Preclinical studies targeting cytotoxic T-lymphocyte-associated protein-4 (CTLA-4), programmed death-ligand 1 (PD-L1), and indolamine 2,3-dioxygenase (IDO) have shown that therapeutic effects are associated with re-activation of CD8+ tumour-infiltrating lymphocytes (TILs) within the TME [10]. This evidence concerns the gene IDO1 and neoplasm.