Mps1 functions at multiple steps to inhibit Cdc20–APC/C. (A) All three groups (Hewitt et al., 2010; Maciejowski et al., 2010; Santaguida et al., 2010) demonstrate that at unattached kinetochores, Mps1 kinase activity is required to recruit other mitotic checkpoint components, including Mad1, Mad2, Bub1, BubR1, Bub3, and the Rod–Zw10–Zwilch complex. Discrepancies exist on exactly which components depend on Mps1 activity (see Table I for details). (B) Hewitt et al. (2010) show that Mps1 kinase activity maintains the recruitment at unattached kinetochores of O-Mad2 to the stably bound Mad1–C-Mad2 template. The molecular mechanism is yet to be elucidated. Mps1 may dimerize and be activated by self phosphorylation at kinetochores followed by quick release into the cytosol. (C) Maciejowski et al. (2010) demonstrate that Mps1 kinase activity in the cytosol promotes the assembly and/or prevents the disassembly of Cdc20–APC/C inhibitory complexes. Although the relative abundance and contribution of specific Cdc20–APC/C inhibitory complexes are unclear, all inhibit Cdc20 to prevent polyubiquitination of the key mitotic regulators cyclin B and securin by APC/C, an event which targets them for degradation as an irreversible trigger for anaphase entry.