AZ3146 is a potent and specific inhibitor of monopolar spindle 1 kinase (Mps1; IC50 = 35 nM), an important regulator of Spindle Assembly Checkpoint (SAC). AZ3146 treated HeLa cells override SAC, and quickly enter mitosis, with a reduced mitosis completion time of 32 minutes. 50% of treated cells entered anaphase without complete chromosomal alignment.