The identities of the second through ninth abdominal segments of Drosophila are specified by two genes of the bithorax complex (BX-C), abdominal-A (abd-A) and Abdominal B (Abd-B). The correct deployment of these two genes requires an extensive region (the iab region) located between the two protein-coding transcription units. We show here that one iab mutation affects the pattern of expression of Abd-B. We also show that most or all of the DNA in this regulatory iab region is transcribed. In blastoderm stage embryos we can define three distinct domains within the iab DNA, each transcribed in a region that extends from a characteristic anterior limit to the posterior end of the segmented part of the embryo. The anterior limits of expression for the three regions are colinear with the sequence of the domains on the chromosome, and lie at about two-segment intervals. We suggest that these early transcription patterns reflect the initial activation of the BX-C.

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