E2F1 and hepatocellular carcinoma: In the RT-PCR analysis, we found PS341 suppressed the expression of Cyclin A2, Cyclin B1, Cyclin D1, CDK1, CDK4 and E2F1 in the mRNA levels in HCC and CRC cells, in which the expression of Cyclin D1 and CDK4 were remarkably inhibited (Fig. 4b,c).