Fig. 5. Summaries of the effects caused by the replacement of Fab-7 by _promscs or scs_prom. (A) The thin horizontal line represents the genomic DNA of the abdominal region of the BX-C marked off in kb according to Karch et al. (Karch et al., 1985). The structures of the abd-A and Abd-B transcription units are shown. The cis-regulatory interaction between the iab domains and their respective target promoters are shown by loops. While iab-2, iab-3 and iab-4 regulate abd-A in PS7, PS8 and PS9 respectively, iab-5, iab-6, iab-7 and iab-8 regulate Abd-B expression in PS10, PS11, PS12 and PS13. The red vertical bar represents the scs^min insulator that prevents Abd-B activation by iab-5 and iab-6. Upon isolation from Abd-B, iab-5 is able to interact with abd-A (Hogga et al., 2001). In _promscs, the promoter on the left of the insulator would be activated by iab-3, iab-4 and iab-5, diverting these regulatory domains from the abd-A promoter. This results in the loss-of-function phenotype of iab-3, iab-4 and iab-5 (loss-of-function of iab-6 is due to the inability of iab-6 to regulate Abd-B properly because of the intervening scs^min insulator) (Hogga et al., 2001). (B) The posterior abdomen of a larva from A4/PS9 to A8/PS13 is shown on the left. On the right, the activity states of iab-4 to iab-8 cis-regulatory domains are shown in their respective segments/parasegments. For example, in A4/PS9, iab-4 is active (red), whereas the remaining iab-5,6,7 and iab-8 are kept inactive by the Pc-G-repressing complex (shown in black). In A5/PS10 the next adjacent cis-regulatory domain iab-5 is activated. In A6/PS11, iab-6 becomes active. As the scs^min insulator (red bar) does not insulate iab-6 fully from Abd-B, we envision that iab-6 activates the scs promoter across the insulator in A6/PS11, leading to transcription through iab-7 and iab-8 (shown by the thick arrow). As transcription would interfere with Pc-G complex silencing, ectopic activation of iab-7 and iab-8 occurs in A6/PS11, giving rise to the Fab-7/8 homeotic transformations.