MPS1/TTK inhibitor is an inhibitor of monopolar spindle 1 (MPS1/TTK; IC50 = 5.8 nM), a kinase involved in mitotic spindle checkpoint signaling that is overexpressed in certain cancerous tumors. It inhibits MPS1 phosphorylation of kinetochore scaffold 1 (KNL1) and increases the rate of mitosis and the number of cells entering anaphase within 15 minutes, indicating MPS1 checkpoint inhibition, when used at a concentration of 100 nM. MPS1/TTK inhibitor (50 and 100 nM) increases the number of missegregated chromosomes, with an increased number of errors at 100 nM compared with 50 nM. It also inhibits colony formation of DLD1, HCT116, and U2OS cells (IC50s = 24.6, 20.1, and 20.6 nM, respectively).