Fig. 6. Precision of the expression domain of the Dpp target gene sal. (A) Sal immunostaining. Same set of disks, orientation, source boundary and scale bar as in Fig. 4A. (B) The sal range x^*=d+w was determined by fitting the sigmoidal function s(x)=s₀(tanh((d-x)/w)+1)/2 (red line) to the Sal profile. Quantification and normalization as for GFP-Dpp (Fig. 4). (C) Sal profiles from the dorsal (black) and ventral compartments (blue). (D) Non-normalized Dpp concentration c^Dpp at x^* from the same wing disks in which x^* was determined. Dorsal profiles, black; ventral, blue. Green horizontal bars in B-D, 5 cell diameters (13 µm).