Staurosporine is a natural product isolated from the bacterium Streptomyces staurosporeus. It is a very potent and broad spectrum protein kinase inhibitor through the prevention of ATP binding to the kinase. It inhibits protein kinase C (IC50=0.7nM), CDK1/cyclin B (IC50=5nM), CDK2/cyclin A (IC50=7nM), CDK4/cyclin D (IC50=3-10nM), CDK5/p25 (IC50=4nM), GSK-3β (IC50=15nM), Pim-1 kinase (IC50=10nM). It induces apoptosis in human neuroblastoma cell lines and chick embryonic neurons. Due to its broad and potent kinase inhibition activities, staurospoine is routinely used as cytotoxic, anti-proliferative reference compound.