In the tumor microenvironment, indoleamine 2,3-dioxygenase 1 (IDO1), a key regulator of T cell function, degrades tryptophan (Trp) to kynurenine (KYN), thereby inhibiting the function of effector T cells and promoting the differentiation and infiltration of immunosuppressive Treg cells [182]. The gene discussed is IDO1; the disease is neoplasm.