The analysis of expression patterns in tryptophan catabolic markers and immune checkpoint regulators in tissue samples revealed significant correlations between core protein and the mRNA expression levels of PD‐L1, CD86, IDO1, and TDO2 in HCC tissue samples collected from patients with HCV infection (Pearson's correlation coefficient, r = 0.7621, 0.7466, 0.7425, and 0.7609, respectively; Figure 7c–f). The gene discussed is IDO1; the disease is hepatocellular carcinoma.