Mitotic segregation of chromosomes depends on correct assembly of the bipolar spindle and mitosis is delayed by the spindle assembly checkpoint (SAC). In the July 19 Nature, Gachet et al., at the National Institute for Medical Research and University College London, describe a different mitotic checkpoint in yeast (Nature 2001, 412:352-355). They investigated the link between organization of the actin cytoskeleton and the cell cycle in the fission yeast Schizosaccharomyces pombe. They synchronized cells in early G2 and then grew them in the presence of latrunculin B (Lat B), an inhibitor of actin polymerization. Lat B-treated cells arrested in mitosis with short and misoriented spindles. Actin depolymerization affected Cut1-mediated cohesin cleavage and sister chromatid separation. The effect of Lat B was independent of SAC activation. Gachet et al. screened yeast mutants to identify checkpoint components and found that cells lacking the Atf1 factor were...