all were defective in cytoplasmic microtubules. The spin- dle position checkpoint was not completely abrogated in these mutants, indicating that the lost proteins are not di- rectly necessary for Bub2p–Bfa1p activity and checkpoint function. Our observations of the ts-arp1 bim1D and ts- arp1 bim1D lte1D mutants show that inappropriate mitotic exit was preceded by the loss of cytoplasmic microtubules from the neck in at least 17/19 cells that divided without any SPB–neck interactions. It therefore seems that certain aspects of cytoplasmic microtubule behavior in these mu- tants are responsible for inactivation of Bub2p–Bfa1p leading to inappropriate mitotic exit. An attractive hypothesis that could potentially account for all of our observations is that interactions of cytoplas- mic microtubules with the neck provide an inhibitory sig- nal through Bub2p–Bfa1p that prevents exit from mitosis. In wild-type cells, penetration of the spindle through the neck leads to disappearance of cytoplasmic microtubule– neck interactions, allowing cells to trigger the mitotic exit pathway. However, if the spindle fails to enter the bud, cy- toplasmic microtubules continue to traverse the neck and thereby inhibit exit from mitosis. In bim1D (and other) mutants, alterations in cytoplasmic microtubule behavior occasionally cause the spindle to fall back into the mother after penetrating the neck, or (less frequently) keep the spindle in the mother but fail to maintain cytoplasmic mi- crotubule–neck interactions. The failure to maintain such contacts then triggers an inappropriate exit from mitosis. If loss of microtubules from the neck allows inappropri- ate mitotic exit, and then the persistence of microtubules in the neck after spindle movement into the neck should pre- vent mitotic exit. In kip3D and bud6D mutants, cytoplasmic microtubules often protrude through the neck after the spindle has moved into the neck (Miller et al., 1998; Straight et al., 1998; Segal et al., 2000). Even after the spin- dle has moved into the neck, mitotic exit is delayed in these mutants (Miller et al., 1998; Straight et al., 1998; Segal et al., 2000). In the bud6D mutant, it is clear that the delay in mi- totic exit occurs only in cells in which cytoplasmic microtu- bules remain in the neck after the spindle is properly posi- tioned (Segal et al., 2000). Together, these observations suggest that the spindle position checkpoint might monitor the presence of cytoplasmic microtubules in the neck. We thank Lea Ann Chen for technical assistance, Anna Zandy for per- forming blind scoring, and Rick Heil-Chapdelaine for helpful discussions. We are especially grateful to Michael Young, Wei-Lih Lee, Danny Lew, and reviewer number 3 for comments on the manuscript. N.R. 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