Disclosed is a method for diagnosing a mood disorder or susceptibility to a mood disorder, including depressive disorders and bipolar disorder, from a biological sample taken from a subject. The method includes detecting markers of monoamine oxidase-A (MAO-A) in the biological sample； determining MAO-A concentration from the markers； and correlating the MAO-A concentration in the biological sample to a control group which does not have a mood disorder in order to diagnose or determine susceptibility to the mood disorder in the subject. Also disclosed is a method of detecting peripheral markers of MAO-A for the diagnosis of a mood disorder or susceptibility to a mood disorder. Also provided are polypeptide markers.
