We found that there were six complement genes, including C1R, C6, C7, C9, CFP, and CFHR3, whose mRNA expressions were significantly lower in HCC samples than in their normal tissue counterparts according to three data sets—TIMER2, GEPIA, and GEO with a fold change >2 or <0.5 and p < .05. The gene discussed is C6; the disease is hepatocellular carcinoma.