At high concentrations in cell culture (1 mM), evidence has been presented that D-1MT/indoximod can elevate transcription of IDO1 leading to increased production of kynurenine in cancer cells (79), but the concentrations used in this study, which exceed by ~100-fold the serum levels of indoximod achieved in patients in clinical trials (50), cast doubt on the physiological relevance of this observation. This evidence concerns the gene IDO1 and cancer.