IDO1 and neoplasm: Finally, we demonstrate that this transporter mediates tryptophan/kynurenine exchange, which, in cooperation with IDO, provides a new metabolic cycle that may contribute to the immune escape of various tumor cells as well as a favorable role of kynurenines in reducing autoimmunity, while at the same time protecting the cells that overproduce IDO from kynurenine accumulation.