IDO1 and head and neck squamous cell carcinoma: To further analyze clinical relevance, a six-gene “interferon-inflammatory immune gene signature” (IFNG signature, consisting of genes IFNG, HLA-DRA, CXCL9, CXCL10, IDO1 and STAT1) that has been identified as predictive for patient response to checkpoint inhibitors in melanoma and HNSCC, was interrogated in HNSCC [3],[22].