Weizmann scientists have identified a microRNA molecule miR135 that controls the levels of serotonin in the brain. Patients suffering from depression have unusually low miR135 levels in their blood. A treatment and diagnostic test are to be developed by miCure Therapeutics.
http://wis-wander.weizmann.ac.il/tiny-molecule-could-help-diagnose-and-treat-mental-disorders#.U6wBdECKhD0