Tryptophan hydroxylase-2 (TPH-2) gene may play an important role in maintaining the normal level of serotonin in the central nervous system [46,47], and, as suggested by Ping et al. (2019) [17], neurotransmission by serotonin in the brain may be associated with changes in white matter integrity in patients with major depressive disorder. The gene discussed is TPH2; the disease is major depressive disorder.