Fig. 4. EXT genes are required for Dpp signaling. Levels of Sal and p-Mad in the wild-type and mutant discs. (A) The Sal and p-Mad levels are normally high in the central region of the wild-type wing disc. In A cells close to the A-P boundary, the Sal and p-Mad levels are much lower than those in the adjacent P cells. A schematic diagram is shown on the right panel. (B-D) In botv⁴²³ (B), sotv³²⁶ (C) and ttv⁵²⁴ mutant cells (marked by the absence of GFP), Sal and p-Mad levels are decreased (red and blue on the right panel, respectively). The A-P boundary is indicated by white arrowheads in B-D, right panel.