Fig. 7. A model for the combinatorial coding of DA3 muscle identity in Drosophila. (A) Col is activated in one (T1-T3 segments) and two (A1-A2 segments) promuscular clusters (Crozatier and Vincent, 1999), in response to positional and mesodermal cues. This first step is probably mediated by clusters of relevant TF-binding sites [light orange boxes (Philippakis et al., 2006)], including Twi-binding sites (+) (Sandmann et al., 2007) that are located within the col upstream region and introns. Col expression subsequently becomes restricted to the DA3/DO5 progenitor (orange cell) by lateral inhibition (Crozatier and Vincent, 1999). We postulate that positive inputs from TFs binding to the -2.6 to -2.3 fragment, including Twi, are sufficient to allow P2.6cl activation in the selected DA3/DO5 progenitor, upon relief of N repression. (B) Following division of the progenitor, restriction of Col expression to the DA3 FC involves positive auto-regulation in this FC and negative regulation by N in the sibling DO5 FC. From this stage, a combination of Nau and Col activity is required for col transcriptional activation in the FCM nuclei, which are recruited by the DA3 FC to form a myofibre, thereby ensuring that all nuclei in the DA3 muscle express the same identity programme.