IDO1 and neoplasm: Constitutively activated STAT3 in tumor cells not only eliminates anti-tumor immune response by continuously promoting IL-6 (91), IL-10 (91) or vascular endothelial growth factor (VEGF) (92) in the TME, but also transcribes and activates key oncogenes involved in immunosuppression such as programmed cell death-ligand 1 (PD-L1) (93), indoleamine 2, 3-dioxygenase 1 (IDO1) (94).