In addition, we found that MRB significantly attenuated CDK4 expression. CDK 4 has been reported to be essential to G1/S progression by binding to cyclins D ( Malumbres and Barbacid 2009 ). Thus CDK4 downregulation has been regarded to be a potential anti-cancer target. From our result, CDK 4 may be a molecular target for MRB-mediated cell growth arrest.