We have identified a honeybee (Apis mellifera) odorant receptor (Or) for the queen substance 9-oxo-2-decenoic acid (9-ODA) from four candidate sex pheromone odorant receptors from the honeybee genome based on their biased expression in drone antennae. Keywords: Tissue Comparison Overall design: Oligonucleotides (70-mer sense-strand) were designed using ArrayOligoSelector3.5 software (43, http://sourceforge.net) using default settings and the Apis mellifera genome (Assemby 2) for the following conceptual cDNA categories: 168 odorant receptors; 49 cytochrome p450s; 19 odorant binding proteins; 17 tetraspanins; 10 gustatory receptors; 10 carboxyl esterases; 7 glutathione S transferases; 6 chemosensory proteins (sensory appendage proteins); 5 lipocalins; 5 visual and non-visual opsins; 4 alpha-arrestins; 4 arrestins; 3 G-protein alpha subunits; 1 Pinocchio ortholog (a Drosophila melanogaster sensory-related protein); 1 sensory neuron membrane protein, and 4 housekeeping genes (cytoplasmic actin, EF1alpha, ribosomal protein 49, ribosomal protein S3), for a total of 313 genes. ArrayControl sense 70-mer oligonucleotides (Ambion) were printed as negative control spots. Four microarray hybridizations (replicates) were performed using mature drone antennal total RNA versus worker antennal total RNA with an even dye swap. Each total RNA pool was derived from an independent collection of 50 sets of antennae.