IDO1 and cancer: These findings have given rise to >90 clinical trials12 that examine the efficacy of inhibitors of two rate-limiting enzymes in the KYN pathway, namely indoleamine 2,3-dioxygenase-1 (IDO1)13 and tryptophan 2,3 dioxygenase (TDO2)14, either alone or in combination with immune checkpoint blockade, especially in the treatment of advanced stages of cancer.