The ISX expression strike enhanced the levels of tryptophan catabolic enzymes, indoleamine 2,3-dioxygenase 1 (IDO1), tryptophan 2,3-dioxygenase, and immune checkpoints regulators PD-L1 and B2-7 (CD86) in hepatocellular carcinoma cells (Wang et al. 2017). Here, IDO1 is linked to hepatocellular carcinoma.