Our results revealed that 27 complement genes, including C1QA, C1QB, C1QC, C1R, C1S, C3, C5, C6, C7, C8A, C8B, C9, CFB, CFD, CFI, CFP, CFHR3, CFHR4, C5AR1, SERPING1, MBL2, MASP1, C4BPA, CLU, CPN1, VSIG4, and CR1, were significantly downregulated in HCC tissues than in normal tissues (p < .05). The gene discussed is C1S; the disease is hepatocellular carcinoma.