Fig. 3. Math1 and Mash1 are transcriptional activators in their neurogenic and neuronal cell-type-specific activities. (A-L) HH14-16 chick neural tubes were electroporated in ovo with expression constructs VP16 (A,B), Math1bHLH-VP16 (C,D), Mash1bHLH-VP16 (E,F), EnR (G,H), Math1bHLH-EnR (I,J) and Mash1bHLH-EnR (K,L), harvested at 24 hours. (A,C,E,G,I,K) Immunofluorescence using anti-myc antibodies to assay for expression of the transgene demonstrates the movement of the cells to the lateral neural tube where differentiating neurons reside. White dots outline the injected side of the neural tube and arrows indicate the lateral (C,E) or medial (I,K) position of the electroporated cells within the neural tube. (M) Quantification of these data shown as a ratio of fluorescence intensity (FI) in the lateral half versus the medial half of the neural tube (M). (B,D,F,H,J,L) Interneuron populations dI1 and dI3 were detected using anti-Lhx2/9 (red) or Islet1 (green). (N) Quantification of these data shown as a ratio of the number of labeled cells on the electroporated side (right, indicated by arrowhead) versus the number of labeled cells on the control side (left). VP16 and EnR on their own have no effect in these assays. Math1bHLH-VP16 and Mash1bHLH-VP16 approximate the activity of full-length Math1 and Mash1 in both the neuronal differentiation and cell-type specification activities. In contrast, Math1bHLH-EnR and Mash1bHLH-EnR have the opposite effect. ^**P<0.001.