Alsterpaullone (Alp, 9-Nitropaullone, NSC 705701) is a potent inhibitor of CDK with IC50 of 35 nM, 15 nM, 200 nM and 40 nM for CDK1/cyclin B, CDK2/cyclin A, CDK2/cyclin E and CDK5/p35, respectively. Alsterpaullone also acts as a potent inhibitor of glycogen synthase kinase-3 (GSK-3) with IC50 of both 4 nM for GSK-3α and GSK-3β. Alsterpaullone induces apoptosis by activation of caspase-9. Alsterpaullone has antitumor activity and possesses potential for the treatment of neurodegenerative and proliferative disorders.