PLK1 and hepatocellular carcinoma: We found that the promoter methylation levels of CDK1, BUB1, ETV4, PLK1, WISP3, CCNB2, CISH, CCNB1, CCNA2, and GAGE2A were significantly decreased in hepatocellular carcinoma compared with normal tissues, while the promoter methylation levels of STK32B, SOX11, HOXC8, and BUB1B were significantly increased compared with normal tissues (Figure 17).