We also observe transcripts from a small number of olfactory receptor pseudogenes, as previously described for three human olfactory receptor pseudogenes [26,44]. Although many fewer pseudogenes than intact genes were represented in our cDNA collection, some neurons in the olfactory epithelium evidently express disrupted olfactory receptors and thus might be unable to respond to odorants or to correctly innervate the olfactory bulb. Wang, Axel and coworkers have shown that an artificial transgenic olfactory receptor gene containing two nonsense mutations can support development of an olfactory neuron, but that pseudogene-expressing neurons fail to converge on a glomerulus in the olfactory bulb [45]. By analogy with an olfactory receptor deletion mutant [45], it is likely that most pseudogene-expressing neurons die or switch to express a different olfactory receptor gene, leaving a small number of pseudogene-expressing neurons in adult mice, but at greatly reduced levels compared to neurons expressing intact olfactory receptors.