A method of treating a bipolar disorder in a subject in need thereof is disclosed. The method comprising administering to the subject sa therapeutically effective amount of a miR-135, a precursor thereof or a nucleic acid molecule encoding said miR-135 or said precursor thereof, thereby treating the bipolar disorder. Methods of diagnosing a mood disorder in a human subject and of monitoring treatment of an anti-depressant drug or a medicament for the treatment of a mood disorder are also disclosed.