GPR139 and major depressive disorder: Gpr139−/− mice also exhibited altered rapid eye movement (REM) sleep under normal conditions, where significantly fewer but longer REM episodes were observed, and showed a minimal response in sleep suppression induced by selective serotonin reuptake inhibitors (SSRIs) [50], suggesting that GPR139 is a potential target of treatment-resistant depression and schizophrenia, as well as other neuropsychiatric disorders.