Differential expression levels among six olfactory receptor genes determined by quantitative PCR. (a) Expression levels of olfactory receptor genes can vary by almost 300-fold (for example, genes A and D). Relative expression levels of six selected olfactory receptor genes (A, AY318555; B, AY318107; C, AY318644; D, AY317365; E, AY317773; and F, AY317797) were determined in olfactory epithelium RNA samples from three mice. Expression levels for each gene were first determined relative to a standard curve made using mouse genomic DNA templates, and then values for each mouse were normalized so that a housekeeping gene, ribosomal S16, had a value of 1 (arbitrary units) (not shown). Error bars show one standard deviation (six replicate reactions). Genes E and F (AY317773 and AY317797) are class I olfactory receptors. Numbers of cDNAs observed in our screen are shown under each gene name. (b) Expression levels of each gene are similar, with some variation, among the three mice sampled. Graphs show pairwise comparisons between the three mice sampled, with relative expression levels (arbitrary units) in one mouse plotted along the x-axis and in a second on the y-axis.