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The mammalian nose is arguably the best chemical detector on the planet, capable of detecting and discriminating among many thousands of compounds. This ability is mediated at the earliest steps by a large family of G protein (heterotrimeric guanine nucleotide-binding protein)–coupled receptors (GPCRs). The capacity to detect and discriminate odors depends on a combinatorial code, in which any given receptor recognizes many odors and any given odor compound might serve as a ligand at multiple receptors. Recent research adds a layer of complexity to the interpretation of this olfactory code, suggesting that the overall effect of a mixture of odorants is not simply equal to the sum of its parts. Rather, individual odorants can act as antagonists at the level of individual GPCRs, thereby suppressing some of the signaling pathways activated by structurally related compounds. Thus, the odor code not only is a function of the pattern of activated receptors, but also may be further sharpened by the action of antagonism. It seems that odor coding is now a division of pharmacology.