Cytokinetics’ efforts in the area of cell cycle control have been focused primarily towards disrupting mitotic function in cancer cells, but also encompass modulation of any activities in the normal cell cycle. To date, our drug discovery and development activities have been directed to the multi-protein systems involved in mitosis, namely the mitotic spindle, and key motor proteins, called mitotic kinesins. The mitotic spindle encompasses many cytoskeletal proteins including tubulin, which forms microtubule filaments, and a sub-group of kinesins known as mitotic kinesins. The highly orchestrated action of the proteins within this structure transports and segregates genetic material during cell proliferation. Unlike current anti-mitotic agents that target tubulin and thus disrupt microtubule function, Cytokinetics’ drug candidates target two well-characterized mitotic kinesins, kinesin spindle protein (KSP) and centrosome-associated protein E (CENP-E). KSP is a mitotic kinesin that functions exclusively in the initial steps of mitosis. The only known role of KSP is to separate spindle poles during prophase of mitosis. Cytokinetics’ expertise in cell cycle control has resulted in two drug candidates, ispinesib and SB-743921, that target and inhibit KSP function. CENP-E is directly involved in coupling the mechanics of mitosis with the mitotic checkpoint signaling machinery, regulating cell-cycle transition from metaphase to anaphase. CENP-E is also essential for prometaphase chromosome movements that contribute to metaphase chromosome alignment. These processes are essential to cell proliferation. Cytokinetics’ expertise has also resulted in one potential drug candidate, GSK-923295, that specifically inhibits CENP-E and which exhibits properties that distinguish it from ispinesib and SB-743921.