Fig. 4. The Rarb Proximal enhancer recapitulates early neural expression and responds transiently to RA. All panels show dorsal views of Rarb2lacZ transgenic embryos at E8-E10.25, histochemically stained for ß-galactosidase activity. (A,B) E8-E8.5 transgenic embryos at the two-somite (A) and seven-somite (B) stages. At both time points, strong reporter expression is present within the neural plate with weaker staining also visible in paraxial and lateral mesoderm. By the seven-somite stage, a diffuse anterior border maps to the vicinity of the Os. (C,D) E9.5 embryos of the genotype Rarb2lacZ/+; Hoxb4^+/+; Hoxd4^+/- (C) and Rarb2lacZ/+; Hoxb4^-/-; Hoxd4^-/- (D) showing that, at this late stage, the neural activity of the proximal enhancer is largely confined to the developing spinal cord with only residual low levels remaining within the caudal hindbrain. This late pattern is unaffected when the functions of both Hoxb4 and Hoxd4 are removed. (E,F) E9.25 transgenic embryos treated with an exogenous pulse of RA, showing a clear ectopic response within r4-r6 at 4 hours (E), but not 24 hours (F), after treatment. The expression pattern in F is similar to that seen in C.