To ask whether susceptibility to malaria is dependent on tryptophan metabolism, we measured kynurenine and other tryptophan metabolites in Pc-infected mice lacking indoleamine 2,3-dioxygenase 1 (Ido1), one of two enzymes that catalyzes the rate-limiting step of the tryptophan pathway (Figure 2A; Fatokun et al., 2013). This evidence concerns the gene IDO1 and malaria.