Fig. 6. Interaction of cdkIs to cdk4 and cdk2 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 or 3 µg of anti-cdk2 polyclonal antibodies, followed by Western blot analysis using anti-p16INK4a rabbit polyclonal antibody (A), antimouse p21Cip1 antibody (B), or p27Kip1 mouse monoclonal antibody (C and D). Protein bands were revealed after ECL detection reaction and exposure to BioMax Kodak films for 310 min. Proteins of interest were scanned and quantified densitometrically by the Phosphorimager SF analyzer. Histograms present arbitrary units of cdkI-cdk4 or cdk2 after data were normalized to the total protein level in 105 epithelial cells in each sample. Numbers on top of columns represent the number of fold increase or decrease compared with the internal control of each group.