Optical density readings were obtained from film autoradiograms in individual nuclei and averaged across serial sections from multiple runs of in situ hybridization histochemistry. Hybridization levels of ³³P-labeled cRNA probes were converted from density units to units of radioactivity using¹⁴C standards exposed on the same sheet of film (see Materials and Methods). In some cases, values represent individual nuclei, and for others they represent average values in groups of nuclei. Hybridization levels are schematically illustrated in ascending order of intensity ranging from levels <100 nCi/gm to levels >600 nCi/gm. Note the high levels of hybridization for the α₁, β₂, and γ₂ cRNA probes, especially in the lateral geniculate and other sensory relay nuclei.