IDO inhibitors, such as 1-methyl-D-trytophan (indoximod) and epacadostat, enhance anti-tumor immunity either alone or in combination with mAbs against CTLA-4 or PD-1 in murine models [137–139] or in studies with CD19-CAR T cells in humanized mouse models [140] and are presently in clinical trials. The gene discussed is IDO1; the disease is neoplasm.