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Cover  The series of images on this month's cover shows how nuclear segregation is disrupted in a budding yeast mutant with impaired cytoplasmic microtubules. Starting from a single mutant cell with a GFP-labeled nucleus, the development of a microcolony is followed by video fluorescence microscopy. In wild-type cells (not shown), the nucleus is positioned close to the mother-daughter cell junction (called the bud neck) early in the cell cycle. The spindle and nucleus move together along the mother-daughter axis, with nuclear elongation into a bar shape concomitant with insertion into the bud. After nuclear division, this results in successful transit of one nucleus into the daughter cell. The process of cytokinesis then separates the cells. In the mutant cells (shown on the cover), the gene coding for Spc72p, a component of the cytoplasmic side of the microtubule organization center (MTOC) has been deleted. Lack of Spc72p prevents the formation of long and stable cytoplasmic microtubules. These microtubules are necessary for nuclear positioning, orientation, and successful nuclear segregation. Formation of nuclear microtubules organized by the nuclear side of the MTOC involved in spindle formation, elongation, and chromosome segregation appears unaffected when comparing spindle elongation kinetics between mutant and wild-type cells (Hoepfner et al., [2002] Mol. Biol. Cell 13, 1366-1380). Spindle elongation in the SPC72 deletion mutant cells takes place in the mother cell and frequently does not occur oriented along the mother-daughter cell axis. Consequently, transit of a nucleus through the mother-bud neck is drastically delayed or even fails, leading to an accumulation of nuclei in mother cells. This segregation defect does not induce a cell-cycle block, and all nuclei in mother cells undergo a simultaneous mitosis in the next cell cycle. Dividing cells with up to 12 nuclei have been observed in the mutant background. Interestingly, daughter cells without a nucleus remain attached to the mother cell, whereas daughter cells receiving a nucleus show cytokinesis with wild-type kinetics, suggesting the presence of a surveillance mechanism that monitors transit of a nucleus through the bud neck.Dominic Hoepfner