Model for KNL1-mediated Aurora B activation. In early mitosis, phosphorylation of outer kinetochore substrates relies on Aurora B kinase activity, which is dependent on the N terminus of KNL1. KNL1-mediated Bub1 kinase activity enhances Aurora B activation at both the inner centromere and the kinetochore. Upon generation of stable kinetochore–MT attachments and sister kinetochore bi-orientation, KNL1 is no longer able to mediate Bub1 kinase activity and Bub1 accumulation, resulting in decreased levels of active Aurora B. During metaphase, reduced levels of active Aurora B at the kinetochore region (DeLuca et al., 2011) lead to further stabilization of kinetochore–MT attachments and checkpoint silencing (mediated in part through PP1 binding; Meadows et al., 2011; Rosenberg et al., 2011).