IDO1 and central nervous system cancer: It has been shown that the expression of IFNG gene signature (including IDO1, CXCL10, CXCL9, HLA-DRA, STAT1, and IFNG), as well as expanded IFNG gene signature (including 18 genes involved in the IFNG response and major downstream pathways), characterizes the T-cell inflamed phenotype well, acts as an effective indicator for screen potential responders to ICB therapy, and is a marker of improved prognosis for most tumors, except for gliomas (Danaher et al., 2018; Qian et al., 2018).