Fig. 3. Regulatory specificity determines cis-interactions at the BX-C. On the B-A-5 construct (top diagram), the IAB5 enhancer was placed 3' of the proximal abd-A-lacZ reporter gene. The distal CAT gene is under the control of the Abd-B promoter. In this configuration the IAB5 enhancer selects Abd-B over abd-A, so that the Abd-B-CAT reporter exhibits a three-stripe IAB5 expression pattern in parasegments 10, 12 and 14; this is readily detected in blastoderm-stage embryos (A) and germband-elongation-stage embryos (B), whereas abd-A-lacZ is silent (C). By contrast, the IAB2 enhancer on the 2-B-A-5 construct (lower diagram) does not interact with its normal endogenous target (abd-A), but directs expression from the Abd-B-CAT gene in parasegments 7 (black arrow), 9, 11 and 13. In blastoderm-stage (D) and germ band elongation-stage embryos (E), a composite IAB2-IAB5-driven expression pattern is detected. The inactivity of the lacZ reporter on this construct (F) suggests that IAB2 only interacts with the most proximal promoter.