a, Classification of the differentially expressed genes in O4-positive immature oligodendrocytes from developing (3-week-old) and aged (1.5-year-old) mouse brains. The top 100 differentially expressed genes were assigned to indicated gene ontology (GO) classes of biological processes. b, The number of oligodendrocytes after ML233 treatment. Cells were incubated with the indicated concentration of ML233 for 3 days. Relative cell number of the culture labeled with Olig2 (n = 4), P = 0.9. Each data point represents biologically independent experiments. Scale bar, 20 μm. Data are the mean ± s.e.m. and were assessed using one‐way ANOVA with post hoc Tukey’s multiple comparison test.