Fig. 8. Model for the segmental transformations of Uab^HH1. The ventral cuticle of a wild-type larva is illustrated on the left, with the positions of segments and parasegments marked. A map of the BX-C is drawn along the bottom, showing the nine regulatory regions responsible for the development of parasegments 5-13. The segmental domains of the BX-C have different structures in different parasegments, progressing from all repressed (black ovals) in PS4 to all active (gray ellipses) in PS13. In Uab^HH1, new transcripts (arrows) appear in PS6-PS12, and these transcripts span the iab-2 and iab-3 regions. Transcription in the mutant animals switches these regulatory regions to the active state (hatched ovals), and their activity transforms the morphology of PS6 and PS7 to that of PS8.