Reduced levels of p27 rescue the peripheral nerve phenotype in Jab1-null mice. (A) Semithin and ultrathin sections of sciatic nerve from P30 Jab1^−/− and double Jab1^−/−p27^−/− mice. (B) Morphometric analysis of the endoneurial area occupied by bundles of unsorted axons, number of axons per each bundle, mean bundle size, and number of sorted myelinated axons in Jab1^−/− and double Jab1^−/−p27^−/− mice (n = 3 mice per genotype). (C) Analysis of fiber type distribution per axon diameter, mean g ratio, and g ratio per axon diameter in WT (n = 3 mice per genotype) and Jab1^−/− and double Jab1^−/−p27^−/− mice (n = 5 mice per genotype). (D) Neurophysiology showing NCV and cMAP in P30 WT, Jab1^−/−, and double Jab1^−/−p27^−/− mice (n = 3 mice per genotype). (E) Quantification of Schwann cell number in P5 sciatic nerves of WT, Jab1^−/−, and Jab1^−/−p27^−/− mice (n = 3 mice per genotype). (F) Percentage quantification of nuclei double positive for BrdU and Ki67 staining on the total number of Ki67-positive nuclei in sciatic nerve sections of P5 WT, Jab1^−/−, and Jab1^−/−p27^−/− mice (n = 3 mice per genotype). A representative immunofluorescence image of Jab1^−/−p27^−/− mice is shown. Bars: (A, top) 20 µm; (A, bottom) 2 µm; (F) 30 µm. Paired Student’s t test: *, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001. Error bars indicate SEM.