Fig. 6. Transcriptional inputs proposed to regulate hh in the A compartment. (A) The wild-type situation. The levels of Ci[rep] (blue) and Ci[act] (brown) are complementary, as these forms are reciprocally controlled by Hh signalling at the AP boundary (black vertical line). Ci[act] induces the expression or the activity of repressor X, which together with Gro forms a repressive complex (green) – thus, the green line follows the brown, except in cases of gro loss of function (C). As there is some repressor complex (blue or green) present at any given position in the A compartment, hh expression (red) is nil. B shows two ci^- clones (grey bars on the AP axis), where both forms of Ci (blue and brown) are eliminated. They both contain low basal levels of hh transcription. This is because of the absence of both repressor and activator complexes within the clones. Note that the brown and blue curves have been shifted anteriorly (to the left) – this is because the loss of ci near the boundary disables the induction of ptc and therefore posterior Hh can diffuse further (Chen and Struhl, 1996) anteriorly to generate Ci[act] and suppress Ci[rep]. C shows two gro^- clones (cross-hatched bars on the AP axis). Those affect only the X-gro repressor activity (drop in green line). As a result of high levels of Ci[act] activator (brown) and lack of repressor (green or blue) within the more posterior (rightmost) clone, hh is expressed at high levels. No effect is expected in the more anterior clone. Again, note that the brown and blue curves have been shifted anteriorwards as a result of high ectopic Hh activity that influences the relative amounts of Ci[act] and Ci[rep]. In B, we assume that the low basal hh levels within the clones are not sufficient to produce a similar effect.