The scientists discovered a large gene family, comprising 1,000 different genes representing about 3 percent of all genes in the body. The olfactory genes give rise to an equivalent number of olfactory receptors located on 5 million cells in a small area in the upper part of the nostrils. The cells are highly specialized to detect molecules of a few inhaled odors.

The ability to detect and identify chemical substances in the environment offers an obvious survival benefit. But the number of odorant receptors varies among species.

For their studies, Axel and Buck used mice, which have about 1,000 odorant receptors. That is about 10 times the number in fish. Humans have about 350 such receptors.

"Smell is absolutely essential for a newborn mammalian pup to find the teats of its mother and obtain milk — without the olfaction the pup does not survive," said the Nobel Assembly, which pointed out that the olfactory receptor area in dogs is about 40 times larger than in humans.

Loss of smell can be a serious handicap for humans, who often lose substantial amounts of weight because food loses its appeal and taste.

Both researchers said that so far their importance has been in understanding basic mechanisms their findings, although they may someday lead to practical applications.

"It is absolutely certain that from understanding basic mechanisms will come information that is critical for treating disease, there is no doubt about that," Buck said. After her postdoctoral training at Columbia, she joined the faculty at Harvard before moving back to her hometown, Seattle, in 2002.

Someday, Axel said, scientists might identify the odor receptors in the nose of malaria-carrying mosquitoes and other disease-causing insects that attracts them to humans. If scientists could block an insect's ability to smell a food source, they might be able to create effective repellents that, unlike pesticides, would not harm other living things and the environment.

Axel, who was born in New York, has spent his entire career at Columbia except for receiving a medical degree from Johns Hopkins.

Both winners have received financial support from the Howard Hughes Medical Institute and the National Institutes of Health.