A potent, cell-permeable, and selective inhibitor of CDKs (IC50s = 4, 70, 35, 850, and 75 nM for cdc2/cyclin B, Cdk2/cyclin A, Cdk2/cyclin E, Cdk4/cyclin D1 and Cdk5-p35, respectively); reversibly arrests synchronized cells in the G1 and G2 phase of the cell cycle, inhibiting both cell proliferation and cell death.