Fig. 4. Small sax clones reveal both signal promotion and antagonism functions. Wing pouch of third instar wing discs (dorsal is upwards; anterior is leftwards) stained for pMad (red), brk-lacZ (blue), GFP (green). AP boundary is indicated by vertical arrows. White lines indicate transect of fluorescence intensity profiles. (A) Wild type. (B-C''') sax-null clones (absence of GFP, B,C) result in reduced pMad levels (B,C'''), but increased pMad levels (asterisks) in cells adjacent to the clone (B'',C''', clone position indicated by the gray bar). (C,C') Ectopic brk expression occurs in some sax mutant cells. Pattern of ectopic brk expression highlights where contribution of Sax to signal promotion is highest and where Tkv-Tkv is unable to mediate sufficient signaling to repress brk.