Our findings for MRB-mediated downregulation of cyclin D1 protein level but not mRNA level indicate that MRB may affect the decrease of protein stability of cyclin D1. MG132 has been used for evaluating cyclin D1 proteasomal degradation because it effectively abolishes cyclin D1 degradation following the inhibition of protein synthesis ( Alao et al. 2006; Lin et al. 2006 ). Thus, the cells were pretreated with MG132 and then co-treated with MRB to evaluate MRB affects cyclin D1 proteasomal degradation. As shown in Fig. 3 A–C, pre-treatment of MG132 blocked MRB-induced decrease of cyclin D1 protein level in HCT116, SW480 and LoVo cells. In addition, cycloheximide is commonly used in studies on protein stability and degradation ( Alao 2007 ). To verify these results, the cells were-pretreated with DMSO or MRB, and then exposed to cy- cloheximide. As shown in Fig. 3 D and E, MRB treatment decreased half-life of cyclin D1 protein in HCT116 and SW480 cells. These data suggest that downregulation of cyclin D protein level by MRB is involved in the proteasomal degradation.