Isoform-specific rescue of Rbfox3 knockdown–induced defects in neuronal differentiation by the Numb-12ex isoform. (A–O) Immunostaining of the E4 neural tube for neuronal markers. The right side of the neural tube in each panel has been transfected with siRbfox3 and the indicated expression construct, as demonstrated by GFP and HA-tagged Numb immunostaining in A–C. Both Numb-12ex-HA and Numb-12in-HA exclude exon 6. Bars, 50 µm. (P) Quantification of marker⁺ cells. Quantification was performed by the same procedure as in Fig. 4 K (mean ± SD, 5 embryos, 3 sections/embryo). *, P < 0.0001; **, P < 0.001 compared with siRbfox3+GFP. (Q) A model for the Rbfox3-mediated regulation of neuronal development. Upon cell cycle exit, (MNR2 expressed) postmitotic neuronal cells express Rbfox3. Rbfox3 binds to the UGCAUG elements on the Numb premRNA and represses exon 12 usage. The resulting Numb-12ex isoform plays an isoform-specific role in promoting neuronal differentiation (Isl1 expression) of postmitotic neurons. Motor neuron markers are used as examples for states of differentiation.