IDO1 and melanoma: As an example, accumulation of serum kynurenine as a result of the IDO/TDO‐tryptophan‐kynurenine pathway is a poor prognostic factor for the overall survival of melanoma and renal cell carcinoma patients treated with nivolumab,355 suggesting that interference with this pathway might improve patient response to PD‐1 and possibly other checkpoint inhibitors.