IDO2 and tricho-dento-osseous syndrome: It should also be noted that there are other enzymes, including tryptophan-2,3-dioxygenase and IDO2, which are closely related to IDO1, yet they could have complementary or differential roles during immunosuppression: TDO triggers the same Kyn/AhR downstream signalling with IDO1 that they show nearly identical bioactivity (76); IDO2 is documented to implement in the IDO1-dependent regulation of T cells (77), but it is also reported that IDO1 and IDO2 may show opposite roles in immune responses (78).