Of the 302 neurons in the C. elegans nervous system, 14 have been characterized as chemosensory, or olfactory, neurons that detect volatile odorants. Each type of chemosensory neuron is tuned to a specific odorant or set of odorants. For example, the AWA olfactory neurons are activated in reaction to diacetyl, thiazoles, and pyrazine. A single olfactory neuron expresses multiple types of olfactory receptors. Researchers test C. elegans via a chemotaxis assay to characterize the different olfactory receptors and their respective odorants. As an example, Sengupta et al (1996) matched the olfactory receptor gene “odr-10” specifically to diacetyl, and therefore to the AWA olfactory neuron.