Indoleamine 2,3-dioxygenase (IDO) is a kind of immune regulation enzyme.IDO is the only rate-limiting enzyme that can catalyze the oxidative clearage of indole ring in tryptophan and its catabolism along the kynurenine pathway.IDO1 is one of the important mechanisms in maternal immune tolerance,which participates in the regulation of maternal-fetal immune relationship.The newly discovered IDO2 also plays an important role in this relationship.The abnormal expression of IDO2 is associated with the male infertility.