Monopolar spindle 1 (Mps1), also known as TTK, is a serine threonine kinase, which ensures proper biorientation of sister chromatids on the mitotic spindle by the activation of the spindle assembly checkpoint (SAC). Mps1 has been shown to function as the key kinase that activates the spindle assembly checkpoint (SAC) to secure proper distribution of chromosomes to daughter cells. Mps1 is a dual specificity protein kinase that is essential for the bipolar attachment of chromosomes to the mitotic spindle and for maintaining the spindle assembly checkpoint until all chromosomes are properly attached. Mps1 is expressed at high levels during mitosis and is abundantly expressed in cancer cells. Disruption of Mps1 function induces aneuploidy and cell death.