IDO1 and neoplasm: The immunomodulatory enzyme IDO1 inhibits the function, proliferation, and activation of T-cells, thus impeding T-cell-dependent immune responses.[19,20] Excessive IDO1 expression in tumors leads to a reduction in Trp and an increase in Kyn metabolites, creating a tumor microenvironment that suppresses the immune system and enables tumor-immune evasion.