IDO1 and cancer: The Trp-catabolic enzymes (TCEs), indoleamine-2,3-dioxygenase 1 and 2 (IDO1 and IDO2) and tryptophan-2,3-dioxygenase (TDO2), which mediate the first step of the kynurenine pathway (KP), are upregulated in diverse cancer entities.