Skp2 deficiency leads to elevated Cyclin D1 transcription and expression, in turn contributing to an increase in HSC cycling. (A) Western blot analysis of p27 and p21 protein expressions in cultured LSK cells from WT and Skp2^−/− mice. (B) Real-time PCR analysis of mRNA level of various Cyclin family genes in LT-HSCs from WT and Skp2^−/− mice. (C) Western blot analysis of Cyclin D1 protein expression in LSK cells from WT and Skp2^−/− mice. (D) The G₁ and G₀ phases of LSK cells isolated from BM cells of Skp2^−/− mice on Cyclin D1 knockdown were determined by flow cytometric analysis (**P < .001 and *P < .01; n = 3).