IDO1 and head and neck squamous cell carcinoma: has reported that patients with metastatic cancer (gastric cancer, head and neck squamous cell carcinoma, and melanoma) that responded to anti-PD-L1/PD-1 immunotherapy had a higher expression level of IFN-γ signaling-related genes (IFNG, CXCL9, CXCL10, HLA-DRA, STAT1, and IDO1) compared with the non-responders (33).