Spindle checkpoint: trapped by the corona, cyclin B1 goes MAD.

The spindle checkpoint protects against aneuploidy by ensuring that dividing cells only proceed with chromosome segregation once all kinetochores are stably attached to spindle microtubules. The checkpoint protein MAD1 localizes to the corona, a structural expansion of the kinetochore forming in the absence of microtubule attachment, but molecular mechanism or functional significance of this localization remains unknown. Recent results now show that cyclin B1 recruits MAD1 to the corona and that this MAD1 pool is required for robust checkpoint signaling.