IDO1 and neoplasm: Apart from the intrinsic immune resistance caused by hyperactivation of TGF-β signaling, the tumor-infiltrating CTLs secrete interferon-gamma (IFN-γ) to activate the JAK/STAT (Janus kinase/signal transducer and activator of transcription) signaling pathway, then elevate the expression of the programmed death ligand 1 (PD-L1) and indoleamine 2,3-dioxygenase 1 (IDO-1), which in turn inactivates CTLs and leads to inducible immune evasion12–14.