Eukaryotes contain a superfamily of microtubule-based motor proteins comprising kinesin and a number of related proteins that are thought to participate in various forms of intracellular motility, including cell division and organelle transport. KIF11 (also known as kinesin family member 11, Eg5 or TRIP5) is a slow, plus-end-directed microtubule-based motor of the BimC kinesin family that is essential for bipolar spindle formation during eukaryotic cell division. When the expression of KIF11 is blocked, centrosome migration halts and cells are arrested in mitosis with monoastral microtubule arrays. KIF11 is phosphorylated on serine during S phase and on both serine and Thr 927 during mitosis, which regulates the association of Eg5 with the spindle apparatus (probably during early prophase). KIF11 is also known to be a member of the thyroid receptor interacting protein (Trip) family, and interacts with the thyroid hormone receptor only in the presence of thyroid hormone.