Olfaction—the sense of smell—is key for all animals to enjoy pleasant stimuli (the scent of food, flowers, perfume) and avoid dangerous substances (smoke, rotting food, leaking cooking gas). This is an interesting sensory modality to study, because at the cellular and neural level it must recognize and categorize thousands of different chemicals. Olfactory neurons express odorant receptor proteins that interact with subsets of these odorants and relay odor-specific information directly to the primary olfactory relay in the brain.

A number of interesting and perplexing questions remain in the field:

1. How do vertebrate olfactory neurons choose to express a single odorant receptor gene from a repertoire of roughly one thousand genes? This transcriptional choice is further restricted to gene expression from either the maternal or paternal chromosome. The mechanism by which this choice is effected—and how the other 999 genes are silenced—is unknown. Choosing to express a single odorant receptor is critical for the neuron to be sensitive to a specific subset of odors that interact with a given odorant receptor.
2. What is the relationship between the chemical nature of an odorant and its perceived smell? There is at present no way to predict how a given odor will smell, given only information about its chemical structure. A lively debate in the field concerns whether the shape and functional groups of an odor are relevant for odor perception, or whether the vibrational energy of the chemical bonds composing the odorant is important.
3. How do odors stimulate recollection of past events? We know that odors are particularly powerful triggers for memories of past events. Although the neuroanatomy of smell is relatively well described, we do not fully understand why and how olfactory cues are more efficient than visual or auditory cues at stimulating recollection of past events.

The prime importance of olfaction to animals makes it possible to study different aspects of these questions in flies, mice, and humans.