According to New York Times report those people in whose families there are instances of close members suffering from depression are found to be more vulnerable to the disease. A person in whom depression is caused due to hereditary presence does not easily response to any kind of medications used for treating depression. Dr. Xiaodong Zhang and Dr. Marc Caron, who are well known biologists at Duke University Medical Center along with a team of other researchers, found that the transformation of a single gene decreases the quantity of serotonin released by brain cells. Serotonin is a hormone produced by the brain which is strongly linked to the mood. An increase in the serotonin production in the brain causes positive feelings in an individual but if the level falls then it leads to depression. According to a research conducted on 87 individuals suffering from severe depression it was found that about 10% of the individuals had the gene mutation which means there is a genetic association, and it is the cause of depression. Due to gene mutation the serotonin in the brain tends to decrease which leads to depression and various other mental problems. Zoloft and Prozac are the most common anti depressant drugs which are very effective in regulating the serotonin level in the human brain and thus help to control the human moods.