Patterning of the Drosophila wing imaginal disc involves a Bmp activity gradient that is created by two Bmp ligands (Gbb and Dpp) and two type 1 receptors (Tkv and Sax). On p. 3295, Bangi and Wharton report that Sax shapes this gradient by both antagonizing and promoting Bmp signalling. Previous data have suggested that Sax mediates Gbb signalling while Tkv mediates Dpp signalling; but, puzzlingly, loss of sax function does not have the same effect as loss of gbbfunction. The researchers now show that in the absence of saxfunction, the distribution of the downstream effector of Bmp signalling (pMad)is consistent with an increase in Bmp activity, not a decrease, as expected if Sax just transduces Gbb signals. The researchers propose that Sax has two functions: when complexed with itself, Sax antagonizes Bmp signalling by sequestering Gbb; when complexed with Tkv, it facilitates signalling. The balance between these functions, they suggest, will shape and stabilize the Bmp activity gradient across the developing wing.