According to the World Health Organization, mood disorders such as depression affect about 10% of the world’s population, and many scientists around the world have invested a great deal of effort in understanding these disorders, yet the molecular and cellular mechanisms behind mental disorders are still only partially understood. None of the existing antidepressants are good enough: 60-70% of patients don’t get any help, and for the other 30-40%, help for the disease is often incomplete, and they have to take the medication for a very long time before they get any effect. In addition, some of the drugs show many side effects associated with the drugs. Thus, there is a clear need for new and better medications, and the first step is to better understand the disease process. Recently, Alon Chen, a professor at the Weizmann Institute, along with his doctoral student Orna Issler, studied the molecular mechanisms of the brain’s serotonin system, which, among other things, is involved in the development of depression and anxiety disorders when it is dysregulated. chen and his colleagues study the role of microRNA molecules in the serotonin-producing neuronal cells. role in serotonin-producing neuronal cells. The findings are published in the journal Neuron. The scientists noted that serotonin-producing nerve cells in brain regions had elevated levels of miR135 when given antidepressant compounds. Mice genetically engineered to produce higher than average amounts of miR135 turned out to be more resistant to constant stress, and the mice did not acquire any chronic stress-related behaviors such as anxiety or depression. In contrast, mice with low levels of miR135 expression showed more of these behaviors; moreover, the mice were weakly responsive to antidepressants. In other words, the brain needs appropriate miR135 levels, low miR135 levels but enough to ensure a healthy stress response, and high miR135 levels but enough to avoid depression or anxiety. When tested with human blood samples, the researchers found that depressed subjects had abnormally low levels of miR135 in their blood. the scientists found that the three genes involved in the production of miR135 were all located in regions of the genome known to be associated with bipolar affective disorder. These results suggest that miR135 may be a potentially therapeutic molecule.