They discovered a family of about 1,000 genes that give rise to a huge variety of proteins that sense particular smells. These proteins are found are found in cells in the nose, which communicate with the brain.

Axel, 58, of the Howard Hughes Medical Institute and Columbia University in New York, shared the prize with Buck, 57, of the Fred Hutchinson Cancer Research Center in Seattle.

They reported the gene discoveries jointly in 1991 and have since worked independently shedding further light on the olfactory system.

The medicine prize includes a check for $1.3 million, but it's the aura of prestige a Nobel Prize confers that candidates crave most.

There are no set guidelines for deciding who wins. Alfred Nobel, the inventor of dynamite who endowed the awards that bear his name, simply said the winner "shall have made the most important discovery within the domain of physiology or medicine."

The Nobel Assembly at Stockholm's Karolinska Institutet, 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 prize winners were Briton Sir Peter Mansfield and American Paul C. Lauterbur for discoveries that led to the development of MRI, which is used by doctors to get a detailed look into their patients' bodies.

The award for medicine opens a week of Nobel Prizes that culminates Oct. 11 with the economics prize. The peace prize, the only one bestowed in Oslo, Norway, will be announced Oct. 8. The physics award will be announced Tuesday and the chemistry prize will be announced Wednesday in the Swedish capital.

A date for the Nobel Prize in literature has not yet been set by the Swedish Academy, but is likely to fall on Thursday, Nobel watchers said.

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