TDO2 and hepatocellular carcinoma: HCC cells and other cells within their microenvironment (e.g., immune cells) highly express the two key rate-limiting enzymes in the tryptophan catabolic pathway—the kynurenine pathway: indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase 2 (TDO2).