WASHINGTON (Reuters) - A gene associated with depression and other forms of mental illness may enlarge an area of the brain that handles negative emotions, U.S. researchers reported on Wednesday.

The study is one in a number which shows that the brains of people with depression are structurally different than the brains of people who are not depressed.

Writing in the journal Biological Psychiatry, researchers from the University of Texas Southwestern Medical Center looked at a serotonin transporter gene, which has two forms, or variants -- short, or SERT-s, and long, SERT-l.

People with two SERT-s genes had pulvinars, a brain region which handles negative emotions, that were 20 percent larger and contained 20 percent more nerve cells than people with either one or two SERT-l genes.

The gene also affects serotonin, a message-carrying chemical or neurotransmitter associated with mood, and one targeted by certain classes of antidepressant drugs, said the researchers, who had studied the brains of 49 people who had died.

The gene is a serotonin transporter since when brain cells release serotonin, the gene brings it back into the cell.

Depression drugs slow this process down, making serotonin available to the cells for longer.

Depression is a common condition, affecting about 121 million people worldwide, according to the World Health Organization.

In the United States an estimated 21 million American adults -- or 9.5 percent of the population -- have depression at some point.

Dr. Dwight German, a professor of psychiatry who worked on the study, said similar studies have shown that certain other areas of the brain are smaller in people with the SERT-s gene.

German's team estimated that about 17 percent of the population has two copies of the SERT-s gene. These people appear to be more sensitive to emotional stimuli and more likely to experience depression than people with one or no SERT-s genes, they said.

Some experts believe that antidepressants help remodel the brain.

"The brain is wired differently in people who have depression, and probably from the point of view of treatment, we should try to identify these people as early as possible and intervene before the 'hard-wiring' gets altered," German said.

Many brain regions are involved in depression, and studies show that several different types of treatment, including drugs and cognitive therapy, are usually needed before patients can be cured.