Fig. 2. CDK4 activity in vivo and in vitro. IP kinase assays were performed on whole-liver homogenates (A) and lysates of fetal hepatocytes cultured for the indicated times [B, E19 (filled bars) and E21 (unfilled bars)]. Activity was measured as incorporation of 32P from [-32P]ATP into GST-pRb fusion protein. Representative autoradiograms are shown (lower panels) with corresponding densitometry (graphs). Error bars, SE. Results were confirmed in three additional experiments. C1, minus antibody control; C2, minus sample control; C3, minus substrate control.