Fig. 1. Cis interactions in Abd-B. (A) Abd-B has an extended 5' regulatory region that can be subdivided into four regulatory domains, termed infra-abdominal (iab)-5, iab-6, iab-7 and iab-8. Each of these controls the development of a corresponding abdominal parasegment (PS). For example, iab-5 regulates Abd-B function in PS10 (roughly the fifth abdominal segment, or A5), iab-6 controls A6, and so on. Neighboring iab genes are separated by domain boundary elements such as Fab-7 and Fab-8, which function to prevent cross-regulatory interference between neighboring iab genes. These elements are also potent insulators that can block enhancer-promoter interactions in transgenic constructs. The PTS element has an anti-insulator activity that allows an enhancer to activate a promoter despite an intervening insulator. In the Abd-B locus, PTS may mediate enhancer-promoter interactions by allowing enhancers to overcome the intervening Fab elements and activates Abd-B promoter. (B) Detailed map of the 9.5 and 5.3 kb tmr elements. B, BamHI; H, HindIII; P, PstI; and R, EcoRI.