BAY 1161909 is a potent and highly selective inhibitor of monopolar spindle 1 (Mps1) kinase activity. Human Mps1 is a serine threonine kinase which functions as a core component of the spindle assembly checkpoint (SAC), a key surveillance mechanism that monitors the attachment of spindle microtubules to the kinetochores of the chromosomes during pro-metaphase and halts the transitions to anaphase until all chromosomes are bi-oriented, fully attached, and correctly tensed at the metaphase plate. Mps1 is expressed in the mitosis phase of the cell cycle in proliferating cells. Overexpression of Mps1 has been observed in several cancer cell lines and tumor types including lung and breast cancers.