IDO1 and congenital rubella syndrome: The gene expression of IDO1, TPH2, and SLC3A1 in the duodenum, as well as that of TPH2, and SLC3A1 in the colon, was significantly increased (p < 0.05), (Figure 4A–H) by dietary tryptophan supplementation under CRS, indicating the activation of intestinal tryptophan, Kyn, and 5-HT metabolism.