The process, called RNA interference, also is being studied for treating such conditions as hepatitis virus infection and heart disease. It already widely used in basic science as a method to study the function of genes.

Fire, 47, of Stanford University, and Mello, 45, of the University of Massachusetts Medical School in Worcester, published their seminal work in 1998.

RNA interference occurs naturally in plants, animals and humans. The Karolinska Institute in Stockholm, which awarded the $1.4 million prize, said it is important for regulating the activity of genes and helps defend against viral infection.

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

Erna Moller, a member of the Nobel committee, said their research helped shed new light on a complicated process that had confused researchers for years.

"It was like opening the blinds in the morning," she said. "Suddenly you can see everything clearly."

Genes produce their effect by sending molecules called messenger RNA to the protein-making machinery of a cell. In RNA interference, certain molecules trigger the destruction of RNA from a particular gene, so that no protein is produced. Thus the gene is effectively silenced.

For instance, a gene causing high blood cholesterol levels was recently shown to be silenced in animals through RNA interference.

The prize for Mello and Fire came remarkably quickly after they did the work. Nobels are generally given decades after the research they honor.

Mello, reached at his home in Shrewsbury, Mass., said the award came as a "big surprise."

"I knew it was a possibility, but I didn't really expect it for perhaps a few more years. Both Andrew and I are fairly young, 40 or so, and it's only been about eight years since the discovery."

He said he would try to get to work Monday but expected to accomplish "not a lot."

Fire, reached in California, said he was awakened by a call from the Nobel committee.

`At first I was very excited.... Then I thought I must be dreaming or maybe it was the wrong number," he said. But then he confirmed the good news by checking the Nobel Web site.

"It makes me feel great. It makes me feel incredibly indebted at the same time," he said. "You realize how many other people have been major parts of our efforts."

Fire conducted his research while at the Washington-based Carnegie Institution.

The announcement opened this year's series of prize announcements. It will be followed by Nobel prizes for physics, chemistry, literature, peace and economics.

Last year's medicine prize went to Australians Barry J. Marshall and Robin Warren for discovering that bacteria, not stress, causes ulcers.

Alfred Nobel, the Swedish inventor of dynamite, established the prizes in his will in the categories of literature, peace, medicine, physics and chemistry. The economics prize is technically not a Nobel but a 1968 creation of Sweden's central bank.

Winners receive a check, handshakes with Scandinavian royalty, and a banquet on Dec. 10 _ the anniversary of Nobel's death in 1896. All prizes are handed out in Stockholm except for the peace prize, which is presented in Oslo.

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