For patients with brain metastasis attributable to lung cancer, the expression of IDO-1 is markedly increased (72), and a phase II clinical trial (NCT02085070) has shown a 33% response rate after the administration of pembrolizumab (anti-PD-1 mAb) in patients with lung cancer brain metastasis, thus supporting that the higher expression of IDO-1 is associated with better response to anti-PD-1 treatment (6). Here, IDO1 is linked to lung cancer.