Cell-permeable. A potent inhibitor of CDK1/cyclin B (IC₅₀ =35 nM). Also inhibits CDK2/cyclin A, CDK2/cyclin E (IC₅₀ = 200 nM), CDK5/p25 (IC₅₀ = 40 nM), CDK5/p35 (IC₅₀ = 40 nM) and GSK-3β.