IDO1 and neoplasm: Cytokines are important players in the tumor microenvironment, and cytokines/chemokines are used to establish connections between various immune cells, such as IL-6 (52), IL-10 (53), IL-12 (54), IL-35 (55), epidermal growth factor (EGF), vascular endothelial growth factor (VEGF), tumor necrosis factor alpha (TNF-α) (56), interferon γ (IFN-γ) (57), indoleamine-2,3-dioxygenase (IDO) (58), and transforming growth factor beta (TGF-β) (59).