Olfactory discrimination is achieved through the action of olfactory neurons with diverse chemical specificities. In C. elegans, at least ten different types of chemosensory neurons respond to different chemicals. The odr-7 gene is required for the function of one pair of chemosensory neurons called AWA neurons. odr-7 null mutants fail to respond to all odorants detected by the AWA neurons, while a missense mutation in odr-7 causes a specific defect in one odorant response. odr-7 encodes a protein with similarity to the DNA-binding domain of the nuclear receptor genes; it is expressed predominantly in the AWA neurons. odr-7 may regulate the expression of olfactory signaling molecules that define a single type of olfactory neuron.