Purvalanol A is a potent, cell-permeable, and selective inhibitor of cyclin-dependent kinases (CDKs) with IC50 values of 4, 70, 35, 850, and 75 nM for cdc2/cyclin B, Cdk2/cyclin A, Cdk2/cyclin E, Cdk4/cyclin D1 and Cdk5-p35, respectively. Purvalanol A reversibly arrests synchronized cells in the G1 and G2 phase of the cell cycle, inhibiting both cell proliferation and cell death. At 10 μM, purvalanol A potently suppresses the anchorage-independent growth of c-Src-transformed cells as well as HT-29 and SW48 human colon cancer cells.