Owing to the strong rational to combine an IDO1 inhibitor with ICI, ipilimumab (anti-CTLA4 antibody), nivolumab (anti-PD-1 antibody), or pembrolizumab (anti-PD-1 antibody) have been evaluated in combination with epacadostat for the treatment of melanoma, non-small cell lung cancer (NSCLC), colorectal cancer (CRC), head and neck squamous cell carcinoma (HNSCC) and renal-cell carcinoma (RCC). Here, IDO1 is linked to head and neck squamous cell carcinoma.