STAT3 and head and neck squamous cell carcinoma: In head and neck squamous cell carcinoma (HNSCC), phosphorylated STAT3 can directly bind to the ARG1 promoter region of MDSCs to promote transcription, thereby contributing to the immunosuppressive effect of MDSCs.188 ARG2 has also been reported to promote melanoma tumor metastasis through the H2O2-STAT3 pathway (Fig. 8).189 Multiple tissue-derived tumor samples show a large number of Arg1-positive myeloid cells in the tumor microenvironment.