1. Howard Hughes Medical Institute, Programs in Developmental Biology, Neuroscience, and Genetics, Department of Anatomy and Department of Biochemistry and Biophysics, The University of California, San Francisco, California 94143-0452, USA

Correspondence to: Cornelia I. Bargmann Correspondence and requests for materials should be addressed to C.I.B. (e-mail: Email: cori@itsa.ucsf.edu).

Caenorhabditis elegans senses at least five attractive odours with a single pair of olfactory neurons, AWC, but can distinguish among these odours in behavioural assays¹. The two AWC neurons are structurally and functionally similar, but the G-protein-coupled receptor STR-2 is randomly expressed in either the left or the right AWC neuron, never in both². Here we describe the isolation of a mutant, ky542, with specific defects in odour discrimination and odour chemotaxis. ky542 is an allele of nsy-1, a neuronal symmetry, or Nsy, mutant in which STR-2 is expressed in both AWC neurons². Other Nsy mutants exhibit discrimination and olfactory defects like those of nsy-1 mutants. Laser ablation of the AWC neuron that does not express STR-2 (AWC^OFF) recapitulates the behavioural phenotype of Nsy mutants, whereas laser ablation of the STR-2-expressing AWC neuron (AWC^ON) causes different chemotaxis defects. We propose that odour discrimination can be achieved by segregating the detection of different odours into distinct olfactory neurons or into unique combinations of olfactory neurons.