The study identifies the complete set of 57 odorant receptor (DOR) genes in Drosophila, revealing a molecular basis for the organization of the peripheral olfactory system. Each DOR gene is expressed in a conserved, spatially restricted pattern of olfactory sensory neurons, with individual neurons likely expressing only one receptor gene. Neurons expressing a given receptor project axons to one or two spatially invariant glomeruli in the antennal lobe, forming a two-dimensional map of receptor activation. This map allows the brain to encode the quality of an odor through distinct spatial patterns of activity in the antennal lobe. In Drosophila, olfactory perception is similar to that in vertebrates, with sensory neurons expressing a single receptor gene and projecting to specific glomeruli. This organization is consistent with anatomical and functional studies showing that different odors elicit distinct patterns of glomerular activity. The study also reveals that individual sensory neurons express only one receptor gene, a finding that contrasts with the C. elegans system, where neurons may express multiple receptor genes. The DOR genes are widely dispersed in the genome, with most existing as single genes on each of the Drosophila chromosomes. The expression of DOR genes is restricted to olfactory sensory neurons in the antenna and maxillary palp, with no expression in other regions of the adult fly. The study demonstrates that individual sensory neurons express only one receptor gene, and that axons from neurons expressing a given receptor converge upon one or two spatially invariant glomeruli in the antennal lobe. This spatial organization supports the idea that the brain uses a topographic map to represent odor quality. The study also shows that olfactory sensory neurons in Drosophila project to both ipsilateral and contralateral antennal lobes, indicating that sensory information is processed in both hemispheres of the brain. The spatial arrangement of DOR gene expression in the antenna does not correspond to the spatial arrangement of glomerular targets in the antennal lobe, suggesting that the topographic map in the brain is not a direct reflection of receptor expression in the periphery. Overall, the study provides evidence that the olfactory system in Drosophila is organized in a manner similar to that in vertebrates, with a topographic map of receptor activation in the brain. This organization allows the brain to decode spatial patterns of activity in the antennal lobe to represent the quality of an odor. The findings suggest that the logic of olfactory discrimination has been maintained over 500 million years of evolution, reflecting an efficient solution to the complex problem of olfactory perception.The study identifies the complete set of 57 odorant receptor (DOR) genes in Drosophila, revealing a molecular basis for the organization of the peripheral olfactory system. Each DOR gene is expressed in a conserved, spatially restricted pattern of olfactory sensory neurons, with individual neurons likely expressing only one receptor gene. Neurons expressing a given receptor project axons to one or two spatially invariant glomeruli in the antennal lobe, forming a two-dimensional map of receptor activation. This map allows the brain to encode the quality of an odor through distinct spatial patterns of activity in the antennal lobe. In Drosophila, olfactory perception is similar to that in vertebrates, with sensory neurons expressing a single receptor gene and projecting to specific glomeruli. This organization is consistent with anatomical and functional studies showing that different odors elicit distinct patterns of glomerular activity. The study also reveals that individual sensory neurons express only one receptor gene, a finding that contrasts with the C. elegans system, where neurons may express multiple receptor genes. The DOR genes are widely dispersed in the genome, with most existing as single genes on each of the Drosophila chromosomes. The expression of DOR genes is restricted to olfactory sensory neurons in the antenna and maxillary palp, with no expression in other regions of the adult fly. The study demonstrates that individual sensory neurons express only one receptor gene, and that axons from neurons expressing a given receptor converge upon one or two spatially invariant glomeruli in the antennal lobe. This spatial organization supports the idea that the brain uses a topographic map to represent odor quality. The study also shows that olfactory sensory neurons in Drosophila project to both ipsilateral and contralateral antennal lobes, indicating that sensory information is processed in both hemispheres of the brain. The spatial arrangement of DOR gene expression in the antenna does not correspond to the spatial arrangement of glomerular targets in the antennal lobe, suggesting that the topographic map in the brain is not a direct reflection of receptor expression in the periphery. Overall, the study provides evidence that the olfactory system in Drosophila is organized in a manner similar to that in vertebrates, with a topographic map of receptor activation in the brain. This organization allows the brain to decode spatial patterns of activity in the antennal lobe to represent the quality of an odor. The findings suggest that the logic of olfactory discrimination has been maintained over 500 million years of evolution, reflecting an efficient solution to the complex problem of olfactory perception.