Fig. 5. A constitutively active RAR, but not RA, can rescue the effects of FGF gene loss-of-function on posterior markers. Embryos were microinjected unilaterally at the two cell stage with ß-galactosidase mRNA as lineage tracer and (A,D) 1 ng of XFD mRNA, (B,E) 1 ng of XFD mRNA then treated with 10^–6 M atRA, or (C,F) 1 ng of XFD and 1 ng VP16-XRAR2 mRNA. When control embryos reached stage 18, the embryos were fixed, stained for ß-galactosidase activity and processed for whole-mount in situ hybridization with either HOXB9 (A-C) or XCAD3 (D-F) probes.