IDO1’s catalytic action causes significant immunosuppressive effects in TME because of the reduction of the essential amino acid tryptophan and the accumulation of immune-active kynurenines, which promote the differentiation of T regulatory cells (Tregs), the suppression of effector T and NK cells, and the differentiation of tolerogenic dendritic cells (DCs), with a resulting tumor immune tolerance (3, 4). The gene discussed is IDO1; the disease is neoplasm.