In the brain, glutamate is synthesized from glutamine by glutaminase, and metabolized to glutamine by glutamine synthetase (GS) in astrocytes.5, 6, 7, 8 Accumulating evidence suggests that abnormalities in the glutamatergic neurotransmission via the N-methyl-d-aspartate receptor (NMDA-R) have a key role in the pathophysiology of major depressive disorder (MDD).5, 6, 8, 9, 10, 11, 12 Hashimoto et al.13 reported increased levels of glutamate in the prefrontal cortex in postmortem brain samples from MDD and bipolar disorder, suggesting a role of the glutamatergic system in mood disorders. Here, GLUL is linked to bipolar disorder.