A microRNA, miR-1202, is downregulated in the brains of individuals with major depressive disorder. Patients who responded to treatment with antidepressants had lower levels of miR-1202 at baseline than did non-responders, and expression of miR-1202 increased during the course of treatment in antidepressant-responsive patients. miR-1202 regulates glutamate receptor 4 expression, which could explain these observations. miR-1202 could be a target for treating depression, or used as a biomarker to predict response.