Anterior-to-posterior distribution of β-galactosidase (X-gal staining, top),OR-Z6 (in situ hybridization,middle), and OR37E (in situ hybridization, bottom) in separate but adjacent nose sections of the same P21 H-lacZ6 mouse. Starting at the organ of Masera (0 mm), 15 μm coronal cryosections were collected, and the labeled cells of every 10th section were counted and plotted along the anterior-to-posterior axis. The three expression profiles shown are nearly bilaterally symmetrical, and each of the three neuron phenotypes shows the highest density of reactive ORNs in the mid-to-caudal aspect of the OE at ∼2.6 mm posterior to the organ of Masera. ORNs expressing the lacZ transgene were approximately twice as numerous as wereOR-Z6⁺ neurons. Whereas a small number of β-gal⁺ ORNs was evident in the region up to 1.2 mm posterior to the organ of Masera, almost noOR-Z6⁺ or mOR37E⁺ neurons were found in this anterior aspect. The unusually large number of mOR37E⁺ neurons is caused by cross-hybridization. The riboprobe used derives from the mOR37E coding region that is highly homologous among the four members of this subfamily, all of which are expressed in the same zone (Strotmann et al., 2000). The total numbers of labeled ORNs in the left or right nose of the H-lacZ6 mouse analyzed were as follows: X-gal⁺, 5680 or 5780;OR-Z6⁺, 3400 or 3240; and mOR37E⁺, 10,230 or 10,410, respectively.