CDK4 and infection: To identify the mechanisms related to the growth inhibition and differentiation of glioma cells secondary to BMPR-IB overexpression, we first examined the mRNA expression of several cell cycle regulatory genes, including p21, p27Kip1, Skp2, Cdk2, Cdk4 and p53, by real-time RT-PCR and found increased expression of p21, p27, and Cdk2 mRNA, decreased expression of Skp2 mRNA, and consistent expression of p53 mRNA after 96 h of BMPR-IB infection of U87 and U251 cells (Figure 5A).