IDO1 and chronic kidney disease: Due to this correlation in serum and saliva kynurenine found by Buczko et al. [25] and the association with inflammation and IDO expression both in kidney failure during chronic kidney disease and kidney transplant rejection [22,26,27], and due to the findings that kynurenine is discussed as a biomarker for renal allograft failure in the literature [28,29], here, we focus on kynurenine determined from saliva as a potential biomarker for kidney transplant rejection.