The kinesin-like spindle protein Eg5 (also known as kinesin-5, kinesin family protein 11, or Kif11) is a motor protein that is essential for establishing a bipolar spindle during mitosis both in normal and tumor cells. Ispinesib is a cell-permeable, allosteric inhibitor of Eg5 (Ki app = 2.3 nM) with >10,000-fold selectivity for Eg5 over a range of other mitotic kinesins. It induces a monopolar spindle phenotype, leading to the activation of a spindle assembly checkpoint, mitotic arrest, and subsequent cell death (GI50s = 22-82 nM in colon, pancreas, prostrate, and lung cancer cells in vitro). At 10 mg/kg, ispinesib produces tumor regression of breast cancer cell xenografts in mice. It has also been used to halt the growth of treatment-resistant glioblastoma tumor-initiating cells, to prevent tumor initiation and self-renewal of a cancer stem cell population (EC50 = 1.15 nM), and to reduce glioma cell invasion.