Fig. 7. Generation of ectopic Phox2b⁺ motoneurons requires the combined activities of Hox and Nkx2 factors. Dorsal views of whole-mounts (A-E) or cross-sections through r1 (G-I) of stage 21-22 chick embryos electroporated, on the right-hand side, with the vectors indicated above each panel, and assayed for Phox2b in situ hybridization (A-E,G) or Isl1/2 immunohistochemistry (H). Hoxa2 (A) or Hoxb1 (B) misexpression induces ectopic Phox2b expression only in ventral r1. Conversely, Nkx2.2 misexpression (C) induces ectopic Phox2b expression at dorsal levels, though not in r1. Co-electroporation of Nkx2.2 with Hoxa2 (D) or Hoxb1 (E) additionally induces ectopic Phox2b⁺ (G), Isl1/2⁺ (H) motoneurons in dorsal r1. (I) GFP fluorescence, showing the dorsoventral distribution of electroporated cells. (F) Summary showing, on AP and DV coordinate axes, the requirement for the combined activities of Hox and Nkx2.2 proteins to induce ectopic Phox2b expression in the hindbrain. GOF, gain of function; r, rhombomere.