A model depicting the proposed distinctions between local and global MPF activation in wild-type and cut12.1 cells. The cartoons show the two types of signaling event that we believe the cut12.1 mutant phenotype and relationship between cut12⁺ and cdc25⁺ have revealed. (A) In addition to a global signaling event that commits the cell to mitosis (green), we propose that local activation of MPF (red) is required on individual SPBs to promote the insertion of these SPBs into the nuclear envelope (gray lines). (B) In cut12.1 mutants, the old (orange) but not the new SPB (gray) is competent to promote local activation. It is unclear at present whether the signal for global commitment in cut12.1 mutant cells comes from the old or new SPB, so this signal is omitted from this panel.