Indirubin-3'-monoxime-5-sulphonic acid is a potent and selective inhibitor of CDK1, CDK5, and GSK-3β with IC50s of 5 nM, 7 nM, and 80 nM, respectively[1]. CDK1|5 nM (IC50)|CDK5|7 nM (IC50)|GSK-3β|80 nM (IC50)
[1]. Leclerc S, et al. Indirubins inhibit glycogen synthase kinase-3 beta and CDK5/p25, two protein kinases involved in abnormal tau phosphorylation in Alzheimer's disease. A property common to most cyclin-dependent kinase inhibitors. J Biol Chem. 2001 Jan 5;276(1):251-60.