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Fig. 2. Effect of RA treatment and somite grafting on Hoxb4 and Hoxb9 expression in the neural tube. Dorsal views of stage 15 (A-C,G) stage 19 (D,H) and stage 14 (E,F) embryos hybridized with Hoxb4 (A-D) or Hoxb9 (E-H). (A,E) Untreated embryos. (B,F) Retinoic acid treated embryos. Exogenous application of retinoic acid causes anterior shift of the expression domain and creates a new anterior limit of Hoxb4 expression (black arrow in B) while Hoxb9 does not show any anterior shift (F). (C,G) Embryos electroporated with a dnRAR expressing construct unilaterally on the left side of the neural tube. dnRAR causes down-regulation of endogenous Hoxb4 expression (white arrowheads in C) while Hoxb9 expression is not affected (G). (D,H) Hoxb4 and Hoxb9 expression in grafted embryos, whereby posterior somites 23-25 of a stage 15 donor embryo were transposed into an anterior region of a stage 15 host embryo at the level of somite 7-9 and cultured for 36hrs. The grafted somites induce upregulation of Hoxb4 (*, D) while there is no change in the pattern of Hoxb9 expression (*, H). Black arrowheads in H show position of graft. OV, otic vesicle; SM, somite grafts.