The honey bee genome sequence reveals a remarkable expansion of the insect odorant receptor (Or) family relative to the repertoires of the flies Drosophila melanogaster and Anopheles gambiae, which have 62 and 79 Ors respectively. A total of 170 Or genes were annotated in the bee, of which seven are pseudogenes. These constitute five bee-specific subfamilies in an insect Or family tree, one of which has expanded to a total of 157 genes encoding proteins with 15%-99% amino acid identity. Most of the Or genes are in tandem arrays, including one with 60 genes. This bee-specific expansion of the Or repertoire presumably underlies their remarkable olfactory abilities, including perception of several pheromone blends, kin recognition signals, and diverse floral odors. The number of Apis mellifera Ors is approximately equal to the number of glomeruli in the bee antennal lobe (160-170), consistent with a general one-receptor/one-neuron/one-glomerulus relationship. The bee genome encodes just 10 gustatory receptors (Grs) compared with the D. melanogaster and A. gambiae repertoires of 68 and 76 Grs, respectively. A lack of Gr gene family expansion primarily accounts for this difference. A nurturing hive environment and a mutualistic relationship with plants may explain the lack of Gr family expansion. The Or family is the most dramatic example of gene family expansion in the bee genome, and characterizing their caste-and sex-specific gene expression may provide clues to their specific roles in detection of pheromone, kin, and floral odors.[Supplemental material is available online at www.genome.org.]The olfactory abilities of the honey bee have long been admired and studied. Similar to most insects, they employ olfaction in many contexts, most prominently in locating flowers and in social communication. A foraging worker bee may encounter a bewildering number of flowers to choose from, and yet they can discriminate between them using subtle olfactory cues (for review, see Galizia and Menzel 2001), even sensing differences between plant genotypes (see Wright et al. 2002). After only a single visit to a reward, the odor associated with the reward can be correctly identified 90% of the time (Menzel and Erber 1978). Many of the studies of learning and memory in bees therefore employ olfaction as the sensory modality, and much is known about the organization of the antennal lobe and neural processing of olfactory information. In particular, the number of glomeruli in the antennal lobe has been estimated at ∼165 with variation from 160-170 (see Galizia and Menzel 2001). This relatively high number of glomeruli compared with 47 or 48 for Drosophila melanogaster (Laissue et al. 1999;, ∼60 for Anopheles gambiae (R. Ignell, pers. comm.), and 60-66 for moths (Berg et al. 2002;Ai and Kanzaki 2004;Skiri et al. 2005) fits well with the enhanced olfactory range and discriminatory abilities of the honey bee. Drosophila olfactory receptor neurons (ORNs) generally express a single olfactory receptor protein (Or; in addition to the widely-e...