Fig. 3. Ectopic expression of Hoxb1 in r6 is associated with activation of r4-like facial branchiomotoneuron differentiation and migration. (A-F) Transverse sections of ventral r6 in E11.5 control (A,C,E) and Hoxa3^-/-d3^-/- double mutant embryos (B,D,F) labeled for Hoxb1 (green), Nkx2.2 (red), Isl1/2 (red) and Phox2b (green). The arrows in B, D, F represent ectopic progenitors in the ventricular layer of the neuroepithelium in r6. (G,H) Transverse sections of ventral r6 and r7 in E11.5 control (G,I) and Hoxa3^-/-d3^-/- double mutant embryos (H,J) labeled for Hoxb1 (green) and Hoxb4 (red). (K,L) Coronal sections of E10.5 control and Hoxa3^-/-d3^-/- double mutant embryos labeled for Hoxb1 (green) and Hoxb4 (red). The arrows in J and L represent ectopic Hoxb1-expressing cells in r7 that have presumably migrated from r6.