Abstract

Odorant receptors (ORs) belong to a large gene family of rhodopsin-like G protein–coupled receptors (GPCRs). The mouse OR gene family is composed of ∼1000 OR genes, and the human OR gene family is composed of ∼400 OR genes. The OR genes are spread throughout the genome, and can be found in clusters or as solitary genes in almost all chromosomes. These chemosensory GPCRs are expressed in highly specialized cells, the olfactory sensory neurons of the nose. Each one of these neurons expresses a single OR gene out of the complete repertoire of genes. In addition, only one of the two homologous alleles of the chosen OR gene, the maternal or the paternal, is expressed per neuron. Here we review recent findings that help to elucidate the mechanisms underlying monogenic and monoallelic expression of OR genes.