Loss of SKP2 stabilizes target proteins. (A) Detection by Western blot of p21^Cip1, p27^Kip1, p130, cyclin E, and β-actin protein levels in BM extracts from Skp2^−/− and Skp2^+/+ mice in a representative experiment. Middle, quantification of the bands normalized by β-actin. n = 3-8 from 2 independent experiments. (B) Histograms in the top panel shows intensity of fluorescence for p27^Kip1 and p130 in the same BM samples processed for Western blot in panel A. Bar graph at the bottom shows average fold increase (in MIF) of p27^Kip1 and p130 in BM cells in Skp2^−/− vs Skp2^+/+ mice. Of note, MIF is measured in a logarithmic scale; thus, small differences in fluorescence are equivalent to greater differences in the number of molecules per cell, as seen by comparison of intracytoplasmic labeling in panel B with the levels of protein in the immunoblots in panel A. (C) Expression levels for p27^Kip1, p130, and p57^Kip2 were determined in specific subsets by immunophenotypical staining with antibody directed to surface markers followed by intracytoplasmic staining. Histograms show intensity of fluorescence of p27^Kip1, p130, and p57^Kip2 in LSK and Lin⁻ populations in Skp2^+/+ and Skp2^−/− mice in a representative experiment. Histogram overlaid are intensity of fluorescence in Skp2^−/− cells (black) and Skp2^+/+ cells (gray). IgG control is shown in white. (D) Bar graphs show average fold increase in MIF of p27^Kip1, p130, and p57^Kip2, cyclin E, and c-Myc in LSK and Lin⁻ populations in SKP2^−/− mice. n = 3-9 in 2-4 independent experiments. Data are expressed as mean ± SEM *P < .05 vs Skp2^+/+ **P < .005 vs Skp2^+/+.