Individual genes do not cause depression, but they are thought to increase the probability of an individual having a depression in the face of other accumulating risk factors, such as other genes and environmental stressors.
One gene that has been shown to increase the risk for depression in the context of multiple stressful life events is the gene for the serotonin transporter protein. This gene is responsible for making the protein that is targeted by all current drug treatments for depression. In a number of studies it has been shown that people who inherit one form of this gene, called SLC6A4, are at up to four times the risk of depression if they experience unusual stresses in their lives. Basic science experiments and imaging studies in normal people suggest that the way this form of the gene affects risk for depression is by impacting on the development of a system in the brain that mediates how negative environmental stresses and threats feel. The effects of this serotonin gene on this brain system are thought to occur early in development, where the shaping of brain systems related to how the environment is experienced emotionally is critically determined.