IDO1-mediated Kyn accumulation activates the aryl hydrocarbon receptor (AhR) pathway, promoting the differentiation of naive T cells into regulatory T (Treg) cells, and proliferation of other immunosuppressive cells, including myeloid-derived suppressor cells (MDSC), thus restraining anti-tumor immune responses [4,5,6,7]. The gene discussed is IDO1; the disease is neoplasm.