Tumor cells produce immunosuppressive cytokines (e.g., IL-10, TGF-β, PGE2), recruit Tregs, activate negative regulatory pathways (e.g., PD-1/PD-L1, CD80/CTLA4) or express immunomodulatory molecules (e.g., IDO), thus establishing an immunosuppressive microenvironment. The gene discussed is IDO1; the disease is neoplasm.