STOCKHOLM, Sweden — Americans Andrew Fire and Craig Mello won the Nobel Prize in physiology or medicine today for discovering a way to turn off the effect of specific genes.

"RNA interference" is already being widely used in basic science as a method to study the function of genes and it is being studied as a treatment for virus infections, heart diseases, cancer and several other conditions.

Fire, of Stanford University, and Mello, 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 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.

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 silenced.

Fire, who conducted the research while at the Carnegie Institution, said he was honored that the work "has received such positive attention."

"Science is a group effort. Please recognize that the recent progress in the field of RNA-based gene silencing has involved original scientific inquiry from research groups around the world," he said in a statement.

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 Marshall and Robin Warren for discovering that bacteria, not stress, causes ulcers.