The human nose can distinguish one trillion different scents—an extraordinary feat that requires 10 million specialized nerve cells, or neurons, in the nose, and a family of more than 400 dedicated genes. But precisely how these genes and neurons work in concert to pick out a particular scent has long puzzled scientists. This is in large part because the gene activity inside each neuron—where each of these 10 million neurons only chooses to activate one of these hundreds of dedicated genes—seemed far too simple to account for the sheer number of scents that the nose must parse.