IDO1 and major depressive disorder: Previous clinical studies suggested that depression may be the result of a disorders of expression and polymorphism located in genes encoding enzymes involved in the tryptophan catabolism pathway, including Tph1/Tph2 (tryptophan hydroxylase 1/2), Ido1 (indoleamine 2,3-dioxygenase 1), Kat1/Kat2 (kynurenine aminotransferase 1/2), Kmo (kynurenine 3-monooxygenase), and Kynu (kynureninase) [5,6,7,8,9,10].