The Kyn generated by this reaction is further converted into the high-energy substrate NAD (+) And ATP, which energize cell activity, and in tumor cells, IDO activity significantly increases, which causes the lack of Trp in TME and the accumulation of downstream product Kyn, while the depletion of Trp and the excessive accumulation of Kyn induce effects T cell apoptosis, dysfunction and induction of immunosuppressive regulatory T cells. This evidence concerns the gene IDO1 and neoplasm.