Downregulation of Olig2 and TCF4 expression in Olig1 mutants. Spinal cord sections from E18.5 (a) wild-type and (b) Olig1 mutant embryos were subjected to immunofluorescence labeling (in red) with anti-Olig2 antibody. The number of Olig2+ cells was significantly reduced in the mutants. (c) Statistical analysis of Olig2+ cells in the Olig1+/- and Olig1-/- spinal cords compared to the wild-type (wt). Values were presented as mean ± standard deviation. (d) Regulation of the largest module shows that the Olig1 regulates the expression of Olig2. (e, f) Spinal cord sections from E18.5 wild-type (e) and Olig1 mutant (f) embryos were subject to in situ RNA hybridization with TCF4 antisense riboprobe. Expression of TCF4 was not detected in the mutants at this stage.