HCN subunits are present in immature, developing OSNs at E13 and colocalize with GAP43. At P2, some colocalization (yellow) with GAP43 persists with HCN2 and HCN1. However, primarily HCN and OMP are coexpressed in the mature OSNs (OMP+) which are located apically in the epithelium. A–A″, E13; GAP-43. B–B″, P2; GAP-43. C–C″, P2; OMP. A–C, HCN1. A′–C′, HCN2. A″–C″, HCN4. Scale bars, 25 μm; scale bar in A applies to A–B″; scale bar in C applies to C–C″.