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Fig. 5. The Rarb Distal enhancer specifies the late r6/r7 border and requires inputs from Hoxb4 and Hoxd4. Panels show dorsal views (A) or lateral views (B-H) of Rarb1lacZ transgenic embryos at E8.5-E10.5, histochemically stained for ß-galactosidase activity. (A,B) An E8.5 transgenic embryo (A) and a E9.5 transgenic embryo (B), indicating that distal enhancer activity is initiated after E8.5 and by E9.5 is restricted to the dorsal neural tube with a sharp border at r6/r7. (C,D) Transgenic embryos treated with a single dose of RA at E9.25 and allowed to develop in utero for either 4 hours (C) or 24 hours (D). No ectopic response to RA is detected in either case. (E-H) E10.5 transgenic embryos carrying various combinations of Hoxb4 and Hoxd4 loss-of-function alleles. Strong distal enhancer activity is seen in Rarb1lacZ/+; Hoxb4+/+; Hoxd4+/- (E), weak distal enhancer activity in Rarb1lacZ/+; Hoxb4+/-; Hoxd4+/- (F) and Rarb1lacZ/+; Hoxb4+/-; Hoxd4-/- (G) and no detectable distal enhancer activity in Rarb1lacZ/+; Hoxb4-/-; Hoxd4-/- embryos (H).