STOCKHOLM — Two British and an American researcher won this year's Nobel Prize in medicine or physiology for discoveries concerning how genes regulate organ development and a process of cell suicide.

Sydney Brenner and John E. Sulston of Britain and American H. Robert Horvitz shared the prize, worth about $1 million.

The laureates have identified key genes regulating organ development and programmed cell death. The discoveries have shed new light on the development of many illnesses, including AIDS, neurodegenerative diseases and strokes.

The announcement opened a week of Nobel Prizes that culminates Friday with the prestigious peace prize, the only one revealed in Oslo, Norway.

The physics award will be announced Tuesday and the chemistry and economics awards Wednesday in the Swedish capital.

As in years past, the date for the literature prize has not been set. But it always falls on a Thursday, usually the same week as the other awards.

The award committees make their decisions in deep secrecy.

Alfred Nobel, the Swedish industrialist and inventor of dynamite, left only vague guidelines in his will establishing the prizes, first awarded in 1901.

For the prize Monday, he simply stated the winner ``shall have made the most important discovery within the domain of physiology or medicine.''

The 18 lifetime members of the Swedish Academy who choose the literature laureate make their final decision at one of their weekly meetings, only setting the date early in the same week to keep the world guessing.

Kaj Schueler, a literary editor at Swedish daily newspaper Svenska Dagbladet, predicted the academy's choice would be a surprise since last year's award went to perennial favourite V.S. Naipaul.

``I also think it's time for them to pick a poet,'' Schueler said, declining to single out any names. ``The last poet they had was the Polish writer Wislawa Szymborska in 1996. Since then they've had playwrights and prose writers.''

The only public hints are for the peace prize.

The five-member awards committee never reveals the candidates, but sometimes those making the nominations announce their choices.

With the world still reeling from last year's Sept. 11 terrorist attacks and concerned about U.S. plans for a war in Iraq, no clear favourites have emerged.

Among the nominees were Afghan President Hamid Karzai, who has sought to unify his country after the hardline Taliban was ousted by U.S.-led air strikes, former New York City mayor Rudolph Giuliani, the Salvation Army and the U.S. Peace Corps.

U.S. President George w. Bush and British Prime Minister Tony Blair were nominated for leading the war against terrorism but were seen as unlikely winners in the wake of their efforts to convince the world of the need to overthrow Saddam Hussein.

The Nobel Assembly at the world-renowned Karolinska Institute, which selects the medicine prize winner, invites nominations from previous recipients, professors of medicine and other professionals worldwide before whittling down its choices in the fall.

Last year's winners were Leland H. Hartwell of the United States and R. Timothy Hunt and Paul M. Nurse from Britain for discovering key regulators of the process that lets cells divide, which is expected to lead to new cancer treatments.

The awards always are presented on Dec. 10, the anniversary of Nobel's death in 1896.