Several lines of studies have demonstrated that the overexpression of IDO1 in either tumor cells or in other cells within TME is associated with a more aggressive cancer phenotype, advanced disease stage and worse clinical outcome of various cancers including ovarian carcinoma, CRC, endometrial cancer, melanoma, cervical cancer, glioblastoma, lung adenocarcinoma, diffuse large B-cell lymphoma (DLBCL)[25–29, 31–36, 38]. The gene discussed is IDO1; the disease is cancer.