The depletion of cellular Trp along with the generation of downstream Kyn induced by IDO1 can activate various immunosuppressive cells (regulatory T cells (Tregs)21, tumour-associated macrophages (TAMs)22, and myeloid-derived suppressor cells (MDSCs)23 while rendering the anergy and apoptosis of effector T cells21,24,25, yielding immunosuppression in tumours. The gene discussed is IDO1; the disease is neoplasm.