Fig. 5. cdk4-, cyclin D1-, and cyclin D2-associated kinase in early mammary preneoplasia at different hormonal conditions. Tissue protein extracts (500 µg) per sample were precleared with normal rabbit serum and then immunoprecipitated with either 4 µg of anti-cdk4 polyclonal antibody, 3 µg of anti-cyclin D1 polyclonal antibody, or anti-cyclin D2 monoclonal antibody. Full-length pRb (p110RB) was used as substrate in all of the assays, as described in "Materials and Methods." Histograms represent relative levels of phosphorylated pRB (substrate), scanned and quantified densitometrically by the Phosphorimager SF analyzer, and results were normalized as described earlier. Numbers on the top of the columns are fold increase or decrease compared with the internal control in each group.