Indirubin is a cyclin-dependent kinase inhibitor which functions by competing with ATP for binding to the catalytic subunit. Indirubin is used in traditional Chinese medicine for anti-inflammatory, anti-tumor, and neuroprotective effects. Indirubin inhibits glycogen synthase kinase-3 (IC50 = 2.5 μM) and cyclin-dependent kinases 1 and 5 (IC50 = 10 μM for both isoforms). Indirubin also inhibits CDK1/5 with IC50 of 10 μM and 5.5 μM. Indirubin exhibits antiproliferative activity leading to G2/M arrest in many cell lines and G1/S arrest in Jurkat cells. Indirubin induces cell cycle arrest and inhibits cell proliferation.