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Olfaction, the sense of smell, is the detection of chemicals dissolved in air (or, by animals that breathe water, in water). In vertebrates it is located in the nose. The importance and sensitivity of smell varies among different organisms: most mammals have a good sense of smell, whereas most birds don't. Among birds, it is important in the tubenoses. Among mammals it is well developed in the carnivores and ungulates, who must always be aware of each other, and in those, such as moless, who smell for their food. It is less well developed in the catarrhine primates, and nonexistent in cetaceans, who in compensation have a sensitive and well-developed sense of taste. The lack of olfaction is called anosmia.

Insects have the sense of smell on their antennae. In many species it is highly tuned to pheromones; a male silkworm moth, for example, can smell a single molecule of bombykol.

Many vertebrates have an auxiliary olfactory sense organ called Jacobson's organ or the vomeronasal organ, located in the vomer, between the nose and the mouth. Snakes use it to smell prey, sticking their tongue out and touching it to the organ. Some mammals make a face called flehmen to direct air to this organ, which detects pheromones. In humans it is subliminal.

Mammals generally have about 1000 genes for odor receptors. Each receptor cell in the nose expresses one of these genes. Each gene is expressed by thousands of cells, whose axons converge in the olfactory bulb. Humans have 347 functional odor receptor genes; the rest have nonsense mutations. This number was determined by analyzing the genome in the Human Genome Project; the number may vary among ethnic groups, and does vary among individuals. For example, some people can smell amyl acetate (which smells like bananas), whereas some others can not.