For two of the class I genes we had shown to be expressed, we determined relative transcript levels using quantitative RT-PCR (see below). Expression levels were similar to those observed for genes that were represented in our cDNA collection, suggesting that class I olfactory receptors are not under-represented in the olfactory epithelium, and that the dearth of class I cDNAs in our screen is likely to be due to bias in the libraries and/or hybridization probes.