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The anti-tumor activity of diosgenin, a new steroidal constituent present in fenugreek, on two human breast cancer cell lines, MCF-7 and Hs578T, was studied. Diosgenin treatment resulted in cell growth inhibition, cell cycle arrest, and apoptosis in concentration- and time-dependent manners in both cell lines. Western blot analyses of whole cell lysates for cell cycle proteins showed that diosgenin altered phosphorylated cyclin checkpoint1 (p-Chk1^Ser345) and cyclin B expression, which resulted in G2/M phase blockade. Mechanistically, Cdc25C-Cdc2 signaling was involved in inactivating Chk1^Ser345 by p53-dependence in MCF-7 cells and p21-dependence in Hs578T cells that are p53-deficient. Moreover, diosgenin induced a significant loss of the mitochondrial membrane potential in breast cancer cells, and prominently affected cell death through down-regulation of the anti-apoptotic protein, Bcl-2. This released cytochrome c and activated the caspase signaling cascade. Taken together, these findings reveal that the anti-proliferative activity of diosgenin involves the induction of G2/M phase arrest via modulating the Cdc25C-Cdc2-cyclin B pathway and mitochondria-mediated apoptosis in human breast cancer cell lines. This suggests the potential usefulness of diosgenin in treating breast cancer.