IDO1 and neoplasm: Interestingly, the STAT3/IL-6 axis is assumed to regulate the constitutive expression of IDO1 in tumor cells (16), leading to the hypothesis that this axis could regulate the high-level expression of IDO1 in the tumor microenvironment and adjacent tissues of HPV-related tumors (27, 45–48), since both molecules are co-expressed in TC-1 cell mouse model or cervical cancer patients.