IDO1 and hepatocellular carcinoma: Indoleamine-2,3-dioxygenase 1 (IDO1) is an intracellular enzyme expressed by HCC and is the rate-limiting enzyme that catalyzes the metabolism of Tryptophan (Trp) along the kynurenic acid (Kyn) pathway, which leads to the inhibition of T cells and is responsible for tumor cells to escape monitoring and clearance of the immune system (6).