actin structures in other groups, except for Lat B and Reversine groups, is significantly enhanced after the metaphase arrest. Treatment with Reversine induces anaphase entry, suggesting that the formation of the ring-like structure is associated with SAC or metaphase-anaphase transition, while Plk1 also participates in the transition. On the other hand, deconstruction of astral microtubule or loss of motion power of Myosin can lead to a more apparent ring-like structure, suggesting potential relations between microtubule, actin filaments and Myosin in the formation of the ring-like structure. Treatment of chemical compounds alters spindle position and symmetric division The reported participation of F-actin structure in the regulation of spindle Salianic acid A chemical information positioning is the precondition of contractile ring formation and symmetric division. To explore the consequence of attenuated formation of the ring-like structure by the treatment of drugs, we measured the spindle position with the same groups of cells in Fig. 3 and Fig. 4 and cell area ratio of daughter cells in anaphase. To measure the spindle position, we drew two circles to determine the centroid of spindle or cell on the Z = 0 plane. The first circle highlights the spindle while the other one outlines the cell. Coordinates of the centroids of the circles are used to calculate the offset distance between them. Dividing the distance by the radius of the cell gives the relative distance which is expressed as D/R ratio. From the viewpoint of the model, the curves of D/R along time represent the average motion 
of two spindle poles regardless of the dynamics in midzone. To measure the cell area ratio of daughter cells in anaphase, we drew two polygons to measure the area of each cell and the ratio equals the larger area divided by the smaller area. The image is projected on Z axis according to the maximum intensity. We measured the groups treated with these drugs and made time course curves of spindle position. Comparing with relative intensity of ring-like F-actin structure of each group displayed in Fig. 3D, the spindle position of the DMSO group is stable along time, while the spindles of the groups treated with BI2536, Reversine and Lat B are getting further away from the center of cell over time. The spindle position of the Blebbistatin group shows a slight recovery over the same period. Considering the relative intensity, the delay in the formation of ring-like F-actin structure is correlated with the perturbation of spindle position, which is probably due to less obstruction in cytoplasm. The inhibition of Myosin by Blebbistatin delays accurate spindle positioning, suggesting another pathway regulating spindle positioning. Together, these results indicated that the mitotic kinases such as Plk1 and Mps1 and motile Myosin are essential for accurate spindle positioning, which is coupled with the formation of the ring-like F-actin structure. To assess how the drug treatment influences spindle PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19655565 position in a single cell over time, we examined the spindle position by living cell images of each group. We found that the spindle positions in the cells treated with drugs except for DMSO have higher variance than the spindle position in DMSO group. Both the formation of ring-like F-actin structure and the spindle position in Lat B group are perturbed. However, we should not attribute the perturbed positioning totally to the perturbed formation of ring-like F-actin structure, because the cortical Fac