Our results revealed that SL4 treatment of MCF-7 and MDA-MB-231 cells resulted in downregulation of cdc2 along with an increase in the level of the inhibitory Tyr15-phosphorylated cdc2, suggesting that regulation of the CDK1 complex is the main mechanism by which SL4 induces G2/M phase arrest in breast cancer cells. The gene discussed is CDK1; the disease is breast cancer.