IDO2 and major depressive disorder: Further elucidation of the mechanism of monoamine neurotransmitter decline revealed that the activity of indoleamine 2, 3‐dioxygenase (IDO) in the serum of patients with depression significantly increased, and the breakdown rate of tryptophan accelerated; this inhibited the metabolism of tryptophan in the 5‐HT pathway and reduced the 5‐HT concentration in the synaptic cleft, accelerating the occurrence of depression [38].