Gustatory and odorant receptor which mediates acceptance or avoidance behavior, depending on its substrates. Gr21a and Gr63a together are sufficient for carbon dioxide detection and avoidance behavior. It is possible that the CO(2) receptors Gr63a and Gr21a activate the TRPC channels through Galpha49B and Plc21C. This innate olfactory avoidance behavior can be inhibited by inhibitory interactions of the odors such as 1-hexanol and 2,3-butanedione with Gr21a and Gr63a. Gr21a and Gr63a probably form a heterodimer that responds to CO(2). Expressed in the adult labellar chemosensory neurons. Carbon dioxide-responsive neurons coexpress Gr21a and Gr63a in a pair of chemosensory receptors at both larval and adult life stages. A single bilateral neuron, expressing the Gr21a receptor, is responsible for CO(2) detection in larvae. The Myb-MuvB complex mediates neuron-specific expression of the carbon dioxide receptor genes Gr63a and Gr21a. Impairs CO(2) avoidance of larvae. Belongs to the insect chemoreceptor superfamily. Gustatory receptor (GR) family. Gr21a subfamily. Truncated N-terminus.