Smell genes must wander

Choosing which 1 of 1,300 odorant receptor genes to express depends on a random interchromosomal encounter, acccording to Stavros Lomvardas, Richard Axel, and colleagues (Columbia University, New York, NY). The receptor gene that nestles nearest the single enhancer available in the genome is the single gene that gets expressed in a given cell.

Various other models for odorant receptor choice had already been deemed unlikely or incorrect. A unique set of transcription factors for each receptor sounded too complicated. And irreversible DNA recombination to place a single receptor gene at a single expression site was clearly not happening, as mice cloned from the nucleus of a mature olfactory neuron expressed the full repertoire of odorant receptors.

The Columbia group started with the H enhancer, which was known...