Fig. 3. Hoxb4 and Hoxd4 regulate the hindbrain expression of Rarb at late but not at early stages. All panels show the expression of Rarb mRNA in dorsal views of E9.0-E10.5 embryos lacking from 0 to 4 wild-type alleles of Hoxb4 and Hoxd4. The wild-type position of the neural Rarb expression border at r6/r7 (arrowhead) is indicated. (A) E9.0 embryos with Hoxb4^+/-; Hoxd4^-/- (left) and Hoxb4^-/-; Hoxd4^-/- (right) genotypes showing wild-type patterns of Rarb expression. At this intermediate stage, although the Rarb neural border lies in the vicinity of the forming r6/r7 junction, it is not as sharp as at E9.5. (B) E10.5 Hoxb4^+/+; Hoxd4^-/- embryo with a normal neural border of Rarb expression at r6/r7. (C,D) E9.5 embryos that are wild type (C) or Hoxb4^-/-; Hoxd4^-/- (D) showing abnormal regression of the Rarb border associated with loss of function of both Hox paralogues. (E,F) E10.5 embryos that are wild type (E) or Hoxb4^-/-; Hoxd4^-/- (F), showing that, by this late stage, removing the activity of both Hox paralogues leads to loss of most Rarb expression from the hindbrain.