IC261, also known as SU5607, is a potent and selective CK1 inhibitor. IC261 triggers the mitotic checkpoint and induces p53-dependent postmitotic effects. At low micromolar concentrations IC261 inhibits cytokinesis causing a transient mitotic arrest. IC261 induces cell cycle arrest and apoptosis of human cancer cells via CK1δ/ɛ and Wnt/β-catenin independent inhibition of mitotic spindle formation. IC261 binds to tubulin with an affinity similar to colchicine and is a potent inhibitor of microtubule polymerization. This activity accounts for many of the diverse biological effects of IC261 and, most importantly, for its selective cancer cell killing.