Er81 regulates the expression of Nrg1-Ig, but not Nrg1-CRD, in DRG neurons. A–C, Isoform-specific in situ hybridization against Nrg1-Ig shows a significant decrease in the number of cells expressing Nrg1-Ig transcript in L4/L5 DRGs from P3–P5 Er81 mutants in an apoptosis-deficient background relative to controls (6.36 ± 0.65 Nrg1-Ig+ cells per 100,000 μm2 in Er81+/−;Bax−/− controls, 4.09 ± 0.48 Nrg1-Ig+ cells per 100,000 μm2 in Er81+/−;Bax−/− controls; p = 0.04). Some positive cells are indicated by arrows. D–F, Isoform-specific in situ hybridization against Nrg1-CRD shows no difference in the number of DRG neurons expressing a high level of Nrg1-CRD transcript between controls and mutants (9.94 ± 1.76 Nrg1-CRD+ cells per 100,000 μm2 in Er81+/−;Bax−/− controls, 10.47 ± 0.77 Nrg1-CRD+ cells per 100,000 μm2 in Er81+/−;Bax−/− controls; p = 0.79). N = 3 animals per genotype, ≥8 sections per animal. *p < 0.05. Not significant: p ≥ 0.05. Error bars indicate SEM. Scale bar, 100 μm.