IDO is a key negative feedback enzyme in immunosuppressive TME formation (Campesato et al., 2020), which exerts its immunosuppressive function by facilitating the enzymatic process of conversion of L-tryptophan (Trp) to L-kynurenine (Kyn), which accumulates to provide the necessary nutrients for rapid tumor growth, and the degradation of the Trp interferes with the survival and activity of CTLs (Su et al., 2018). This evidence concerns the gene IDO1 and neoplasm.