Indirubin-3'-monoxime is a potent inhibitor of GSK-3β (IC50=22 nM)1. Indirubin-3'-monoxime also inhibits CDK1 (p34cdc2; IC50=180 nM) and CDK5 (IC50=100 nM)1. Indirubin-3'-monoxime reversibly arrests asynchronous HBL-100 cells at G22. This compound induces apoptosis in the mammary carcinoma cell line MCF-7 (10 μM). Indirubin-3'-monoxime is an inhibitor of Cdk2, cyclin A, cyclin B, cyclin E and p35.