The metaphase-to-anaphase transition is triggered by the Anaphase-Promoting Complex (APC), an E3 ubiquitin ligase that targets proteins for degradation, leading to sister chromatid separation and mitotic exit. The function of APC is controlled by the spindle checkpoint that delays anaphase onset in the presence of any chromosome that has not established bipolar attachment to the mitotic spindle. In this way, the checkpoint ensures accurate chromosome segregation. The spindle checkpoint is mostly activated from kinetochores that are not attached to microtubules or not under tension that is normally generated from bipolar attachment. These kinetochores recruit several spindle checkpoint proteins to assemble an inhibitory complex composed of checkpoint proteins Mad2, Bub3, and Mad3/BubR1. This complex binds and inhibits Cdc20, an activator and substrate adaptor for APC. In addition, the checkpoint complex promotes Cdc20 degradation, thus lowering Cdc20 protein level upon checkpoint activation. This dual inhibition on Cdc20 likely ensures that the spindle checkpoint is sustained even when the cell contains only a single unattached kinetochore.