IDO1 and hepatocellular carcinoma: Similar to IDO, TDO is expressed by a bevy of different tumor types including glioblastoma, melanoma, ovarian carcinoma, hepatic carcinoma, breast cancer, non-small-cell lung cancer, lymphoid and myeloid cancers, renal cell carcinoma, and bladder cancer, with high expression levels associated with a more rapid rate of tumor progression and/or decreased patient survival (33–36).