IDO1 expressing immune cells, especially macrophages, were found to be more abundant in malignant tissues and associated with worse prognosis of many cancer types, such as penile squamous cell carcinoma (Zhou et al., 2020), oral squamous cell carcinoma of advanced stages (Struckmeier et al., 2023), and classical Hodgkin lymphoma (Karihtala et al., 2020). Here, IDO1 is linked to oral cavity squamous cell carcinoma.