We propose herein that hepatitis C virus (HCV) infection promotes hepatocellular carcinoma (HCC) by augmentation of the tryptophan (Trp)-2,3-dioxygenase-kynurenine-aryl hydrocarbon receptor (TDO–Kyn–AhR) pathway, resulting in suppression of both anti-HCV and anti-tumour immune responses, as well as in tumorigenesis. The gene discussed is AHR; the disease is hepatocellular carcinoma.