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Fig. 5 The combination of pax3a IR1 and IR2 is less susceptible to signaling perturbations than either enhancer alone. (A-G) GFP expression driven by a pax3a IR2+IR1 composite enhancer is shown under various treatments. dkk1-mCherry overexpression beginning with a shield stage heat shock mildly decreases enhancer activity (60%, n=42) (B versus A). bmp2b overexpression beginning with a shield stage heat shock causes increased enhancer activity, a shift medially in the trunk region and expansion in the posterior (87%, n=31) (C versus A). SU5402 treatment starting at 7 hpf drastically decreases the activity of the composite enhancer (100%, n=14) (E versus D). wnt8a-MO injection causes a more severe decrease in activity than dkk1 overexpression (80%, n=10) (G versus F). (H) A model for pax3a IR1 and IR2 activity. IR1 is activated by Wnt and FGF signaling in a wide band surrounding the NPB (green). IR2 activity is precisely positioned at the NPB by BMP signaling and a repressive neural plate factor and is amplified by FGF signaling (yellow). All pictures are dorsal trunk views with anterior upwards.