IDO1 and renal cell carcinoma: IDO1 converts L-tryptophan to N-formyl-L-kynurenine in tryptophan metabolism, and the three predicted MGPs are: “N-formyl-L-kynurenine-VHL-tryptophan metabolism” and “anthranilate-VHL-tryptophan metabolism” for the RCC samples collected in this study, and “N-formylanthranilate-VHL-tryptophan metabolism” for Kidney-RCC.