"This year's Nobel laureates have discovered a fundamental mechanism for controlling the flow of genetic information," the jury declared.

Their discovery, called RNA interference and which occurs in plants, animals and humans, was published in 1998.

That leaves a bare eight years between publication and a Nobel award, which approximates to a record for fast-track recognition. A Nobel is typically awarded decades later, when history proves that the research was truly groundbreaking.

"RNA interference is already being widely used in basic science as a method to study the function of genes and it may lead to novel therapies in the future," the jury said.

Mello said he was stunned by the Nobel Committee's speedy recognition.

"I was very surprised, mainly because I'm fairly young and I thought maybe there were so many other discoveries worthy of a Nobel prize," Mello, born in 1960 and a professor of molecular medicine at the University of Massachusetts Medical School, told Swedish Radio.

"I just assumed it was something that might come several years from now," he said just after receiving a telephone call from the Nobel committee in the middle of the night.

"It's still sinking in I think, I can hardly believe it."

Fire, a 47-year-old professor at Stanford University School of Medicine in California, said he was "very happy" to be honoured.

"At first of course one doesn't believe it. It could be a dream or a mistake or something like that. I guess it's not," he told the radio.

Genes make proteins, the molecules that comprise and maintain all the body's tissues. They set the protein-making machinery in motion through a gofer molecule called messenger RNA, or mRNA.

In 1998, Fire and Mello, working together on nematode earthworms, discovered a mechanism that interferes with mRNA -- RNA interference (RNAi). RNAi, they discovered, is a natural molecular switch, regulating gene expression in plants and animals as well as humans.

By "silencing" over-active or malfunctioning genes, researchers hope to be able to devise a new generation of treatments for virus infections, cardiovascular disease, hormonal disorders and a range of inherited health problems.

It is also hoped that their research will be used in agriculture.

"Their discovery clarified many confusing and contradictory experimental observations ... (and) heralded the start of a new research field," the Nobel committee said.

The pair published their discovery in the journal Nature in 1998, and in 2002 the US medical journal Science named RNAi as the breakthrough of the year.

But the science is very new and analysts caution that technical problems and safety concerns remain to be resolved before RNAi therapies enter the medical vocabulary.

Last year, the Medicine Prize went to Australians Barry J. Marshall and J. Robin Warren for their pioneering research on stomach ulcers, overturning conventional wisdom to prove they are caused by bacteria and not spicy food or stress, and best treated with antibiotics.

Fire and Mello will each receive a gold medal and a diploma and will share the prize sum of 10 million Swedish kronor (1.37 million dollars, 1.07 million euros).

The Nobel prizes, founded by Swedish industrialist Alfred Nobel, the inventor of dynamite, were first awarded in 1901.

The Physics prize will be announced on Tuesday and Chemistry on Wednesday. The Economics prize, awarded by Sweden's central bank, the Riksbank, is scheduled for October 9.

The Literature prize is traditionally awarded on a Thursday, though the actual date is only announced 48 hours in advance. It is expected to be announced on either October 5 or October 12.

The announcement of the Peace Prize will wrap up the Nobel season on October 13.

The formal awarding of the prizes will take place in Stockholm on December 10.