By Julie Steenhuysen

CHICAGO (Reuters) - When it comes to a man's body odour, the fragrance -- or stench -- is in the nose of the beholder, according to U.S. researchers who suggest a single gene may determine how people perceive body odour.

The study, published online on Sunday in the journal Nature, helps explain why the same sweaty man can smell like vanilla to some, like urine to others and for about a third of adults, have no smell at all.

"This is the first time that any human odorant receptor is associated with how we experience odours," Hiroaki Matsunami of Duke University in North Carolina said in a telephone interview.

Matsunami and colleagues at Duke and Rockefeller University in New York focused on the chemical androstenone, which is created when the body breaks down the male sex hormone testosterone.

Androstenone is in the sweat of men and women, but it is more highly concentrated in men. How one perceives its smell appears to have a lot to do with variations in one odour receptor gene called OR7D4.

"It is well known that people have different perceptions to androstenone. But people didn't know what was the basis of it," Matsunami said.

To find out, researchers in Matsunami's lab tested sweat chemicals on most of the 400 known odour receptors used by the nose to sniff out smells and chemicals.

They found the OR7D4 gene reacted strongly with the sex steroid androstenone. Next, they tested whether variations in this gene had an impact on how people perceived the smell of androstenone in male sweat.

They took blood samples and sequenced the DNA of 400 people who participated in a smell perception test done in Leslie Vosshall's lab at Rockefeller.

What they found is slight genetic variations determine whether androstenone has a pungent smell, a sweet, vanilla-like smell or no smell at all.

The role of androstenone is not well understood in humans, but in pigs it sends a powerful sex signal that puts sows in the mood for love.

"It facilitates the courtship behaviour in females," Matsunami said.

"There is some evidence published showing this chemical can modify the mood or hormone levels in humans," he said. "What we don't know is whether the receptor we found was in any way involved in this process."

He and colleagues will further study this aspect to understand how smelling these chemicals might affect human social and sexual behaviour.