Another EAA, tryptophan, follows a metabolic fate that has acquired great visibility in the last years due to its implication in cancer immunotherapy: kynurenine produced from tryptophan by the enzyme indoleamine 2,3-dioxygenase (IDO) induces immunosuppression (Fallarino et al., 2003; Greene et al., 2019) by binding to and activating the transcription factor aryl hydrocarbon receptor (AhR) (DiNatale et al., 2010; Opitz et al., 2011). The gene discussed is IDO1; the disease is cancer.