In situ hybridization (Figure 4) shows that increased numbers of expressing cells account for some, but not all, of the difference in transcript levels between two of the genes tested by real-time PCR (genes A and D in Figure 3). We hybridized alternate coronal serial sections spanning an entire olfactory epithelium of a young mouse (P6) with probes for gene A and gene D. Southern blot and BLAST analyses show that both probes are likely to hybridize to their intended target genes and no others (not shown). Gene A is expressed in zone 4 of the epithelium according to the nomenclature of Sullivan et al. [32] (Figure 4a). The expression pattern of gene D does not correspond to any of the four 'classical' olfactory epithelial zones [14,15,32]: positive cells are found in regions of endoturbinates II and III and ectoturbinate 3, resembling the expression pattern seen previously for the OR37 subfamily and ORZ6 olfactory receptors [33,34] (Figure 4b). Counting the total number of positive cells in alternate sections across the entire epithelium, we find that gene A is expressed in 2,905 cells, about 12 times more cells than gene D, which is expressed in a total of 249 cells. This 12-fold difference in numbers of expressing cells does not account for the almost 300-fold difference in RNA levels observed by real-time PCR, implying that the transcript level per expressing cell for gene A is about 25 times higher than transcript level in each expressing cell for gene D. We note that hybridization intensities per positive neuron appear stronger for gene A than gene D after comparable exposure times, in accordance with the idea that transcript levels are higher per cell. Thus, we suggest that expression in more cells and in higher levels per cell together account for the almost 300-fold higher olfactory epithelial RNA levels of gene A relative to gene D (Figure 3).