9. Which of the following mechanisms may explain these results?

Choose TRUE or FALSE. A. Elevated activity levels of CDK4-cyclin D. False This complex is active before crossing the restriction point.

B. Inhibition of CDK2-cyclin A activity. False This complex is active during S phase but is not involved in S phase initiation.

C. Partial phosphorylation of pRb. True pRb is phosphorylated by CDK4-cyclin D and then by CDK2-cyclin E. If cells have crossed the R point but not the G1/S transition point, probably those sites on pRb phosphorylated by CDK4-cyclin D are phosphorylated, whereas those phosphorylated by CDK2-cyclin E are not phosphorylated.

D. Inhibition of CDK2-cyclin E activity. True