IDO1 and head and neck squamous cell carcinoma: A comparison of IDO1 expression data from TCGA shows that HPV‐positive HNSCCs have significantly higher IDO1 expression when compared to HPV‐negative HNSCCs (P = .001) and adjacent normal tissue (P < .001) (Figure 3)68 although the IDO expression is also elevated in HPV‐negative HNSCC when compared to normal tissue.