Fig. 3. Cyclin D1-associated kinase activity. IP kinase assays for GST-pRb phosphorylation were performed on E19, E21, and adult whole liver homogenates immunoprecipitated with cyclin D1 antibodies. Densitometry is shown as the mean plus SE. Lower panel, a representative autoradiogram. Control conditions are those described for Fig. 2 . Similar results were obtained in two additional experiments.