/or CDK4-immunoprecipitation reaction. The activities of isolated CDK2 and CDK4 were assayed using the appropriate substrate (histone H1 and GST-pRb) and visualized by autoradiography. Data are representative of at least three independent replicates.