Setting Limits

The ability to perceive the chemical world is a universal feature of life. In mammals, the olfactory system detects and discriminates between vast numbers of distinct volatile molecules (recently estimated at an astonishing > 1012 in humans) providing critical information that allows an organism to respond appropriately to changes in its environment. The universe of volatile odorants is accommodated by a large gene family of greater than 1,000 odorant receptors (ORs), employed by the olfactory sensory neurons (OSNs) to commence a cascade that ends in the perception of smell. The ORs are seven-transmembrane G-protein coupled receptors encoded by genes found in large and small arrays that are scattered throughout the genome. ORs are expressed in the olfactory epithelium in an intriguing manner: each OSN chooses a single OR family member and transcribes it from only one allele. This monogenic and monoallelic mode of gene regulation has captivated the interest of neuroscientists and biologists across disciplines. Stochastic Choice