We noted that a small number of one-cell embryos could not be inhibited atmetaphase but entered anaphase. Indeed, not all embryos can express the exogenous mRNA smoothly or in time, thus the translated protein may not be sufficient or it may be sufficient but delayed in the recruitment to the kinetochores. In HeLa cells, depletion of Mad2 shortens the duration of mitosis from 60 to 100 min and roughly halves the interval from NEBD to anaphase, leading to chromosome missegregation. To determine whether depletion of SAC would accelerate mitotic progression, we examined the duration of mitosis and found that it was approximately 30 min shorter in SAC-depleted embryos compared with controls. To further explore the role of SAC as spindle checkpoint proteins, we cultured one-cell embryos with nocodazole to determine whether the metaphase-anaphase transition was blocked by this drug. We found that in the presence of nocodazole, mitosis in the nocodazole-control embryos was severely arrested. However, nocodazole treatment did not result in significant alterations in SAC depleted embryos. Thus, depletion of SAC abrogates the metaphase arrest induced by nocodazole, which is consistent with findings in somatic cells and oocytes, further indicating that SAC Hyperforin functions as checkpoint in early embryos. We surmised that the accelerated progression observed in embryos following depletion of SAC might result in aneuploidy. We therefore performed chromosome spreading and found that a large number of SAC depleted embryos exhibited incorrect numbers of chromosomes, which provided clear evidence for aneuploidy. In fact, the actual rates of aneuploidy may even be 1,5-Isoquinolinediol underestimated, because only one blastomere of the two-cell embryos was used for chromosome spreading to avoid mixture of chromosomes from two blastomeres. The vast majority of control embryos displayed normal chromosome alignment with only approximately 1/14 being misaligned. In contrast, following SAC depletion, misalignment was found in approximately half of the embryos. Of these embryos, some were severely misaligned, whereas others displayed a few lagging chromosomes. Mitosis in somatic cells may provide an explanation for this phenotype. Following 30%Mad2 depletion, HeLa cells fail to arrest at metaphase in the presence of nocodazole and prematurely inactivate Cdk1. Following reduction in Mad2 levels, however, spindle assembly and chromosome condensation are seriously perturbed, cyclin B is degraded and mitosis is shortened by approximately.