Washington ? In the first study to show a direct genetic link between emotional stress and depression, researchers found that people with a certain type of brain chemistry gene were more vulnerable to deep depression after traumatic events such as the death of a family member.

The study, appearing this week in the journal Science, focused on two forms of a gene called 5-HTT that helps regulate serotonin, a brain chemical. Researchers found that adults who carried a short form of this gene were more prone to slip into depression after experiencing serious life events than were adults who carried a long form of 5-HTT.

Experts said the study was the first to show a proven direct genetic link between emotionally distressing events and the onset of clinical depression.

“It is a very important discovery and a real advance for the field,” said Dr. Thomas R. Insel, director of the National Institute of Mental Health. “The effects of stress on depression only get played out in certain individuals and those are individuals … with a particular variation of this single gene.”

Insel said the finding had no immediate clinical application, but it gave a fundamental new understanding of the link between genes and depression.

The World Health Organization has identified depression as the fourth-leading cause of disease burden, which is defined as years patients must live with a disability. It’s estimated that about 121 million people worldwide suffer from depression. The disorder is now being diagnosed more frequently and the WHO estimates depression will become the leading cause of disease burden worldwide by the year 2020.