The three bright-field images illustrate the criteria by which olfactory neurons coexpressing thelacZ transgene and OR-Z6 were identified.OR-Z6 in situ hybridization was performed using a digoxigenin-labeled OR-Z6 riboprobe, and hybridization signals were detected and visualized using an alkaline phosphatase-conjugated anti-digoxigenin antibody and a substrate that developed a purple precipitate. Left, Hybridization signals were most prominent in the apical cytoplasmic portion and appeared triangular (arrows). In addition we observed a thin line surrounding the nucleus; the nucleus itself was usually white. Right, In contrast, ORNs that solely express the lacZ transgene (arrow) were gray, including the cytoplasm covering the nucleus. Expression of β-galactosidase was detected using a primary anti-β-galactosidase antibody and visualized using a substrate yielding a gray precipitate.Middle, The double-labeled neuron shows a combination of both features described. The gray cytoplasm covering the nucleus indicates β-galactosidase expression (arrow), whereas the peak-shaped purple precipitate in the apical cytoplasm corresponds to OR-Z6 in situ hybridization (arrowhead). The three differently labeled neurons shown derive from the same coronal cryosection of a 35-d-old H-lacZ6 mouse and were closely associated on endoturbinate-II. Scale bar, 20 μm.