Monastrol is a small, cell-permeable molecule that arrests cells in mitosis by specifically inhibiting Eg5, a member of the Kinesin-5 family. Monastrol treatment of dividing cells results in spindle collapse and cell cycle arrest with a monoastral spindle, which is similar to the phenotype observed when Eg5 is inhibited by anti-Eg5 antibodies. Monastrol is an allosteric inhibitor of the mitotic kinesin Eg5 that exhibits an antiproliferative effect against several cell lines. Monastrol treatment can decrease cell viability in MCF-7 tumor cells. Real-time cell growth kinetic analysis showed a decrease in the proliferation of MCF-7 cells exposed to monastrol.