Most previous studies have shown the relationship between IDO activity at baseline and prognosis of patients with acute myeloid leukemia (29, 30), breast cancer (31), cervical cancer (32), lymphoma (33–35), colorectal cancer (36), glioma (37), lung cancer (22, 23, 38–41), and melanoma (42, 43) (Table 3). Here, IDO1 is linked to lung cancer.