Cyclin D1 level can be regulated by multiple mechanisms. One is through transcriptional regulation. The present results showed that cyclin D1 mRNA level did not be changed by MRB treatment, which indicates that MRB-mediated downregulation may be inde- pendent on transcription. Another mechanism to regulating cyclin D1 level is through the activation of proteasomal degradation and cyclin D1 proteasomal degradation has been regarded as one of important anti-cancer mechanisms, previously reported with cur- cumin ( Mukhopadhyay et al. 2002 ), retinoic acid ( Spinella et al. 1999 ) and troglitazone ( Huang et al. 2005 ). In our results, MRB-mediated cyclin D1 downregulation was attenuated in presence of MG132, as a proteasome inhibitor. In addition, MRB treatment de- creased half-life of cyclin D1 protein in the cells exposed to CHX. These results indicate that MRB-mediated cyclin D1 downregula- tion may be attributed to activating proteasomal degradation.