Stockholm, Sweden ? An American and two Britons won this year’s Nobel Prize in medicine Monday for discoveries about how genes regulate organ growth and a process of programmed cell suicide. Their findings shed light on the development of many illnesses, including AIDS and strokes.

Britons Sydney Brenner, 75, and John E. Sulston, 60, and American H. Robert Horvitz, 55, shared the prize, worth about $1 million.

Working with tiny worms, the laureates identified key genes regulating organ development and programmed cell death, a necessary process for pruning excess cells. Many cancer treatment strategies are now aimed at stimulating the cell-death process to kill cancerous cells.

Brenner, a professor at the Salk Institute for Biological Studies in San Diego, Calif., is also the founder of the Molecular Sciences Institute in Berkeley. He showed that the tiny transparent worm C. elegans was useful for studying how cells specialize and organs develop. His work “laid the foundation for this year’s prize,” the awards committee said.

Brenner also demonstrated that a chemical could produce specific genetic mutations in the worm, allowing different mutations to be linked to specific effects on organ development.

Sulston, who is retired from the Wellcome Trust Sanger Institute in Cambridge, England, discovered that certain cells in the developing worm are destined to die through programmed cell death. He described visible steps in the cell-death process and demonstrated the first mutations of genes that participate in that process, the committee said.

Horvitz, of the Massachusetts Institute of Technology, identified the first two “death genes” in the worms and showed that humans have a gene similar to one of them, the awards committee said. Scientists now know that most genes controlling cell death in the worms have counterparts in humans.

Information about programmed cell death has helped scientists understand how some viruses and bacteria invade human cells, the Nobel committee said. In conditions such as AIDS, stroke and heart attack, cells are lost because of excessive cell death. In other diseases like cancer, cell death is reduced, leading to the survival of cells that are normally destined to die.

The award for medicine 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 today 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.