Abstract: ObjectiveTo explore the synergistic effects of herceptin and 9-cis RA on the cell cycle progression of HER2/neu-positive breast cancer cells and associated molecular mechanism.MethodsAfter the treatment of MDA-MB453 cells with hercept in and 9-cis RA,flow cytometry assay was employed to detect the inhibitory effects on cell cycle prog ression.After that,mRNAand protein expression of cyclin E,CDK₂,and P27 were analyzed by RT-PCR and Western blotting.At the same time,the bingding affinity of cyclin E and CDK₂ was tested by immunopr ecepitation assay.ResultsHerceptin and 9-cis RAsy nergistically blocked the cell cycle progression of MDA-MB-453 cells in G₀/G₁ phase in vitro.The expression of cyclin Eand CDK₂ were no tably down-regulated as well as the cyclin E/CDK₂ binding affinity,while the P27 protein expression was remar kably up-regulated by the co-treatment.ConclusionHerceptin and 9-cis RAcould exert synergistic inhibitory effects on cell cycle progression of ErbB2-positive breast cancer cells by influencing the expression and activity of cyclin E,CDK₂ and P27.