The typical hyperactivation of the IDO1 pathway in an inflammatory microenvironment, which is usually characterized by increased release of IFNγ [33], could also account for the statistically significant association between a high Kyn/Trp ratio and a high density of TILs at the tumor site among the squamous cell carcinoma group of the current study. Here, IDO1 is linked to squamous cell carcinoma.