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,