IDO1 and autosomal dominant polycystic kidney disease: Hopp et al. identified tryptophan metabolites as being highly associated with disease progression in the ADPKD model, the Pkd1RC/RC mouse29, and building on this foundation, Nguyen et al. demonstrated that knockout of the key tryptophan metabolism enzyme, indoleamine 2,3-dioxygenase 1 (IDO1), in this model reduced cystic index and kidney to body weight ratios54.