GRIK4 (glutamate receptor, ionotropic, kainate 4) is the gene that encodes KA1, a type of neurotransmitter receptor subunit that joins together with other subunits to form a glutamate receptor. These receptors are located on cells in the brain and are involved in enhancing cell-cell communication, which may play a role in major depressive disorder. This type of receptor primarily binds glutamate, a major neurotransmitter involved in developmental growth, learning and memory. Although the effect of variations in GRIK4 on KA1 function and expression are not fully known, KA1 may play a role in modulating the therapeutic effect of antidepressants. Mutations in GRIK4 may impact patients with major depressive disorder who take citalopram (Celexa®).^[49] Genetic variant tested for GRIK4 is c.83-10039T>C.