In addition, other studies have shown that depression in the inflammatory state is often accompanied by a disorder of the tryptophan–kynurenine (TRP–KYN) metabolic pathway, while indoleamine 2,3-dioxygenase (IDO), an enzyme involved in tryptophan metabolism, is a key factor connecting inflammation and depression [18,19]. The gene discussed is IDO2; the disease is depressive disorder.