IDO1 and colitis: Since it has been previously shown that the serum tryptophan depletion during colitis is associated with the up-regulation of indoleamine 2,3-dioxygenase 1 (Ido1) gene expression, one of the enzymes that catalyze the rate-limiting step in the kynurenine pathway [47,48], we measured the expression level of this enzyme in the intestinal samples taken from the test animals.