The OL population is significantly diminished in R-Ras1^−/−, R-Ras2^−/−, and R-Ras1^−/−;R-Ras2^−/− mice. A, Immunolabeling with anti-Olig2 (green) of longitudinal optic nerve sections from control, R-Ras1^−/−, R-Ras2^−/−, and R-Ras1^−/−;R-Ras2^−/− adult mice (P90) showing a reduction in the OL population in the mutant mice compared with the controls. B, Simplified scheme of the visual tract showing the region of analysis. C, Quantification of the density of Olig2-positive cells per square millimeter showing a 19.1 ± 7.5% decrease in R-Ras1^−/− (*p = 0.02), a 33.5 ± 2.1% reduction in R-Ras2^−/− (***p < 0.0001), and a 42.8 ± 11.8% reduction (**p = 0.003) in R-Ras1^−/−;R-Ras2^−/− mice relative to controls. Reduction of OLs in the R-Ras1^−/− mutant mice was significantly less than that observed in the R-Ras2^−/− (p = 0.003) and R-Ras1^−/−;R-Ras2^−/− (p = 0.02) mutant mice. The bar graph represents the mean and SD for the number of Olig2-positive cells per square millimeter from six different experiments (n = 6). Scale bars, 75 μm.