Two Americans won the Nobel Prize in medicine yesterday for discovering a way to silence specific genes, a revolutionary finding that scientists are scrambling to harness for fighting illnesses as diverse as cancer, heart disease, and AIDS. Andrew Fire, 47, of Stanford University, and Craig Mello, 45, of the University of Massachusetts Medical School in Worcester, will share the $1.4 million prize.

They were honored remarkably swiftly for work that they published together just eight years ago. It shed light on a process called RNA interference, which occurs in plants, animals, and humans. It’s important for regulating gene activity and helping defend against viruses. RNA is short for ribonucleic acid.

It is “a fundamental mechanism for controlling the flow of genetic information,” the Karolinska Institute in Stockholm, which awarded the prize, said.

Since the discovery, scientists have already made RNA interference a standard lab tool for studying what genes do. And they’re working to use it to develop treatments against a long list of illnesses, including asthma, cystic fibrosis, diabetes, flu, Parkinson’s and Huntington’s diseases, and age-related macular degeneration, a major cause of blindness.

“This has been such a revolution in biomedicine, everybody is using it,” Thomas Cech said. Mr. Cech is the president of the Howard Hughes Medical Institute, for which Mr. Mello is an investigator.

“It’s so important that people almost take it for granted already, even though it was discovered fairly recently,” Mr. Cech, who won a Nobel in 1989 for RNA research, said.

Nobel prizes are generally awarded decades after the work that they honor, so a prize now for a finding published in 1998 is striking.

But it’s appropriate, the president of the Cold Spring Harbor Laboratory in Cold Spring Harbor, N.Y., Bruce Stillman, said, because the work “is recognized now as one of the really revolutionary changes in the way we think about how genes are controlled.”

Genes produce their effect by sending molecules called messenger RNA to the protein-making machinery of a cell. The messenger RNA directs that machinery to produce a particular protein.

In RNA interference, certain molecules trigger the destruction or inactivation of the messenger RNA from a particular gene, so that no protein is produced. Thus, the gene is effectively silenced.

For instance, researchers have shown they can lower cholesterol levels in lab animals by suppressing a gene through RNA interference. Could this ability to block disease-promoting genes produce new treatments?

“In principle, it works. In controlled laboratory conditions, it works,” Mr. Cech said. “And the next five years will tell whether this is a whole new class of pharmaceuticals with the potential to defeat numerous human diseases.”

Mr. Mello, who is still doing research on RNA interference, said, “There’s a lot of work to do” to turn the basic work into drug treatments.

Ironically, when the congratulatory call from the Nobel committee reached his home at 4:40 in the morning, he was checking the blood level of his diabetic 6-year-old daughter.

“You don’t really appreciate how important the work of the last 50 years or so of modern molecular medicine … has been until you know somebody who is alive and well because of it,” he said.

Mr. Fire said the award showed the importance of publicly funding basic research that doesn’t appear to have any near-term payoff. He also said he had not decided how to use his half of the prize money. Mr. Fire, who was working for the Washington-based Carnegie Institution at the time of the discovery, and Mr. Mello did their groundbreaking experiment in a tiny worm called C. elegans.

They found that they could block the effect of a specific gene by injecting worms with a particular double-stranded version of RNA. Usually RNA has only one strand.

“That was the breakthrough that set the whole field on fire, no pun intended,” Mr. Cech said.