Chromosome passenger complex (CPC). (A), Localization of the CPC during mitosis in Xenopus XTC cells: green, tubulin; blue, Dapi; red, Aurora B. Arrow points to midzone localization of Aurora B (reproduced with permission from Bolton et al,  ASCB). (B), Model of the CPC depicting the relationship of survivin and borealin to INCENP's N-terminal region. The C-terminus of INCENP contains the "IN Box" that tightly binds Aurora B. (C), Model of the two-step activation of CPC Aurora B kinase activity. Initial phosphorylation of the T-loop on Aurora B results in partial activation. Phosphorylation of INCENP at Serine 850 results in full activation. Structural and biochemical studies suggest that Aurora B is trans-autoactivated (c-terminus of INCENP shown in blue). (D), Aurora B phosphorylation target motifs.