IDO1 and pancreatic neoplasm: In pancreatic cancer tissues, levels of 3-IAA, 5-HT, KYNU, IDO, and TPH1 are significantly elevated, while levels of MAOA, 3-hydroxyanthranilic acid 3,4-dioxygenase (HAAO), and quinolinic acid phosphoribosyltransferase (QPRT) are markedly downregulated (Fig. 1) [77–79, 87].