Emerging functions of force-producing kinetochore motors.

More than two decades of research has resulted in the identification of some 60 microtubule motor proteins, several of which have been implicated in mitosis. Although some kinesin super-family proteins function as microtubule depolymerases at kinetochores, such as Kinesin-8 and -13, it is now appreciated that there are only two force-producing kinetochore associated motors, the plus end-directed microtubule motor CENP-E and the minus end-directed microtubule motor cytoplasmic dynein. Defining their roles at kinetochores has been hampered by the complexity of mitosis itself, and a multiplicity of mitotic roles, at least for cytoplasmic dynein. Nonetheless, recent advances have served to define the primary roles of the two kinetochore motors in detail.