The sense of smell allows chemicals to be perceived as diverse scents. We used single-neuron RNA sequencing to explore the developmental mechanisms that shape this ability as nasal olfactory neurons mature in mice. Most mature neurons expressed only one of the ˜1000 odorant receptor genes (Olfrs) available, and at a high level. However, many immature neurons expressed low levels of multipleOlfrs.CoexpressedOlfrslocalized to overlapping zones of the nasal epithelium, suggesting regional biases, but not to single genomic loci. A single immature neuron could expressOlfrsfrom up to seven different chromosomes. The mature state in which expression ofOlfrgenes is restricted to one per neuron emerges over a developmental progression that appears to be independent of neuronal activity involving sensory transduction molecules.