The disappointing efficacy in GBM is likely due to the immunosuppressive and immune evasive tumour microenvironment (TME), which is influenced by suppressive factors such as Transforming Growth Factor-β (TGF-β), hypoxia, lactate, Prostaglandin E2 (PGE2), Indoleamine 2,3-dioxygenase (IDO), and adenosine [12,13,14,15,16,17]. The gene discussed is IDO1; the disease is neoplasm.