IDO1 and neoplasm: Only a few studies have investigated the role of IDO1 and IL4I1 in HNSCC so far, with limited insights into the interplay of IDO1 and IL4l1, but they have demonstrated that IDO1 expression in HNSCC is positively correlated with several immune-related molecules, and that IL4I1 is a metabolic immune checkpoint that activates the aryl hydrocarbon receptor (AHR) and promotes tumor progression [18,19].