Indirubin-5-sulfonate shows inhibitory activity against GSK-3β. Indirubin-5-sulfonate is a cyclin-dependent kinase (CDK) inhibitor, with IC50 values of 55 nM, 35 nM, 150 nM, 300 nM and 65 nM for CDK1/cyclin B, CDK2/cyclin A, CDK2/cyclin E, CDK4/cyclin D1,