We have identified and sequenced 1,264 odorant receptor cDNAs from 419 olfactory receptor genes, confirming their expression in the olfactory epithelium. We have thus validated the similarity-based prediction of over one-third of the intact olfactory receptor genes annotated in the mouse genome [1,2], thereby vastly increasing the proportion of the family for which experimental evidence of olfactory function is available. We have not found cDNAs for all olfactory receptor genes or an even phylogenetic distribution of cDNAs, probably because the libraries and/or our screen are biased toward certain olfactory receptor subfamilies. Using RT-PCR with both degenerate and specific primers, we have confirmed olfactory expression of a number of additional olfactory receptors, bringing the total number of olfactory receptor genes verified in this study to 436, and ensuring that almost all phylogenetic clades have at least one representative with evidence of olfactory function.